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GEOGUIDE 1GUIDE TODESIGNCeotechnkal ControlEngineering Development DepartmentHONG KONGJuly 1982


Reprography by the Government Printer, Hong Kong


-1-This Guide to Retaining Wall Design is the first Guide to be- -—""produced by the Geotechnical Control Office.It will be found useful tothose engaged upon the design and construction of retaining walls and otherearth retaining structures in Hong Kong and* to a lesser extent, elsewhere.This Guide should best be read in conjunction with the Geotechnical Manualfor Slopes (Geotechnical Control Office, 1979), to which extensive referenceis made*The Guide has been modelled largely on the Retaining Wall DesignNotes published by the Ministry of Works and Development, New Zealand (1973),and the extensive use of that document is acknowledged. Many parts of thatdocument^ however, have been considerably revised and modified to make themmore specifically applicable to Hong Kong conditions. In this regard, itshould be noted that the emphasis in the Guide is on design methods whichare appropriate to the residual soils prevalent in Hong Kong.Many staff members of the Geotechnical Control Office havecontributed in some way to the preparation of this Guide, but the maincontributions were made by Mr. J.C. Rut ledge, Mr. J.C. Shelton, and Mr. G.E.Powell. Responsibility for the statements made in this document, however,lie with the Geotechnical Control Office.It is hoped that practitioners will feel free to comment on thecontent of this Guide to Retaining Wall Design, so that additions andimprovements can be made to future editions.E.W. BrandPrincipal Government Geotechnical Engineer


CONTENTSPage NoFOREWORDCHAPTER 1 : INTRODUCTION1.1 SCOPE OF THIS PESIGN GUIVE1.2 RETAINING WALL PESIGN PRINCIPLES1.2.1 tn.


-4-CHAPTER4 : EFFECTS CF SURCHARGES4.14.24.3CHAPTERS5.75.25.35.45.5CHAPTERS6.16.26.3UNIFORM SURCHARGESLIME LOAVSPOINT LOAVSEFFECTS OF WATERGENERALEFFECT OF WATER ON EARTK PRESSURES5.2.7 Static. WateA Leve£5.2.2 flowing WateA.DRAINAGE PROt/ISIONSFILTER REQUIREMENTS5.4.7 Graded F-t&te/ti5.4.2 Geo;tex£c£e&CONTROL OF GROUNPWATERSTABILITY OF RETAINING MALLSGENERALSLICING STABILITY6.2.7 8a4£ wAhouJL a Ker/6.2.2 Sa6e wtc#i a. fCw/6.2.3 SLLcUng on a Rock foundationOl/ERTURNING STABILITY6.3.7 GeneAoJL6.3.2 facto*. o Safety again&tPage. No31313232333334343435373738404141424242434343446.46.56.66.76.B6.3.3FOUNDATION BEARING PRESSURE6.4.7 Gvn.


- 5-Page NoCHAPTER/ : SHEET RETAINING STRUCTURES si7.1 GENERAL 517.2 STRUTTED EXCAVATIONS 517.3 ANCHORED FLEXIBLE WALLS 537.3.7 Walts, Anchored neoA £he Top 537.3.2 Multiple.-anc.hoK.zd {Haiti, 547.3.3 Ej^eoti 0)J AncAo/i Inctination 557.4 CANTILEl/EREP WALLS 55CHAPTER 8 : REINFORCED EARTH RETAINING WALLS 5?CHAPTERS : CRIB WALLS 599.7 GENERAL 599.2 PESIGN 609.3 BACKFILL 619.4 PROI/ISION OF DRAINAGE 619.5 MULTIPLE PEPTH WALLS 619.6 WALLS CURVED IN PLAN 61CHAPTER 10 : SETTLEMENTS ADJACENT TO LARGE 63EXCAVATIONS70.7 GENERAL 6370.2 MINIMIZING SETTLEMENTS 6370.3 PREDICTIONS OF SETTLEMENT 64CHAPTER 11 : SOME ASPECTS OF REINFORCED CONCRETE 67DESIGN AND RETAILING77.7 INTROPUCTION 6777.2 GENERAL NOTES 6777.2.7 Ccdea 6777.2.2 Ultimate. S&ie.ngtk 01 Linut State. Vtei.gn 6777.2.3 CoveA to Re^otcewien* 6777.3 TOE PESIGN 6877.4 STEM DESIGN 6877.4.7 Stem Loading 6877.4.2 Bending Moment* and SheaA. Po-tcei


~ 6 ~Page. No11.5 HEEL SLAB VESIGN 6911.5.1 Loading 6911.5.2 tfee£ Slab* ion. CountM-Aovt \HaJUU> 6911.6 COUNTERFORT PESIGN 6911.7 KEV PESIGN 7011.S CURTAILMENT ANP ANCHORAGE OF REINFORCEMENT 7077.9 DETAILING OF REINFORCEP CONCRETE CORNERS ANPJOINTS7111.9.1 BocfegAxjund 7111.9.2 R&oi^o/uUng Ste.e£ Ve&uLing Raeommewdotconi 7377.70 JOINTS 7411.10.1 VeAJMiat Joint* {,0*. LonQituucLLnaJt Movement 7411.10.2 HonJ-zonXaJi Cam>&U!ic£ion 3oin&> 7577.77 CONTROL OF CRACKING 7577.72 APPEARANCE' OF RETAINING WALLS 76APPENDIX A : SYMBOLS 83APPENDIX B : LIST OF TABLES 87APPENDIX C : LIST OF FIGURES 89FIGURES 91 to 15377


- 7-1INDUCTION1.1 SCOPE Of THIS VES1GM GtlWEThese notes are Intended as a Guide for use in the estimation ofearth pressure forces 9 and for the design and construction of retainingwalls and other earth retaining structures In Hong Kong. Recommended methodsare given for most aspects of design 9 except for reinforced concrete, whereguidance is given on only a few special points. Throughout the Guide,reference is made to relevant textbooks, Codes and published papers, and thereader should consult those original documents for more detailed coverage ofparticular aspects of the subject matter.It Is important to remember that engineering judgement shouldalways be exercised in applying the theories and design methods given In theGuide. In particular, the practitioner must be aware of the limitations onthe basic assumptions employed In a particularly theoretical or computationalmethod.The material contained in this Guide has been arranged so as toprovide the maximum convenience to the user. In this respect, it should benoted that :(a) Full references to the published material cited in the textare given in alphabetical order on pages 77 to 81.(b) A list of symbols used in the text and figures Is given onpages 83 to 86.(c) A list of tables contained in the text is given on page 87 .(d) A list of figures is given on pages 89 to 90 .(e) The 32 figures referred to in the text are collected togetheron pages 91 to 153at the back of this Guide.


-8-7.2 RETAIA/IWG WALL PESIGW PRINCIPLES1.2.1 fsmz-AtandUng R^taiyung Watt*In the design of free-standing retaining walls , the followingaspects need to be investigated :(a) the stability of the soil around the wall*(b) the stability of the retaining wall itself ,(c) the structural strength of the wall; and(d) damage to adjacent structures due to wall construction*The magnitude of the earth pressure which will be exerted on awall is dependent on the amount of movement that the wall undergoes.It is usual to assume for free-standing retaining walls thatsufficient outward movement occurs to allow active (minimum) earth pressuresto develop. The designer must ensure that sufficient movement can take placewithout affecting the serviceability or appearance of the wall.Where it is not possible for the required outward movement tooccur 9 for instance due to wall or foundation rigidity, higher pressureswill develop and the wall must be designed for these. Further guidance onthis matter is given in Section 3.2.7.2*. 2 Otk&iIf a structure prevents outward movement of the soil* the wallwill usually be subject to static earth pressures greater than active. Thisoccurs where a wall retaining earth is part of a more extensive structure,such a basement wall in a building or an abutment wall of a portalstructure. It also occurs when the wall is connected to another structure,such as a bridge abutment connected to the superstructure.1.3 LOAV CASES1.3J BaAJjc. Loading*The basic pressure loading to be considered for design is :Normal loading - static earth pressure 4- water pressure +pressure due to live loads or surcharge.


-9 -In general, the resulting design pressure for earth retaining structuresshould not be less than the pressure due to a fluid of unit weight 5kN/m 3 .It should be noted that, in accordance with Chapter 4 of Volume Vof the Civil Engineering Manual., (Public Works Department, Hong Kong, 1977),highway structures should be designed to withstand seismic forces correspondingto ground accelerations of O.G7g. It may be assumed that conventionalcantilever, counterfort and gravity retaining walls of normal proportionsand detailing will have adequate resistance to withstand such an acceleration.Further guidance may be obtained from the paper by Seed & Whitman (1970).7.3.2The possible occurrence of other design cases, or variations of theone above, caused by construction sequence or future development of surroundingareas should also be considered. For instance, additional surcharges may needto be considered and allowance made for any possible future removal of groundin front of the wall in connection with services, particularly if the passiveresistance of this material is included in the stability calculations.The effect of excavation on the wall bearing capacity may also need to beconsidered.For the determination of earth pressures, it is usual to considera unit length of the cross-section of the wall and retained soil. A unitlength is also used in the structural design of cantilever walls and otherwalls with a uniform cross-section.7.4 SUPPORT Of EXISTING FILL SLOPES '-'Fill slopes constructed in Hong Kong prior to 1977 are likely tohave been end tipped or inadequately compacted. Such slopes may be subjectto liquefaction under conditions of heavy rainfall, vibration or leakage fromservices, and resulting mud flows may have serious consequences. Undercuttingof the slope toe, for retaining wall construction, will increase the risk offailure.


-10-The state of existing fill slopes should established by insitudensity testing in trial pits* in conjunction with GCO probe (ModifiedMackintosh probe) testings to eatablish the insitu dry density and aeral extentof the loose fill. Where appropriate, remedial measures should be carried outto ensure that failure of the fill slope cannot occur by liquefaction.


"11-2SOIL2.1 GEWERALFor all walls higher than 5 metres, especially those with slopingbackfill, the soil properties of the natural ground and backfill should beestimated in advance of design from tests on samples of the materials involved*In addition, special attention should be paid to the determination of groundwater levels, particularly with respect to maximum probable values.For less important walls, an estimation of the soil propertiesmay be made from previous tests on similar materials, A careful visualexamination of the materials, particularly that at the proposed foundation level,should be made and index tests carried out to ensure that the assumed materialtype is correct.2.2 SELECTION AMP USE OF BACKFILLThe ideal backfill for a minimum section wall is a free draininggranular material of high shearing strength. However, the final choice ofmaterial should be based on the costs and availability of such materialsbalanced against the cost of more expensive walls*In general, the use of fine-grained clayey backfills is not recommended.Clays are subject to seasonal variations in moisture content and consequentswelling and shrinkage. This- effect may lead to an increase in pressure againsta wall when these soils are used as backfill. Due to consolidation, longterm settlement problems are considerably greater than with cohesionlessmaterials.For cohesive backfills, special attention must be paid to theprovision of drainage to prevent the build-up of water pressure. Free drainingcohesionless materials may not require the same amount of attention in thisrespect. They may still require protection by properly designed filter layers.


The wall deflection required to produce the active state in cohesivematerials with a significant clay content may be up to 10 times greater thanfor cohesionless materials. This 9 together with the fact that the formergenerally have lower values of shearing strengths means that the amount ofshear strength mobilised for any given wall movement is considerably lowerfor cohesive materials than for cohesionless materials* The correspondingearth pressure on the active side for a particular wall movement will thereforebe higher if cohesive soil is used for backfill.yIn Hong Kong, backfill for retaining walls usually comprises selecteddecomposed granite or decomposed volcanic rock. This material is in generalsuitable for backfill provided that it is properly compacted and drainagemeasures are carefully designed and properly installed to prevent build-upof water pressure.Rock fill is a very suitable material for use as a backfill toretaining walls and consideration should be given to its use when available,In general, the rockfill should be well graded and have a nominal maximum sizeof 200mm. A well-graded densely compacted rockfill should not have more thanabout 2% finer than 75ym if it is to remain free-draining.Movement of soil, due to seepage f into the rockfill needs to beprevented. This may require the provision of properly designed filter layersbetween the soil and the rockfill.It is essential to specify and supervise the placing of backfill toensure that its strength and unit weight properties agree with the designassumptions both for lateral earth pressure and dead weight calculations. Inthis regard s it is particularly important to ensure that the backfill behinda wall and on a slope is properly compacted. The backfill should normallybe compacted in thin layers using light compaction plant for the reasonsoutlined in Section 3.10.The active earth pressure is substantially reduced, particularly fora steeply sloping backfill, if the failure plane occurs in a material with ahigh angle of shearing resistance. In some circumstances* it may be economicalto replace weaker material so that the above situation occurs.


- 13 -2.3 UWIT WEIGHTThe unit weight of soil depends on the specific gravity of thesolid particles and the propertions of solid» air and water in the soil.The average specific gravities of Hong Kong soils in general lie between 2.65and 2.70* although values outside this range are found. The proportion ofthe total volume that is made up of this solid material is dependent on thedegree of compaction or consolidation*As estimate of the unit weight of backfill material to be usedbehind a retaining structure may be obtained from standard laboratorycompaction tests on samples of the material or from records of field testing.The unit weight chosen must correspond to the compaction and moisture conditionsthat will apply in the actual field situation*The unit weight of natural soil should be obtained from undisturbedsamples kept at the field moisture content and volume. For initial designpurposes, dry densities in the range 1750 to 1850kg/m 3 may be assumed forall soils compacted near optimum moisture content.2.4 EFFECTIi/E STRESS AWP PORE PRESSUREAn effective stress may be considered to be the stress transmittedthrough the points of contact between the solid particles of the soil. It -is this stress that determines the shearing resistance of the soil. Theeffective stress, a ? , at any point in a saturated soil mass may be obtainedby subtracting the pressure transmitted by water in the voids 5 u s (porewater pressure) from the total stress, a, thus :a ? = a ~ u ...'..(I)An increased pore water pressure gives a reduced effective stressand therefore a reduced soil shearing resistance. This leads to an increasedforce against a wall in the active case. Conversely, an increase in thenegative pore pressure (i»e. a pore suction) gives an increased shearingresistance and reduces the force against a wall in the active case.


Positive pore water pressure results from a number of factors,the most important in Hong Kong being static water pressure, seepage ofgroundwater or rainfall and seepage from other sources, such as burst orleaking water supply mains. In some soils, shock or vibration can causetransient increases in pore pressure. In low permeability soils 9 changesin pore water pressure can result from changes in total stress due toground loading, dewatering or excavation. These pore pressures dissipatewith times, but may need to be considered in design. Pore water pressuresdue to static water pressure and seepage of water are covered in Chapter 5.Negative pore pressures are present in many partially saturatedsoils in Hong Kong. Soil suction may be destroyed by surface infiltrationor seepage, and* until more information on its magnitudes distribution andbehaviour becomes available 9 its effect on the shear resistance of the soilshould not be used in retaining wall design.2.5 SHEAR STREW6THIn all earth pressure problems the magnitude of earth pressure ona particular structure is a function of the shear strength of the soil.The shear strength is not a unique property of the material but depends uponthe conditions to which the soil is subjected when it is sheared. Where aretaining structure supports a saturated clay soil of low permeability 9 theundrained shear strength can be used to calculate the earth pressure forshort-term stability., because the shear strength of such soil does not changeas it is sheared quickly (i.e. the excess pore water pressures cannotdissipate during shear). However* Hong Kong residual soils are not saturatedand they have relatively high permeabilities. The water contents therefore,can change quite rapidly, with a consequent change in pore pressure and, hence*with a change in shear strength* It is necessary, therefore, for earthpressures in Hong Kong soils to be calculated from shear strengths expressedin terms of effective stresses,The shear strength of a soil is proportional to the effective stresswhich acts on the failure plane. Laboratory tests can be carried out toestablish the relationship between strength* S, effective stress, cr f , and thisis commonly termed the strength envelope. The envelope will generally becurved, but portions of the curve can be approximated by the relationship :


-15-S - c 1 + a s tan 0 f . (2)where c ! and 0 ? are termed the effective strength parameters. These parametersshould be used for earth pressure calculations in Hong Kong soils- It isimportant to note that the design strength parameters must be those determinedin the laboratory for the range of effective stress which is appropriate to thefield situation.Laboratory triaxial tests or shear box tests are commonly used todetermine the strength envelope of a soil. Guidance on these methods ofstrength measurement and on the interpretation of test results can be obtainedfrom Lambe & Whitman (1969) and from the Geotechnical Manual for Slopes(Geotechnical Control Office, 1979).The following two types of triaxial tests can be used :(a) Consolidated-undrained tests with pore pressure measurement(CU tests) carried out on specimens saturated using backpressure.(b) Drained tests (CD tests) on saturated specimens.Shear box tests are simpler to carry out than triaxial tests butonly drained tests can be conducted on Hong Kong residual soils. Care shouldbe taken to ensure that test specimens are soaked for a sufficient period priorto testing and that submergence is maintained during shear«The shear strength of a backfill material depends on its density*and laboratory strength tests should be carried out on specimens compactedto the density that will exist insitu. Where inadequate shear strengthinformation is available at the time of preliminary design 9 the followingvalues may be taken as guidance to the properties of compacted Hong Kongsoils :For decomposed volcanics, c f « 0, 0 f « 35°,Ya 88 1750kg/m 3For decomposed granite, c' « 0 9 0 ? « 39°,Yd ** 1850kg/m 3


-16-2.6 BASE SHEAR RESISTANCEThe amount of shearing resistance available between the base ofthe wall and the foundation soil will depend on the nature of materials usedto construct the base and on the construction technique*The base friction to be used for walls without a key is 20 f /3.When it can be ensured that the excavation of the base will be carried outin the dry season and that disturbance and deterioration of the subsoil isprevented by construction of an adequate blinding layer immediately afterfoundation exposure, and where there is professional site supervision it maybe possible to justify a higher proportion of 0 f . Values of base adhesion,%, used in calculations should be taken as zero unless specific data provingotherwise are available*If a shallow base key is used, the failure plane will generally bethrough the foundation soil (see Figure 1) and, therefore, the shearingresistance may be taken as that of the soil (6^ = 0 f and c^ = c 1 )- Furthercomment on this is given In Section 6.2,2.7 WALL FRICTIONThe magnitude and direction of the developed wall friction dependson the relative movement between the wall and the soil* In the active case,the maximum value of wall friction develops only when the soil wedge movessignificantly downwards relative to the rear face of the wall. In some cases,wall friction cannot develop. These include cases where the wall moves downwith the soil, such as a gravity wall on a yielding foundation or a sheet pilewall with inclined anchors, and cases where the failure surface forms awayfrom the wall, such as in cantilever and counterfort walls (Figure 9).The maximum values of wall friction may be taken as follows :Timber, steel, precast concrete,max. *=Cast in-situ concrete,6 max.In general, the effect of wall friction is to reduce active pressure.The effect is small and often disregarded.


- 17-The effect of wall friction on passive pressures Is large (seeSection 3).Considerable structural movements may be necessary 9 however,, tomobilise maximum wall friction* for which the soil in the passive zone needsto move upwards relative to the structure. Generally 5 maximum wall frictionis only mobilised where the wall tends to move downwards f for example. If awall is founded on compressible soi! 9 or for sheet piled walls with Inclinedtensioned members. Some guidance on the proportion of maximum wall frictionwhich may develop in various cases Is given in Table 1; the residual soils ofHong Kong might be taken to be covered by these data.Table 1.Indicative Proportions of Maximum WallFriction Developed(Granular Soils - Passive Case)(Rowe & Peaker, 1965)Structure TypeDeveloped Proportionof Maximum WallFrictionLooseDenseGravity or free standing walls withhorizontal movement. Sheet pile wallsbearing on hard stratumSheet walls with freedom to move downwardsunder active forces or inclinedanchor loadsWalls where passive soil may settleunder external loadsAnchorage blocks * etc. which havefreedom to move upwards on mobilizationof passive pressure*01.0000.51.000Where a wall will be subjected to significant vibration, wallfriction should not be included.


- 18 -2.8 COEFFICIENT OF SUBGRAPH -REACTIONIn the design of footings and wall foundations, the simplifiedconcept of subgrade can be used to determine wall rotations. This conceptis based on the assumption that the settlement, A* of any element of aloaded area is entirely independent of the load on the adjoining elements.It is further assumed that there is a constant ratio, K s$ between theintensity9 q* of the foundation pressure on the element and the correspondingsettlement, A* given by :i A(3)The foundation pressure s q s is called the subgrade reaction^ and the ratio,K ss is known as the coefficient of subgrade reaction.2.9 PERMEABILITYLumb (1975) has presented values of insitu permeability for HongKong residual soils. These are summarized in Table 2 and may be used togive some guidance. It should be noted, however 9 that other sources ofpermeability test results have revealed values well outside these rangesands when the particular value is critical in design, permeability testsshould be carried out. In this regard, Lumb has noted and subsequentinvestigations have confirmed that laboratory measurements of permeabilityof decomposed volcanics based on small intact tube specimens are two ordersof magnitude lower than values obtained from field tests. Lumb attributedthe difference to the influence of joints. Laboratory results 5 therefore*should be treated with caution*Table 2Insitu Permeabilities of Hong Kong Residual SoilsSoilDecomposed granitesPermeability(m/s)3 x ICf 7 to 4 xID' 5Decomposed volcanics-92 x 10 to 4 xID' 7


- 19-Procedures for determination of insitu permeability are givenin Chapter 2 of the Geotechnical Manual for Slopes.The permeabilities of granular backfill materials 9 in relation toparticle grading, are given in greater detail in Figure 20.


33.1 STATES OF STRESSEMTHThe stresses at any point within a soil mass may be represented onthe Mohr co-ordinate system In terms of shear stress, T, and effective normalstress 5 a 1 . In this system,* the shearing strength of the soil at variouseffective normal stresses gives an envelope of the combinations of shear andnormal stress. When the maximum shearing strength Is fully mobilised alonga surface within a soil mass* a failure condition known as a state of plasticequilibrium is reached. Reference should be made to Section 3.9 In theGeotechnical Manual for Slopes for the plotting of stresses and use of thesystem.Where the combinations of shear and normal stress within a soil massall lie below the limiting envelope, the soil Is in a state of elasticequilibrium (Terzaghi & Peck, 1967). A special condition of elastic equilibriumis the ? at-rest f state, where the soil is prevented from expanding or compressinlaterally with changes In the vertical stress. Any lateral strain In the soilalters its horizontal stress condition. Depending on the strain Involved, thefinal horizontal stress can lie anywhere between two limiting (failure)conditions, known as the active and passive failure states.3.2 AMOUNT AMP TYPE OF WALL MOVEMENTThe earth pressure which acts on an earth retaining structure isstrongly dependent on the lateral deformations which occur in the soil.Hence, unless the deformation conditions can be estimated with reasonableaccuracy, rational prediction of the magnitude and distribution of earthpressure in the structure is not possible.The minimum active pressure which can be exerted against a walloccurs when the wall moves sufficiently far outwards for the soil behind thewall to expand laterally and reach a state of plastic equilibrium. Similarly,the maximum passive pressure occurs when the wall movement is towards thesoil. The amount of movement necessary to reach these failure conditions isdependent primarily on the type of backfill material* Some guidance on thesemovements is given in Table 3.


-22-Table 3Wall Displacements Required toDevelop Active and Passive EarthPressures (Wu, 1975)SoilState of StressType of MovementNecessaryDisplacementSandActiveParallel to wallO.OQ1HActiveRotation about base0.001HPassiveParallel to wall0.05 HPassiveRotation about base> 0.1 HClayActiveParallel to wallO.QQ4HActivePassiveRotation about base0.004H-For wall displacements less than those necessary to produce thefailure conditions, the magnitude of the pressure on the wall lies betweenthe extreme values* Figure 2 shows the typical variation in wall pressurewith movement.For a rigid wall free to translate or rotate about its base, theactive or passive condition occurs if sufficient movement can take place, andthe pressure distribution remains approximately triangular for uniform slopingground (Figure 3(a)).In some cases, rotation about the base or translation of a freestanding wall may be limited by a strong foundation or by some other restraintsuch as occurs in bridge abutments or walls framed^in with the superstructure.Structural deformations for walls are not usually sufficient alone to allowdevelopment of active pressures, and hence the wall is subject to pressuresnear those for at-rest conditions (Figure 3(b)) or those caused by compaction(Section 3*10). Thermal expansion of-the structure may force the retainingwall into the soil producing higher earth pressures (Broms & Ingelson 1971),When the top of the wall is restrained while the base can rotate, notall of the retained soil passes into the active state. Limited movement nearthe top of the wall, together with arching, leads to an approximately parabolicpressure distribution, with a corresponding force on the wall 10 to 15% higherthan the force for the active condition (Figure 3(c)).


-23 -An approximate calculation of the magnitude of the tilting movementthat results from the backfilling of a retaining wall may be obtained bysimulating the foundation soil as a series of springs with an appropriatecoefficient of subgrade reaction (see Section 2.8). The base rotation, 6 b ,(radians) is then given by :T>( for e b $ 7- ) . (4)where V is the vertical component of the foundation bearing pressurese^ is the eccentricity of the load on the baseL, B are length and breadth of the bases respectively,and K s is the coefficient of subgrade reaction (Eqn. 3).Flexible walls allow complex deformations and redistribution of loads,Loads vary on individual supports depending largely on the stiffnesscharacteristics of the supports themselves.Strutted walls have approximate final deformation patterns as shownin Figure 3(d). This profile is strongly influenced by construction detailsand procedures, and so pressure envelopes covering possible actual pressuredistributions are used for retained heights of greater than 6 metres* (Figure24).Compaction of the backfill can produce pressures higher than active.This is discussed in Sections 3.10 & 3.11.3.3 RANKINE EARTH PRESSURE THEORYRankine's equations give the earth pressure on a vertical plane whichis sometimes called the victual back of the wall. The earth pressure on thevertical plane acts in a direction parallel to the ground surface and isdirectly proportional to the vertical distance below the ground surface.The pressure distribution is triangular.Rankine f s conditions are theoretically only applicable to retainingwalls when the wall does not interfere with the formation of any part of thefailure wedges that form on either side of the vertical plane, as shown inFigures 1 & 9 or where an imposed boundary produces the conditions of stressthat would exist in the uninterrupted soil wedges. These conditions require thatthe angle of wall friction is equal to the backfill slope (6 » w).


-24 -Passive calculations using Rankine are not recommended 9 since thedirection of wall friction will be incorrect and an underestimation ofpassive resistance will result.3.4 COULOMB EARTff PRESSURE THEORYCoulomb theory assumes that a wedge of soil bounded by a planarfailure surface slides on the back of the wall- Hence shearing resistance ismobilised on both back of the wall and the failure surface. The resultantpressure can be calculated directly for a range of wall frictions» slopesof wall and backfill slopes*Where the wall friction is at angles other than the backfill slopeangle the equations are an approximation due to the curved nature of theactual failure surface and the fact that static equilibrium is not always .satisfied* The error is slightly on the unsafe side for the active case, andmore serious for the passive case. For simple geometries * the charted valuesof K a given in Figures 4 & 5 (Caquot & Kerisel, 1948) may be used; these wereobtained for the more accurate failure mechanism involving curved failuresurfaces*3.5 TRIAL WEPGE METHODDifficulties arise in the use of charts or equations where theground surface is irregular, where the backfill possesses some cohesion,where water pressures exist in the backfill or where the backfill comprisesmore than one soil type.The simplest approach for earth pressure determination in thesecases is to use a graphical procedure making the assumption of planar failuresurfaces based on Coulomb theory* The method is very powerful in thatsolutions to most active pressure problems are possible and it also has theadvantage that the designer can see the solution developing and gains anappreciation of the significance of the contributory factors involved.There are, however, certain limitations in the use of the method for thedetermination of passive pressures. The procedure is known as the Trial WedgeMethod or the Coulomb Wedge Method,


-25-The method is outlined in Figures 6,7 & 8. The backfill isdivided into wedges by selecting planes through the heel of the wall. Theforces acting on each of these wedges are combined in a force polygon so thatthe magnitude of the resultant earth pressure can be obtained. A force polygonis constructed, although the forces acting on the wedge are in general not inmoment equilibrium. This method is therefore an approximation with the sameassumptions as the equations for Coulomb's conditions, and, for a groundsurface with a uniform slope, gives the same result. When the wall frictioncorresponds to that implied by the Rankine case, the value of earth pressureobtained from the Trial Wedge Method is equal to that obtained from Rankine f sequation.Figure 8 shows the general method of dealing with active pressuresin more complex ground conditions using the Trial Wedge Method. It shouldbe noted that the method can be rather laborious in these situations.The adhesion of the soil to the back of the wall in cohesive soilsis usually neglected, since its value is difficult to determine and thesimplification is conservative. For the active case, the maximum value of theearth pressure calculated for the various wedges is required. This is obtainedby interpolating between the calculated values (see Figure 6). For the passivecase, the required minimum value is similarly obtained. The direction of theresultant earth pressure in the force polygons should be obtained by consideringthe direction of the relative movement between the wall and soil. For caseswhere this force acts parallel to the ground surface, a substitute constantslope should be used for soil both with and without cohesion (Figure 10).Theoretically, in cohesive soils, tension exists to a depth Y 0 belowboth horizontal and sloping ground surfaces.Y 0 =^tan (45° + f) (5)where c is the cohesion of the soil in terms of total stress,Y is the bulk unit weight of the soil, and0 is the angle of shearing resistance of the soil in terms of total stress


-26 -Vertical tension cracks will develop in this zone since soil cannotsustain tension and will become water filled- One of these cracks will extenddown to the failure surface and so reduce the length on which cohesion acts,The effect of this s together with the slightly smaller wedge weight, is thesame as neglecting the reduction in total pressure provided by the tensionzone according to the Rankine and Coulomb equations. Figure 7 shows the wedgeanalysis for this case.For an irregular ground surface the pressure distribution against thewall is not triangular. However* if the ground does not depart significantlyfrom a plane surface 9 a linear pressure distribution may be assumed, and theconstruction given in Figure 11 used to determine the point of application ofthe active force, A more accurate method is given in Figure 12. The lattershould be used when there are abrupt changes in the ground surf ace, or thereare non-uniform surcharges involved.3.6 PASSIVE EARTH PRESSURESThe shape of the failure surface for passive failure is curved, morestrongly when wall friction is present. Both Coulomb and the Trial Wedgetheories assume plane failure surfaces and lead to substantial errors incalculated values of passive resistance.Methods using curved failure surfaces, such as log-spiral andcircular* may be used without introduction of significant error. Caquot &Kerisel(1948) have presented charts for simple geometries (Figures 4 & 5)based on a combination of log-spiral and a plane. For more complex geometries,passive pressure may be calculated using the circular arc method outlined inFigure 13. This method is quite laborious for even relatively simpleconditions.The trial wedge method may be used to determine passive resistance.However,, serious overestimation of the passive pressure results when the angleof wall friction 6 is greater than 20 f /3 (Morgenstern & Eisenstein* 1970),Care should be taken then to ensure that 6 is not overestimated* as the erroris on the unsafe side, and the trial wedge method should not be used for thedetermination of passive pressures when 6 > 0'/3.


-27-3.7 EARTH PRESSURES FOR SMALL WALL VEflECTWMSFor certain wall types, such as propped cantilevers and anchoreddiaphragm walls, only small wall movements occur and elastic conditions apply,Where no lateral movement takes place from the insitu condition,the f at-rest* earth pressure applies. For the case of a vertical wall anda horizontal ground surf ace, it has been shown empirically by Jaky (1944) thatthe coefficient of ! at-rest f earth pressure, K Os for normally consolidatedmaterials may be taken as :K 0 » 1 - sin 0 ! (6)where 0 1stress.Is the angle of shearing resistance of the soil In terms of effectiveBecause of the lack of data on the values of K Q for Hong Kong soils,values adopted for design should not be less than 0.5 even for soils with highfriction angles. It should be noted that, in some situations, values muchhigher than K 0 = 0.5 may be found.For a sloping ground surface 9 K 0 varies from that given by equation(6). The Danish Code (Danish Geotechnical Institute, 1978) suggests for avertical wall and ground sloping at an angle, o>, that the f at-rest ? earthpressure coefficient is K 0 (1 + sin w). For other wall angles and backfillslopes, it may assumed that the at-rest pressure coefficient varies proportionallyto the f active 1 earth pressure coefficient, K a . 'At-rest 1 earth pressures,except for over-consolidated soils, may be assumed to increase linearly withdepth from zero at the ground surface* The total at-rest earth pressure forceis given by P 0 = ^K o H 2 . This acts at H/3 from the base of the wall or fromthe bottom of the key for walls with keys.In cohesionless soils, full f at-rest 1 earth pressures occur only withthe most rigidly supported walls (see Section 3*10). In highly plastic clays,pressures approaching at-rest may develop unless wall movement can continuewith time.


-28-3.8 INFLUENCE OF GEOMETRICAL SHAPE Of RETAWING STRUCTURE ON WALL FRICTIONWhen relative movement can occur between a wall and the supportedsoil* the effect of wall friction must be taken into account. In some casesthe wall is free to m'ove with the soil* such as in the case of laggingbetween soldier piles. In these cases little or no wall friction is mobilised.When the outer failure surface from the heel of the wall intersectsor lies within the wall Coulomb ? s conditions apply. Rankine's conditions onlyapply to cases where this failure surface does not intersect the wall, as shownin Figure 9.3.9 INFLUENCE OF LIMITED BACKFILLThe methods given above assume that the soil is homogeneous for asufficient distance behind the wall to enable an inner failure surface toform in the position where static equilibrium is satisfied (Figure 12) . Wherean excavation is made to accommodate the wall, the undisturbed insitu materialmay have a strength differing from the backfill. If equations are used* theposition of two failure planes should be calculated, one using the propertiesof the backfill material and one using the properties of the undisturbed material.If both fall within the physical limit of the backfill, the critical failureplane is obviously the one calculated using the backfill properties.Similarly, if they both come within the undisturbed material, the critical oneis that for the undisturbed material properties.Two other possible situations may arise: firstly where critical failureplanes occur in both materials , in which case the one giving the maximum earthpressure is used, and secondly where the failure plane calculated with thebackfill properties would fall within the undisturbed material,and the failureplane for undisturbed material would fall within the backfill. In the lattercase, which occurs when the undisturbed material has a high strength, thebackfill may be assumed to slide on the physical boundary between the twomaterials. The earth pressure equations do not apply in this case, but thewedge method may be used with the already selected failure plane and thebackfill soil properties. The total pressure thus calculated is less thanthe active value assuming uniform material behind the wall. The variation ofpressure with depth is not linear, and should be determined by the proceduregiven in Figure 12.


-29 -The boundary between the two materials should be constructed so thatthere is no inherent loss of strength on the surface. Benching the insitumaterial ensures that the failure surface is almost entirely through insitu orwell compacted material.3. JO PRESSURES PROPUCE1? EV COMPACTIONProper compaction of- backfill to a retaining wall is necessary inorder to increase the backfill shearing strength and to prevent its excessivesettlement later. Care should be taken to ensure that the compaction processdoes not cause damage to the wall, as pressures produced by compaction canvary considerably in magnitude and distribution and can be much larger thanthose predicted using classical earth pressure theories,Aggour & Brown (1974) give guidance on the formulation of numericalsolutions to compaction problems and include in their paper graphical solutionswhich indicate the influence of some factors affecting residual pressures , e.g.backfill geometry, wall flexibility , end wall restraint.Broms^ (1971) has presented a method for the determination of lateralearth pressures due to compaction against unyielding structures and proposesthe earth pressure distribution shown in Figure 14 (i) for use in design. Theassociated data relating to the figure are for a limited range of compactionplant.Ingold (1979) has presented a simple analytical method which can beused to give a working approximation of compaction induced pressures forroutine designs. The method is based on the following assumptions :(a)(b)An idealised stress path is followed in the compaction processBelow a critical depth Z c there is no reduction in horizontalstress after removal of compactive force. Ingold shows thatapproximate values of Z c may be obtained from the followingexpressions :(7)(c)The increase in vertical effective stress, A0 r v* at a depthZ due to a dead weight of vibratory roller applying a unitweight p/unit length may be obtained from the expression :


-30-(8)The depth 9 h c » below which active pressure due to the weightof the overlying soil exceeds the compaction induced pressure is obtainedfromih c .-L /ITK a fj ny ..... (9)The effect of compaction on lateral pressure is shown in Figure14(ii)(a) & (b) and the resulting pressure distribution for use in design^based on this simplified theory* is shown in Figure 14(ii)(c). Ingold f s designpressure distribution can be seen to be very similar to that of Broms shown inFigure 14(1).3.77 EFFECTS OF COMPACTION ON CONVENTIONAL WALL DESIGNThe lateral pressures induced by compaction (Figure 14) can be upto twice the active pressures obtained by conventional analysis. Thesecompaction pressures lead to higher structural loads» which may cause distressor result in serviceability problems with a wall.If movement of the wall is allowed to take place these compactioninducedpressures are reduced. Translations or rotations of the order ofH/500 are sufficient to reduce the pressures to near the active state. Thefinal pressure distribution is parabolic rather than triangular 9 and thus theline of thrust is raised.It is satisfactory to use the active pressure distribution whendetermining the factor of safety against sliding. The bending moments aftersliding has taken place may still be up to 50% higher than those predictedusing a triangular active pressure distribution. Calculations of bearingpressures and overturning moments should take into account the higher positionof the line of thrust.of the above.Reference should be made to Ingold (1979) for more detailed discussion


- 31-ifOF4.1 UNIFORM SURCHARGESLoads Imposed on the soil behind the wall should be allowed for indesign.Uniform surcharge loads may be converted to an equivalent height offill and the earth pressures calculated for the correspondingly greater height.In this case the depth of the tension zones in cohesive material is calculatedfrom the top of the equivalent additional fill. The distribution of pressurefor the greater height is determined by the procedures given in Chapter 3. Thetotal lateral earth pressure is calculated from the pressure diagram* neglectingthe part in tension and/or the part In the height of fill equivalent to thesurcharge., as shown in Figure 12*Buildings with shallow foundation may be as a unifromsurcharge of lOkPa per storey.The standard loadings for highway structures in Hong Kong areexpressed in terms of HA and HB loading as defined in BS 5400 : Part 2 : 1978.In the absence of more exact calculations 9 the nominal load due to live loadsurcharge may be taken from Table 4.The two loading cases shown in Figure 16 need to be considered.Table 4Suggested Surcharge Loads to be Used In the Design ofRetaining Structures (Public Works Departments 1977)Road classUrban trunkRural trunk(Road likely to be regularlyused by heavy industrialtraffic)Primary distributorRural main roadDistrict and local distributorsOther rural roadsAccess Roads * CarparksFootpaths * isolated from roadsPlay areasType of live loadingHA 4- 45 units of HBHA -f 37% units of HBNote : It is recommended that t' lese surcharges be applied to the1 in 10 year storm condiI: ion.HAEquivalentsurcharge20kPaISkPalOkPa5kPa'


-32 -4.2 LIWE LOADSWhere there is a superimposed line load running for a considerablelength parallel to the wall, the Wedge Method of design may be used* and theweight per unit length of this load can be added to the weight of theparticular trial wedge to which it is applied, A step thus appears in theactive force locus s as the weight of the trial wedge suddenly increases whenthe line load is included. The increased total earth pressure will be givenfrom the trial wedge procedures but the line load will also change the pointof application of this total pressure. The method given in Figure 15 may beused to give the distribution of pressure*When the line load is small compared to the active earth pressure 9the effect of the line load on its own should be determined by the method givenin Figure 15.experiment.This is based on stresses in an elastic medium modified byThe pressures thus determined are superimposed on those due toactive earth pressure and other pressures as appropriate.4.3 POINT LOADSPoint loads cannot be taken into account by trial wedge procedures.The method based on Boussinesq's equations given in Figure 15 may be used,but it should be noted that the method is only approximate as the stiffnessof the wall is not taken into account.


-33 -5EFFECTS OF WAI5.1 GENERALThe presence of water behind a wall has a marked effect on thepressures applied to the wall, Waen the phreatic surface intersects the wall,a hydrostatic pressure is exerted against the wall, together with upliftpressures along the base of the wall. Even when there is no water in directcontact with the wall,, such as when adequate drainage is provided 9 there is anincreased pressure on the wall due to the increased earth pressure (Section5.2). The effect of water behind the wall is significant; the total forcemay be more than double that applied for dry backfill- Many recorded wallfailures can be attributed to the presence of water*The height to which water can rise in the backfills and the volumeof flow, are both of prime concern. To determine these the ground waterconditions must be established. These may be best derived from theobservation of groundwater conditions prior to construction using piezometersand by applying the principles outlined in this Section and in Chapter 4 ofthe Geotechnical Manual for Slopes, Notwithstanding the results of groundwatermonitoring, the groundwater level assumed for design should be not lower thanone-third of the retained height.The effect of leakage from services can be significant. There isevidence from field measurements and failures in Hong Kong that this leakagecontributes substantially to both perched and main groundwater tables. Theprovisions for services outlined in Sections 9.18 & 9.19 of the GeotechnicalManual for Slopes are appropriate for retaining walls, and these should beapplied.Where inadequate drainage is provided behind a retaining structure,there may be a damming effect which would result in raising groundwater levelslocally and in the general area. Such a rise may adversely affect thestability of slopes and retaining walls. Effective drainage measures shouldalways be provided in such cases.


5.2 EFFECT Of WATER ON EARTH PRESSURES5.2.1 S^otcc Wat&i Leve£When a soil is submerged , its effective unit weight is reduced toY f = Y sat ~Yw* The lateral earth pressure should,, in this case § becalculated using Y ? in equations or charts. Alternatively s in graphicalprocedures such as the trial wedge method^ all forces acting on the soilwedge * including the hydrostatic normal uplift pressure on the failure planeand the lateral hydrostatic pressure , may be included in the trial wedgeprocedure. This is illustrated in Figure 6 to 8.In low permeability cohesive soils 9 the pore water pressures set upduring construction may be in excess of any hydrostatic pore pressure , so anundrained analysis may be more appropriate.When tension cracks occur 5 lateral hydrostatic water pressure shouldbe included for the full depth of the crack* as given in Section 3.5 or forH/2* whichever is less* Full lateral water pressure must be allowed forbelow the invert of the lowest weep holes or other drainage outlets.5,2.2 FlowingIf 'the water in the soil voids is flowing, the pore water pressuresare changed from the hydrostatic values to values determined by die seepage .patternThese values have to be used in a trial wedge solution to determine the earthpressure.The actual flow pattern developed is very dependent on theuniformity and homogeneity of the ground , and on the position of any drains.Figure 17 (a) shows the flow net produced by steady seepage into a verticaldrain when the phreatic surface is below ground level and the backfilluniform and isotropic. Rainfall of intensity equal to or greater than thepermeability of the backfill will change this flow net to that shown inFigure 17(b) if there is no surface protection to prevent infiltration.There is a significant increase in water pressure on the failure surface for thislatter case. It is thus desirable, for this drainage arrangement, to preventwater entering the backfill from the surface. Figure 17(c) shows the flownet due to heavy rainfall infiltration into an inclined drain. The effectof this drainage arrangement is to reduce the water pressure in the backfilltro zero; this is therefore a very effective drainage measure,


-35-The pore water pressures normal to the active or passive wedgefailure surface affect the forces acting on a wall. The resultant thrust onthe failure surface» determined from a flow net* is applied in the forcepolygon for the soil wedge together with any lateral water pressure at thewall as shown in Figures 6 to 8* The method of determining water pressuresfrom the flow net* and hence the water force 9 is shown in Figure 17.For methods of dealing with seepage through anisotropic andnon-homogeneous backfills* reference may be made to Cedergren (1977).5.3 PRAXMAGE PROVISIONSWater pressures must be included in the forces acting on the wallunless suitable drainage is provided„ Good practice requires that drainageis always provided.For walls less than 2 metres high,, drainage material is usuallyonly provided on the back face of the wall* with weep holes to relieve waterpressure. In some low risk situations, it may be geotechnically tolerableand economically advantageous to omit the drain and design for the hydrostaticwater pressure.With correctly designed inclined drainage systems, such as thoseshown in Figures 18(a) & (c), water pressures maybe neglected both on the wallitself and on the soil failure plane. Alternative drainage details as shownin Figures 18(b) & (d) may be used. In these cases, the appropriate waterpressure should be considered in design. Hydrostatic pressure will act onthe wall below the lowest drainage outlet.For a drain to be effective it must be able to carry the design flowof water without backing up or blocking. This design flow should include theflows from leaking or burst service conduits where appropriate.To prevent blockage, the drain must be protected by an adequatefilter, designed according to the rules given in Section 5.4.


-36-The rate of seepage into the drain from the soil can bedetermined from a flow net together with a knowledge of the permeabilitiesof the soils involved and a flow-net. Methods for determining permeabilitiesare outlined in Section 2.9.The water flow rate that the drainage layer can accommodate dependson the permeability of the drainage medium, the thickness of the drain and thehydraulic gradient in the drain. In some cases, it may be intended that thefilter itself should act as a drain; if so, it should be designed to haveadequate capacity.By the use of a conventional flow net sketch, the approximate rateof flow into the drain may be estimated. Using an appropriate value ofhydraulic gradient, i, and the value of permeability for the drainage material,k^, the required area of drainage material, A, normal to the direction offlow can be determined by application of Darcy's law :where q


-37-A common practice in Hong Kong is to use no-fines concrete orhand-packed rubble as a drainage layer behind retaining walls. Thesematerials should be protected with a transition zone of gravel or crushedrock which will not migrate into the voids of the rubble or no-fines concrete,and which conforms to the filter design rules applied to the soil-protectingfilter. However, when this is done, it is probable that the hand-packedrubble or no-fines concrete can be omitted and that the transition zone canbe used as the drainage layer.It should be noted that a clean well-graded rock backfill protectedby an appropriate filter would be an excellent solution in any location whereseepage from the soil or leakage from service conduits may be a problem.5.4 FILTER REQUIREMENTS5.4.1 Gxaddd fWLdUAll drainage that is provided should be adequately protected byproperly designed filter layers against blockage due to the movement of thefiner soil particles. Filters should be more permeable than the protectedsoil, and filter materials should be transported and placed carefully so thatsegregation, and contamination by fines, does not occur.Table 5 gives details of the normal filter design criteria applicableto soils in Hong Kong. Where the base soil contains a large percentage ofgravel or larger sized particles, the finer fraction should be used for thefilter design.Where filter materials are used in conjunction with a coarser freedrainagematerial such as crushed rock, the grading of the coarser materialshould conform to the filter design criteria given in Table 5, to protectthe filter from erosion.


-38-Table 5Filter Criteria to be Used in Hong Kong(Geotechnical Manual for Slopes, 1979)Rule Number(1)D 15 F C « 5 x D 85 SfFilter Design RuleD 15 F C < 20 x Di 5 S f(3)(4)(5)(6)D 15 F f > 5 x D 15 S CD 50 F C < 25 x D 50 S fUniformity coefficient 4 < J&fil. < 20D 10 FShould not be gap graded(7)Maximum particle sizes 75mm(8)Not more than 5%. to pass 63pm sieve, andthis fraction to be cohesionless* For well-graded base soil this criterion can be extended to40 xIn this table 9 D-^F is used to designate the 15% size of the filtermaterial (i.e. the size of the sieve that allows 15% by weight of the filtermaterial to pass through it) . Similarly* Bg5S designates the size of sievethat allows 85% by weight of the base soil to pass through it.indicates the D size on the coarse side of the filter envelope.indicates the DJQ size on the fine side of the filter envelope-When certain gradings of decomposed volcanic materials with anappreciable fines content are being used as backfill s the filter design mayrequire special care.Reference should be made to Section 4.19 of the GeotechnicalManual for Slopes for further discussion on the design of filters.5.4.2In some cases» it may be possible to use man-made fibrous wovenand non-woven fabrics, known as geotextiles, to protect the drainagefacilities.


- 39 -As yet, there Is very little experience In Hong Kong with the longtermperformance of fabric filters for permanent drainage measures.Consequently, it is recommended that they should only be used in low risksituations, as defined in Table 2.1 of the Geotechnical Manual for Slopes,and where failure could not be expected to occur even if total blockage ofthe fabric occurred. It is also recommended that they should only be used inlocations where they can be replaced if found to be defective after a periodin operation.There are objections to the use of some of these materials, suchas serious deterioration on exposure to sunlight and ultra-violet light,clogging due to movement of fines, reduction in permeability due to compression,constructional difficulties and materials forming planes of weakness in theworks. If these objections are overcome by attention to design, constructionand quality control, then the availability of geotextiles provides newopportunities for innovative filter/drain design and construction.Fabric filters should be properly designed to be in filterrelationship with the surrounding soil. Care must be taken to select ageotextile which is appropriate to the grading of the soil it is intended toprotect and has adequate drainage capacity for the particular application.A summary of design criteria for fabric filters is given in the book byRankilor (1981).Available literature suggests that fabrics with an equivalentopening size of less than 150ym (or an open area of less than 4%) and thethicker non-woven fabrics, may be more prone to clogging than other varieties.The use of these types should therefore be avoided unless the satisfactoryperformance of the particular soil/fabric/Irainage-medium system has beendemonstrated by permeability test. On the other hand, some of the very thinfabric varieties exhibit quite large visible gaps caused by unevendistribution of fibres, and the use of such defective materials should alsobe avoided.During construction, stringent measures are required to ensure thatthe manufacturer's instructions concerning storage and handling are strictlyfollowed, and that storage, placement and backfilling of fabrics arecarefully controlled to avoid excessive exposure to ultra-violet light,mechcnical damage and ineffective overlapping. It is prudent to use twolayers of fabric as a precaution against impairment of the filter function bymechanical damage during placement.


5.5 CONTROL Of GROUMDWATERGroundwater levels may need to be controlled in excavations fparticularly in sheeted excavations. The dewatering method chosen shouldassure the stability of the excavation and the safety of adjacent structures.Techniques for dewatering are outlined in the Code of Practice for FoundationsCP2004 (British Standards Institution* 1972) and in Terzaghi & Peck (1967).When pumping is carried out inside a sheeted excavation* flow willoccur under the sheeting and up into the excavation. Piping may occur indense sands if the seepage exit gradient at the base of the excavation equalsabout 1.0. Heave associated with groundwater flow may occur in loose sands ifthe uplift force at the sheeting toe exceeds the submerged weight of theoverlying soil column. Both failure modes may be prevented by increasing thedepth of penetration of the sheeting.Design charts for determining the stability against piping inexcavations are given in Figure 21.


6STABILITY OFWALLS6JGENERALThe stability of a free standing retaining structure and the soilcontained by it is determined by computing factors of safety (or stabilityfactors), which may be defined in general terms as :F = Moments or forces aiding stabilitys ' Moments or forces causing instability ^Factors of safety should be calculated for the following separatemodes of failure and should apply to the 1 in 10 year groundwater condition :(a)(b)(c)(d)sliding of the wall outwards from the retaining soil,overturning of the retaining wall about its toe,foundation bearing failure, andlarger scale slope or other failure in the surrounding soil.The forces that produce overturning and sliding also produce thefoundation bearing pressures and, therefore, (a) and (b) above are inter-relatedwith (c) in most soils.In cases where the foundation material is soil, overturning stabilityis usually satisfied if bearing criteria are satisfied. However, overturningstability may be critical for strong foundation materials such as rock, or whenthe base of the wall is propped, or when the base of the wall is small, forinstance with crib walls*In general, to limit settlement and tilting of walls on soil materialsthe resultant of the loading on the base should be within the middle third.For rock foundation material, the resultant should be within the middle halfof the base.When calculating overall stability of a wall, the lateral earthpressure is calculated to the bottom -of the blinding layer, or in the case ofa base with a key, to the bottom of the key where the actual failure mechanismextends to that point.


-02-If the passive resistance of the soil in front of a wall is includedin the calculations for sliding stability, only 50% of the calculated passiveresistance should be used* because of the large deformations required tomobilise the full passive resistance.in Figure 22,Stability criteria for free standing retaining walls are summarised6.2 SLIPIMG STABILITY6*2.1 8o6£ mtkout aSliding occurs along the underside of the base (see Section 2.6for further discussion) .1.5.The factor of safety , F s , against sliding should not be less thanF s (sliding) = v + c h B + 0.5P p _ ..... (12)where ¥ t is the weight of the wallP v is the vertical component of earth pressure forcePjl is the horizontal component of earth pressure force6fo is the angle of base friction% is the adhesion at the base of the wallB is the base width, andPp is the passive pressure force.The effects of water forces should be taken into account in thisequation, including uplift pressures below the wall base, unless drains thatpermanently and effectively eliminate uplift water pressures are provided*6.2.2 Bo6e ettctfc a K&yHuntington (1961) suggests that walls with shallow keys should beanalysed assuming that sliding occurs on a horizontal plane through the soilat the bottom of the key. Both active and passive forces should be adjustedto take into account the depth of the key. The weight of soil in front of thekey and below the base, down to the failure surface, should be included in the


-43 -total weight, W t . Figure 1 shows the forces involved.The factor of safetyagainst sliding should be as given in Section 6.2.1, with the angle of basefriction, S b , replaced by the angle of shearing resistance, 0% of thefoundation soil.6.2.3 Sliding on a Rocfe. foundationIt is possible to analyse the sliding of a retaining wall on arock foundation in a similar manner to sliding of rock along a rock joint.The basic friction angle may be increased by a waviness angle, i w , based onthe measured waviness of the exposed rock surface.The waviness must be of a sufficient size so that shearing throughthe asperity does not occur. In addition, there must be a significantcomponent of the rock surface inclined at i w in the direction of sliding.6.3 OVERTURNING STABILITYMoments calculated about the bottom of the front of the toe shouldgive a factor of safety, F s , against overturning of not less than 2.F s (overturning) « rr 8^ (13)where M r is the algebraic sum of moments resisting overturning andMQ is the algebraic sum of moments causing overturning.For semigravity cantilever and counterfort walls, only theoverturning factor of safety for the wall as a whole is significant. Forcrib walls and solid gravity walls for which the base and the upper portionof the wall are usually separate units, the factor of safety of the upperportion against overturning about its toe should be checked.Passive resistance should not be included in calculations for F s(overturning) for conventional walls.


6.3*2 factor oi Sa{&tt/ agcumtThere are a number of ways in which a factor of safety againstoverturning may be determined , and these lead to significant differences inthe computed value of F s .In order to understand why some of these differences occur , theforces acting on the simple retaining wall illustrated in Figure 22 (a) willbe examined. Dry backfill only is considered* and terms are defined on thediagram*Application of equation (13) gives (Figure 22) :Wt- aF s (overturning) = p ~Xi.It may be noted that, for the usual proportions of solid gravityretaining walls 9 the batter of the back is usually such that the line of actionof P^ passes below the toe. The lever-ram* m, is thus negative and P^contributes to the stability of the wall. A negative value of F s thus indicatesthat the wall cannot overturn.It is usual in retaining wall design to work in terms of thehorizontal and vertical components of the overturning force P^, These forces,multiplied by their respective lever arms and substituted into equation (14)for the simple case as illustrated in Figure 22 (a).It is commonly assumed however that the component P v contributes toresisting overturning and on this basis, the factor of safety becomesT? F — t ***" - - ,rV*s ~ =?Equations (15) and (16) do not, of course, give the same value offactor of safety.


-16-It can be seen that, according to equation (16), the overturningfactor of safety is that number by which the horizontal component of the earthpressure would need to be multiplied to cause overturning* the verticalcomponent of this pressure remaining unchanged. It is unlikely , however, thatthe horizontal component of the resultant earth pressure would increase andthe vertical component remain unchanged. On this basis , it would appear thatthe procedure represented by equation (16) is not logical.Although equation (16) leads to a more conservative result than theprocedure based on equation (15) 5 it is not recommended and the design datagiven in Figure 22 is based on the more logical procedure represented byequation (15). Huntington (1961) discusses this topic.6.3.3 Wain uo^thApplication of an analysis of rotational stability of walls withdeep keys to the real situation is found to be very uncertain, as the forcesacting are dependent on the relative stiffness of the wall and the supportingsoil, and on the deformation that takes place. In view of constructionaldifficulties and likely large deformations, walls with deep keys should ingeneral be avoided (see Section 11.7).6.4 FOUNDATION BEARING PRESSURE6.4.1The ultimate bearing capacity of the foundation soil on which anearth retaining structure rests should generally be determined f ran a theoreticalanalysis of the foundation, using the soil properties obtained from laboratorytests. Where appropriate, these shear strength properties should be reviewedas the construction proceeds. The applied loading should provide a factor ofsafety of 3.0 against ultimate bearing failure.Foundations of retaining walls are usually subjected to inclined andeccentric loads, the foundation itself may be tilted at an angle to thehorizontal and sometimes the wall is founded on sloping ground. A generalexpression for the ultimate bearing capacity of shallow foundations which candeal with these situations has been given by Vesic (1975), and this is presentedin Section 6.4.2.


Other factors which may influence the bearing capacity are thefoundation depth, soil compressibility , scale effects and non-homogeneous soilconditions- These are discussed by Vesic (1975).6.4.2 EwrunQ CapacityThe ultimate bearing capacity of a shallow (D^B) strip foundationis given by :- term relating to effects )- c N c S c i c t c gc Qf coliesion )+ h Y B Y N Y S v i v t y g Y - term relating to influence,T T Y Y Y Tof unit weight of soil J •••+ q N q S a i a t a g a - term relating to surcharge)H Heffects )The bearing capacity factors 9 N c , Ny, N q are functions of the angleof shearing resistance, 0, of the soil and are modified as appropriate usingfactors for the shape of footing, S c , S , Sq, inclination of load, i^, i Y , i q »tilt of footing base , t c , ty> t q , and slope of ground, g c , g Y , g . Valuesfor these factors are given in Figure 23.The above bearing capacity factors have been determined on theassumption that the foundation material is reasonably incompressible, so thatfailure would occur by general shearing. For compressible materials, failureoccurs by local, or punching failure. For these materials Terzaghi (1943)recommended that the value of cohesion used should be reduced to 2c?/3> and theangle of shearing resistance to tan ((2 tan 0 f )/3). A more accurate solutionconsidering both compressibility and size effects is given by Vesic (1975).In using the above expression, it should be noted that foundationsconstructed on the relatively high permeability residual soils usuallyencountered in Hong Kong, decomposed granites and volcanics, require theanalysis of bearing capacity to be carried out in terms of effective stresses.Under these conditions, the contribution to the bearing capacity of thecohesive terms is in general very small and may be neglected.For foundations constructed on saturated clayey soils of lowpermeability, the short-term stability is critical, and they are usuallyanalysed in terms of undrained strength (0* « 0 analysis).


-47-Where a wall is founded on compacted fill overlying either softclay or loose fill, particular care must be taken. Reference should be madeto Vesic (1975).6.4.3 E^ect o& GsioanduwuteA LevelEquation (17) applies when the groundwater table is at a distanceof at least B below the base of the foundation.When the water table is atthe same level as the foundations the submerged unit weight of the soil belowthe foundation should be used.For intermediate levels of the water table,the ultimate bearing capacity should be interpolated between the abovelimiting values.6.5 ECCENTRIC LOAP3When the load on the foundation is eccentric, this substantiallyreduces the bearing capacity. To allow for this, the base width, B, isreduced to an effective width B f given by :B 1 = B - 2e b ..... (18)where e^ is the load eccentricityRFor a footing eccentrically loaded in two directions, the effectivedimensions of the base become such that the centre of an area, A 1 , coincideswith the vertical component, V, of the applied load. Then :A' - B ! x L'where L f « L - 2e ls and B f - B - 2ei, and ej, e^ are the load eccentricitiesin the two-directions.L' and B 1are then used in place of L and B in all equations.The factor of safety is given by :F s (bearing) - *jd±. ..... (19)* ax j_ •VVwhere q a ii. » TT for a rectangular footing, and q^i^5*jr" for a continuousstrip footing (unit length considered).


-08-6.6 FOUNDATIONS CONSTRUCTED ON SLOPING GROUND AW) NEAR SLOPE CRESTSThe ultimate bearing capacity of foundations constructed on slopesis lower than that for foundations constructed on level ground. The groundslope factors of Vesic (1975) 9 given in Figure 23* are devised to take thisinto account.Where a foundation is constructed on the crest of a slope, thebearing capacity increases with distance from the crest to a maximum value atdistances from the crest greater than approximately four times the foundationwidth, No exact solution is available for this case. The procedure outlinedby Bowles (1977) could be applied to the values given by Vesic in Figure 23.Alternatively, as a conservative assumption, a linear variation between thetwo extreme values may be used.The bearing capacity calculations do not consider the fact that thesoil on the slope is already under stress. This is particularly important wherethe inclination of the slope is greater than 0 f /2. The overall stability ofthe slope under the influence of the loaded footing must therefore be checked,in addition to the bearing capacity calculation.6.7 FOUNDATIONS ON ROCKFoundations on continuous sound rock seldom present problems sincethe rock is stronger than most foundation materials. Structural defects anddiscontinuities, or the compressibility of the rock mass below the foundation*usually control the allowable bearing pressure.Where discontinuity-controlled failure mechanisms are possible, jointsurveys should be carried out in the excavation and adjacent slopes*The compressibility of the rock mass below foundation level dependson the frequency of joints and on the amount and type of infiling of thesejoints in the zone of influence of the foundation. RQD (Rock Quality Designation)is defined as :RQD (%) = 100 x Len S th ° Tf ^weathered core £ IQOimnLength of borehole(2Q)


In unweathered rocks, RQD indicates the joint intensity f whereasin weathered rock it gives a measure of the amount of compressible materialbut no indication of the infill compressibility.Ttfhere only tight clean joints are presents the correlation betweenRQD and allowable bearing pressure proposed by Peck et al (1974) 9 given inTable 6, may be used.Table 6. Allowable Bearing Pressure on Jointed Rock(Peck, H'anson £ Thornburn, 1974)RQD(%)Allowable Pressure(kPa)100 3000090 2000075 1200050 650025 30000 1000Note :(1) Use allowable pressure orunconfined compressive strength ofintact rock, whichever is less.(2) RQD is for rock in the zoneof influence of the foundation.For infilled joints deformation will be larger, end estimates ofthe joint infill compressibility may be required* The effect of joint infillingon allowable bearing pressure for a limited range of joint spacing and thicknessis given in the Canadian Foundation Manual (Canadian Geotechnical Society 9 1978),6.8 SLOPE FAILURE IN SURR0UWPING SOILThe overall stability of the ground surrounding the retaining wallshould be investigated, and calculations should be carried out on the fullrange of potential failure surfaces to ensure that an adequate factor of safetyagainst overall slope failure is maintained. The calculations should includethe influence of the surcharge from the wall on the slope. The minimum factorof safety required at a site is dependent on its hazard potential.Reference should be made to Chapter 5 of the Geotechnical Manual forSlopes, where detailed guidance is given on the Risk Category of a slope andthe minimum factor of safety required. The factor of safety should bedetermined for groundwater conditions associated with a 10 year return periodrainfall. Chapter 5 of the Geotechnical Manual also gives guidance on methodsthat may be used for carrying out the analysis.


- 51 -CHAPTER 7SHEET RETAINING STRUCTURES7.1 GEWERAlWalls which have uniform cross-section with depth are considered inthis chapter. These Include flexible sheet structures, such as sheet-piled an


- 52 -Since failure of strutted cuts often occurs by structural failure,particular attention should be paid to the structural detailing of theinternal strutting. Guidance on the structural design of such walls * togetherwith typical details of connections and strutting systems, are given byGoldberg et al (1975). Struts must be sufficient for all stages ofconstruction*The distribution of pressure on a strutted excavation is complex,and it is normal to use a pressure envelope covering the normal range pressuredistributions. The envelopes (Figure 24) given by Peck (1969), and the JapanSociety of Civil Engineers (1977), together with loadings from groundwater andsurcharge, should be used to determine strut loads for all internally struttedexcavations. In assessing loading from groundwater, the effect of accidentalbreakage of water carrying services should be considered.The load carried by each internal strut is estimated by assumingthat the sheet pile is simply supported between struts, and that a reactionbelow at the base of the excavation exists. This reaction is provided by thepassive resistance of the soil beneath the cut.The depth of penetration of the wall below the base of the excavationshould be sufficient to provide this reaction.Since the wall moves towards the excavation, it may be assumed thatactive and passive pressures develop against the wall below the excavationlevel, and horizontal equilibrium may be used to determine the depth ofpenetration. The passive resistance should be factored by 2.0.For soft clays, neglible passive resistances develop, and the lowersection of the wall must be designed as a cantilever, and the bending momentand deflection must be checked.The maximum bending movement at, or below, the lowest strut should bechecked against overstressing of the wall.Instability of the base of an excavation can occur due to shearfailure in soft to firm clays (known as base heave)* In granular materials,piping or heave associated with groundwater flow can occur.


-55-The factor of safety with respect to shear failure is given by :where the terms are defined in Figure 25. Where F s is less than 2 substantialdeformations may occur with consequent loss of ground, and the probability offailure exists. Where soft clay extends to considerable depth below theexcavation 9 the effect of increased sheeting stiffness,or depth* is minimal,However driving the sheeting into a hard stratum before commencing theexcavation can appreciably reduce the deformations.Control of the groundwater may be necessary to prevent piping orheave associated with groundwater flow. Methods to achieve this are discussedin Section 5.5.7.3 ANCHOREV FLEXIBLE WALLS7.3.1 WaJUU> Anckosi&d mast the. TopThe deformation of an anchored sheet pile depends on the relativestiffness of the pile/soil system. For a relatively rigid system, such as aheavy pile section in a loose sand, the earth pressure distribution correspondsclosely to the triangular active and passive conditions. The toe of the pileis assumed pinned, and the Free Earth Support design method as outlined byTeng (1962) is appropriate.As the stiffness of the system decreases the pressure distributionalters in such a way as to reduce the bending moment in the pile. As aconsequence, the sheet pile section used may be reduced as compared with aninfinitely stiff wall. Rowe's Theory of Moment Reduction (1952, 1955, 1957)takes this effect into account; it is summarised by Teng (1962) and in CIRIAReport No. 54 (1974).When calculating the toe penetration, it is recommended that nofactor of safety should be applied to the active pressures. The passiveresistance may be factored by 2.0, or, as recommended in the CIRIA report, thefollowing factored values of 0 ? and


-54 -and 6 . tan- ( TL) ..... (22)For sands, F s = 1.5 should be used* which gives an approximate factorof 2.0 on the derived K p values. If, however, the values of 0 f and 6 areuncertain* then F g = 2,0 should be used,For the short term stability of walls in clays , a factor 2.0 £ F s £ 3.0should be applied to the value of undrained cohesion* c, depending on thereliability of the parameters* For long term stability , the factor on tan 0 fcan be taken as 1.2 ^ F s ^ 1.5.given in Chapter 3.Passive and active pressures should be calculated using the methods7.3.2 Mu£tcp£e AnchoredThe multiple-anchored system of wall support results in the retainingstructure being progressively fixed. Consequently * the lateral deformationsare limited to such an extent that failure within the retained soil is unlikley.The earth pressure which finally acts on the wall depends on the relativestiffness of the wall to the soil, the anchor spacing, the anchor yield andthe prestress locked into the anchors at installation.The earth pressure distribution has been shown to be similar to thatobtained for internally braced excavations.similar to that adopted by Peck (Figure 24) is appropriate.A rectangular pressure envelopeThe earth pressurecoefficient may be taken as K 0 . However, it is common to use a value betweendK a and K o , such as (K a 4- K o )/2, in an attempt to control surface movements.Successful designs have been made using triangular pressuredistributions with earth pressure coefficients varying between K a and K 0 .However 9 because of the mechanism involved, the rectangular distribution isconsidered more appropriate (Hanna, 1980). Anchor loads may be checked usingboth distributions, and the worst case taken.The determination of vertical and horizontal spacing of anchors usingthe procedure for internal strut spacing gives acceptable results. Anotherapproach is the semi-empirical design method of James & Jack (1974) which


-55simulates the field construction procedure using triangular pressuredistributions . This method allows determination of the depth of penetrationrequired* and results correspond well to field and laboratory tests.7.3.3 E^ec& o{ Anckoti InclinationAnchors are usually inclined downwards , transmitting the verticalcomponent of the anchor force into the anchored member. This force should beconsidered in design* together with the weight of the member itself (White,1974).A number of cases have been recorded where soldier piles have failedin end bearing due to the vertical component of the anchor force,7.4 CANTlLEVERED WALLSRelatively rigid cantilevered caisson walls are used in Hong Kong.These rely entirely on the development of passive resistance in front of thewall for their stability. As a consequence, considerable movement must occurbefore equilibrium is reached, and deep penetration is required. The deflectionat the top of the wall may be the governing criterion. Such walls should notnormally be used as permanent structures to retain a height of more than 5munless cantilevered from rock.The pressure distribution at failure approximates the classicaltriangular pattern. Full active pressure should be used and the passivepressure should be factored with F s « 3 on tan 0 f and tan 6 (refer to Section2.7 for appropriate values of 6). This higher factor of safety is requiredbecause of the large deformations needed to develop full passive resistance.However 9 if it can be shown that wall deformations will not cause distressto neighbouring structures or services, then a lower factor of safety may beappropriate.The depth of penetration is obtained by taking moments about thetoe. The maximum bending moment may be obtained by taking moments of thepressures, above various cuts, until the maximum value is determined*Installation of a drainage and filter medium behind the wall may bedifficult and so full hydrostatic pressure may have to be considered for thedesign.


- 57-CHAPTER 8REINFORCED EARTH RETAINING WALLSThe technique of reinforced earth Is used for retailing walls invarious parts of the works. Such walls are relatively new to Hong Kong, andthere is little experience under Hong Kong conditions.It is recommended s at present, that designs should be in accordancewith the Technical Memorandum (Bridges) BE 3/78 (Department of Transport, UK,1978). It is also recommended that for the backfill, the grading and plasticityIndex requirements of the Federal Highways Administration (1978), outlined inTable 7 9 should also be met, because of the limited documented experience ofreinforced earth retaining walls constructed using materials with a high finescontent and plasticity index.It is considered that difficulty will be experienced in obtainingsuitable backfill material from natural sources. Decomposed volcanics willnot meet the specifications. For decomposed granites. It is likely that thevariability of grading and plasticity index within a local area will presentdifficulties. Consideration should therefore be given to the use of crusher-runor similar materials. Designers are advised not to commit the design of awall to a reinforced earth system until a sufficient source of fill thatwill meet the specification has been identified.Close supervision is required to ensure that construction proceedsaccording to specification, particularly all aspects of the backfillspecification. Difficulties with later provision of services and thesterilization of land above for building development may preclude the use ofreinforced earth in certain circumstances.


- 58 -Table 7 Minimum Specification for Select Backfill forReinforced Earth Retaining Walls(Federal Highway Administration s 1978)Sieve SizePercentage Pas s ing150mm 10075mm 75 - 10075um 0-25and PI < 6OR If percentage passing 75pm is greater than 25%,and percentage finer than 15ym is less than 15%,material is acceptable if 0 ^ 30 asdetermined by the appropriate test and P.I. < 6.


-59-CHAPTER 9CRIB WALLS9.1 GENERALCrib walling, although commonly used in some countries (e.g. NewZealand, Australia and the U.S.A.), has not been much used in Hong Kong. Thetechnique can provide walls that are economical, aesthetically pleasing, andrelatively rapid to construct.A crib wall structure is made by placing a number of criblike cellstogether and filling them with soil or rock fill to give them strength andweight. The wall essentially acts as a gravity retaining wall. Crib wallunits may be built of precast concrete, steel or of treated timber. Themanufacturers of crib wall units produce design data for crib walls, but ingeneral care must be exercised in the interpretation and application of thisdata.The front face of a crib wall usually consists of a grid of concretemembers so spaced that the soil infill at its angle of repose does not spillthrough the spacers. Horizontal members of such a grid are termed stretchers.The face members are connected by transverse members termed headers to a similargrid of stretchers, parallel to the face, forming the back face of the wall(Figure 26). The minimum thickness of walls should be one metre, except wherethe wall is non-supporting for landscaping. A 1.2 m thickness is usually abetter engineering solution. Additional spacers between the stretchers withinthe front and back grids may be used if the system requires it, and these aretermed false headers or pillow blocks. Headers should in general beprependicular to the face of the wall, although some available systems havevariations to this.The system usually allow for the addition of one or more grids ofmembers parallel to the face and situated behind the structure described above,so forming multiple depth walls of greater height. Such additional grids areconnected to the grid in the front by a header system.


- 60 -9.2 VESIGNThe general design criteris for gravity walls apply to crib walls.The pressures acting on a crib wall should be determined by the methods givenin Chapter 3.cross-section.The resultant should always lie in the middle third of the wallFigure 26 shows the earth pressure distribution acting on atypical wall and some typical construction details.curves which may be used for preliminary design only*Figure 27 gives designTo a great extent, the performance of a crib wall depends on theability of the crib members to contain the enclosed soil. Analysis of thestresses and loadings in the crib members and connections is based on theearth pressure inside the crib. The individual units for crib walls should bedesigned to withstand the jtor^ion, bending moments, shear forces and tensileforces exerted on them. The theoretical determination of the forces on cribunits and the actual strength of the units is difficult and is usually basedon earth pressures from bin pressure theories (Schuster et al, 1975;Tschebotarioff5 1951), the structural form of the crib units and the earthpressure from the backfill. However, it has been found by Schuster et al (1975)that stresses measured in crib wall units are much higher than those predictedusing loads on the units from bin pressure theories. Specification CD209 -Crib walling and Notes (Ministry of Works and Development N.Z., 1980) specifiesthat crib units be able to withstand loadings which imply earth pressures twicethose given by bin pressures. This requirement followed an examination ofsatisfactory and unsatisfactory crib wall units. Good detailing and design isrequired at the connection between units to ensure the satisfactory transfer offorces. Crib wall failures have occurred because of poor steel reinforcementdetailing.The Specification CD209 also gives useful advice on requirements forthe strength and testing of crib units and the construction of crib walls.Careful quality control during manufacture of the crib units is requiredespecially with regard to concrete cover, the placement of steel reinforcement,concrete mix design, and the dimensional tolerances of individual units.Many crib walls have failed because of differential settlement of thewall structure. Because of this, all crib walls should be founded at least 300mmbelow ground level on a cast in-situ reinforced concrete base slab of 150mmminimum thickness over the whole plan area of the wall.


9.3 SACKFULThe crib wall units should always be infilled with a free-drainingmaterial placed and well compacted in layers in a way that does not disturbthe crib units, "Where soil is used 5 a relative compaction of at least 98% toBS 1377 : 1975 Test 12 should be obtained. Where rock fill is used, therelative density to be obtained should be specified. The strength of thecompleted wall depends on the standard of this backfilling.9.4 PROVISION OF PRAIA/AGEAdequate drainage of the whole crib structure is essential. Many ofthe failures in crib walls have occurred because material of low permeabilitywas used as backfill^ thus developing high static or seepage water pressures.A subsoil drain should be installed at the heel of the wall wherever possible,otherwise ponding may occur.9.5 MULTIPLE PEPTH WALLSThe stability of walls of more than single depth should be checkedat the changes from single to double and double to triple 9 etc., to ensurethat the resultant force lies within the middle third of each sectionconsidered, and that the overturning criterion stated in Figure 22 is met.9.6 WALLS CURl/EP IN PLANCrib walls with a convex front face are much more susceptible todamage by transverse deformations than are concave walls.


-63-CHAPTER 10SBTLeeirs ADJACENT TO LARGE EXCAVATIONS10.1 GENERALThe formation of large excavations causes movements of thesurrounding ground and settlement of adjacent ground surfaces, associatedservices and structures. The magnitudes of these settlements and theirdistribution depends on the dimensions of the excavation,the support systememployed, the sequence and timing of the works, and most importantly thecharacteristics of the surrounding soil and the quality of workmanship involved.The process of excavation reduces the vertical load on the excavationbase and the horizontal stress on the sides. The underlying ground movesupward and the ground alongside tends to move inwards. This inwards movementoccurs even at levels below the base of the excavation. The base heave andinward lateral movement cause settlements of the surrounding ground surfacesand structures. The magnitude and distribution of these settlements may causedamage to adjacent structures and services.10.2 MINIMISING SETTLEMENTSPrevention of all settlements is virtually impossible, because someof the movements causing them occur before support can be installed. Thestiffness of ordinary soldier piles or heavy section steel sheet piling is notusually large enough to have a significant effect on the magnitude of the lateraJwall movement. Lateral movements of walls of these types may be limited by theinsertion of supports such as struts or anchors, at relatively close verticalspacing as soon as possible after excavation. In addition, good workmanshipand detailing is required so that soil movement into the excavation isminimised, unfilled voids are not left outside the supports, and losses of finesthrough seepage and changes of water table are prevented.Very stiff walls such as caisson and diaphragm walls will, to someextent, reduce the inward lateral movements associated with an excavation.Under comparable conditions, the intervals between supports need not be as smallas for more flexible types.


It should also be noted that settlements of the same magnitude asthose that occur during excavation can occur on removal of struts. Therefore,care should be taken during this stage of the construction operation to ensurethat these movements are minimised. The location of the supporting systemcomponents needs to take account of works which have to be completed prior tothe removal of the supports. Where possible, allowance should be made forchanges in construction sequence and methods.10.3 PREDICTIONS OF SETTLEMENTSThe estimation of settlement around excavations is a considerableexercise in engineering judgement. There have been several well documentedcase histories published on this topic s and a very useful summary of some ofthese has been provided by Peck (1969) and updated by O'Rourke et al (1976).Figure 28 gives the approximate magnitude of settlements likely to occur inthe vicinity of excavations. It is based on North American experience andshould only be used for approximate guidance in residual soils.The construction of the Mass Transit Railway in Hong Kong hasprovided some data for Hong Kong conditions, and a number of papers have beenpublished. It is not recommended that this data be used to predict movementsin Hong Kong until additional case histories and supportive evidence isavailable to prove the general values given.Morton et al (1980) described the methods of construction, and theground conditions encountered, and they provided information on existingbuildings adjacent to the railway and a description of the measures adopted tomonitor building settlements. The data collected were separated into thatresulting from station wall installation, that from dewatering, and that fromstation box excavation (Figure 29). The following observations were made :(a) Settlements resulting from station wall installation -The magnitude of the ground and building settlements whichoccurred during the installation of permanent walling systemswere under-predicted and were the most significant phenomenaencountered during construction. In the case of diaphragmwalling, settlements of up to 63mm were recorded. Davies& Henkel (1980) suggested that such movements are due tolateral swelling of the decomposed granite during constructionof individual panels of the wall.


-65-(b)Settlements resulting from dewatering - Settlementsoccurred as a result of dewatering for hand dug caissonconstruction and during dewatering to facilitate stationbox excavation. It was estimated that the resultingsettlements varied between 8mm per metre of drawdown forbuildings on shallow foundation and 3mm per metre for thosewith piled foundations.(c) Lateral wall movements and settlement during excavation -The maximum observed lateral movements of station walls wasbetween 9mm and 43mm for secant piles, and 18mm and 58inm fordiaphragm walls. The movement of the station wal?.s» asmeasured by inclinometers, occurred to their full depthseven though the walls in some cases penetrated to some 30mor more, and movements of the toes of up to 20mm were recorded.Despite the relatively large wall deflections, building settlementswere low, and ratios of maximum lateral wall movement to building settlementof the order of 4:1 were reported.Whilst the causes of settlement of adjacent buildings differed fromsite to site, the total settlements recorded were related to their foundationdepths. Figure 29 shows the relationship between total building settlementand depth factor.


-67-CHAPTER USOME ASPECTS OF REINFORCED CONCRETE DESIGN AND DETAILING11.1 INTRODUCTIONThis chapter does not aim to cover all aspects of reinforced concretedesign as it applies to retaining walls. There are, however, several aspectsof the design and detailing which are not adequately covered in the commonlyavailable literature or present Codes and Regulations, and some guidance isgiven here on these. In particulars the junctions between members are oftenpoorly detailed and suggestions are contained in Section 11.9 for improvements.Reference should be made to comprehensive publications on reinforcedconcrete (e.g. Scott et al, 1965; Park & Paulay* 1975) for complete details ofconcrete retaining wall design and detailing.11.2 GENERAL NOTES11. 2.1 Cod&>Reinforced concrete structural design should be in accordance withthe appropriate standard currently used in Hong Kong; either the Building(Construction) Regulations Cap. 123 (Hong Kong Government, Buildings Ordinance),or Chapter 4, Volume V of the PWD Civil Engineering Manual (Public WorksDepartment, Hong Kong, 1977).77.2.2 U£#m£e Stnmgtk 01 Limit State,The Code being used will specify the load factors or partial factorsto be used* A serviceability limit state analysis should always be made toensure that the limits given in Chapter 4, Volume V of the PWD Civil EngineeringManual are not exceeded.77.2.3 Covet toParticular attention should be given to the cover of reinforcement,both in the detailing and during construction. Blinding concrete should alwaysbe used on soil-like materials.


- 68 -11.3 TOE PESIGNShear in a toe is usually the critical loading case.section of the toe may be taken at distance f d fsupport as shown in Figure 32,of reinforcement is important (see Section 11.8)*The criticalout from the face of theThe detailing of the curtailment and anchorage11.4 STEM PESIGN11.4.1 $£w LoadingFor the stem design cantilever and counterfort walls, it is normalpractice to take the earth pressure acting on the vertical plane through therear of the heel as being projected onto the stem (see Figure 1).However* innearly all walls, the earth pressure acting on the structural section of thewall is different from this, because of the lateral pressures that developduring the compacting of the backfill.Such lateral pressures are usually muchhigher than active and can be higher than at-rest pressures.such lateral pressures is discussed in Sections 3.10 £ 3.11.The magnitude ofTherefore, in designing stem of a wall the earth pressures fromcompaction should always be calculated. In many cases, this will be thecritical loading. There is little evidence to show that the deflection ofcantilever walls will reduce the compaction pressures. (See Section 3.11).11 .4.2 Bending MomeJtt6 and Sktcui Fo/ic&6 in the. Sterna oi Count&i&asitThe bottom of a stem, where it joins the heel, should be reinforcedfor vertical spanning action in addition to horizontal spanning action.Horizontal steel should be continuous in both faces.Horizontal bending momentvariations with height should be catered for by varying the reinforcementspacing in preference to changing the bar sizes.Shear forces should be calculated at the face of the counterforts.Shear stresses will usually govern the stem thickness.The bending moments and shear forces in stems should be calculatedby methods which properly take into account the fixity of each edge of thestem slab and the distribution of pressures on the slab. Huntington (1961)gives useful guidance on this based on work done by the US Portland CementAssociation. Bowles (1977) gives similar information.


-69-11.5 HEEL SLAB PESIGW11 .5.1 LoadingThe design loading on the heel slab is shown in Figure 30. Thebearing pressures for use in structural design are not the same as those usedto check the factor of safety against ultimate bearing failure (Section 6.4).They are normally taken as the bearing pressures at working loads,, as follows:(a)If the resultant passes through the base within the middlethirds the toe and heel pressures for structural design maybe calculated fromwhere V is the normal component of the resultant loading onthe base, B is the base width , and L is the length of wallfor which the resultant earth pressure is calculated (usuallyunity) , and e^ is the eccentricity of the load.(b) If the resultant lies outside the middle third :P max2V3( - e b )L11. 5. 2 Hee£ Slab* £01 Connt^ontThe heel slab for counterfort walls should be designed as a slabspanning in two directions. The references given in Section 11.4.2 may beconsulted for this purpose.of the counterforts.As in Section 1 1.4.2, the critical section for shear is at the faceAgain, shear stresses usually govern the heel thickness.11.6 COUNTERFORT t?ESIGMVertical steel in the counterfort is required to carry the net tensileload from each strip of the heel slab into the counterfort.The main momentreinforcement for the wall is usually concentrated at the back of the counterfort,Horizontal steel in the counterfort is required to carry the net load on eachhorizontal strip of stem.The detailing of this steel should be done so as to


-70-provide adequate anchorage between the stem slab and the counterfort (Figure31). Consideration should be given to staggering the laps in these anchoragebars.Cut-off positions for the main tensile steel in the counterfortsare shown in Figure 31.11.7 KEY DESIGNIn general the ratio of depth to thickness of the key should beless than 2.0.will be. Approximately :It is difficult to predict what the force acting on the keyDesign horizontal . . „., . „ total vertical loadsload on key Ji -^ • above blinding layerslidingIt may be assumed that this load acts at one-third of the key heightfrom the bottom of key. The key should be detailed in accordance with Section11.8 & 11.9. Note that tensile stresses are carried from the key into thebottom of the heel slab, and therefore some reinforcement is called for in thatarea.11.S CURTAILMENT AND ANCHORAGE OF REINFORCEMENTThe curtailment of reinforcement in retaining walls is critical* Abar must extend beyond the point where it is theoretically no longer requiredto allow for inaccuracies in loading and analysis, to allow for inaccuraciesin placing bars, and to avoid large cracks at the curtailment section. Suchcracks reduce the resistance to shear forces and introduce high peak stressesin the tension reinforcement. It is recommended that the following requirementsfrom Clause 3.11.7.1 of the Code of Practice for the Structural Use of Concrete,CP110 (British Standards Institution, 1972) should be applied, to all designs,regardless of the code or regulation being used."In any member subject to bending every bar should extend, except atend supports, beyond the point at which it is no longer needed for a distanceequal to the effective depth of the member, or twelve times the size of thebar, whichever is greater. A point at which reinforcement is no longer required


-71-is where the resistance moment of the section, considering only thecontinuing bars, is equal to the required moment.In addition, reinforcementshould not be stopped in a tension zone, unless one of the following conditionsis satisfied :(1) the bars extend an anchorage length appropriate to theirdesign strength (0.87f y ) from the point at which they areno longer required to resist bending, or(2) the shear capacity at the section where the reinforcementstops is greater than twice the shear force actuallypresent, or(3) the continuing bars at the section where the reinforcementstops provide double the area required to resist the momentat that section.One or other of these conditions should be satisfied for allarrangements of ultimate load considered."Although the above clause is worded in terms of ultimate load designits provisions can clearly be used for working stress design as well.11.9 VET AILING Of REINFORCED CONCRETE CORNERS ANP JOINTS11 . 9 . 1Many reinforced concrete walls involve cantilevers that meet atright angles.moments and peak shear forces.At this junction, there is the combination of peak bendingSuch cantilevers and corners must be carefullydetailed to avoid wide crack width,, and so ensure the strength and serviceabilityof the structures.Some guidance on suitable detailing is given in this Chapter.Research work by Nilsson & Losberg (1976) has shown that reinforcementdetails commonly used in cantilever walls have ultimate capacities significantlyless than are usually assumed in calculations, and they result in excessivelywide corner crack widths at what would normally be working loads. Forexample the commonly used detail shown in Figure 32a, even with the additionof diagonal stirrups, had a failure moment of less than 80% of the calculated


- 72 -ultimate capacity 9 and at a load of 55% of the calculated ultimate capacity,there was a corner crack 2.5mm wide. The detail shown in Figure 32b s whilehaving sufficient ultimate moment capacity, had a corner crack 5.3mm wide ata load of 55% of the calculated ultimate capacity. Other commonly useddetails had an even worse performance. These tests were at relatively smallsteel percentages of 0.5 to 0.8%. Swann (1969) carried out a similar seriesof tests at the higher steel percentage of 3% and significantly worse momentcapacities were obtained. Such joints should be capable of resisting a momentat least as large as the calculated failure moment in adjacent cross sections.The cracks that form in the inside of corners should have acceptable crackwidths for loads in the working range. Also the reinforcement in cornersshould be easy to fabricate and position, and this should normally avoid theneed for stirrups or ties.For the reinforcement of corners subjected to an opening bendingmoment, Nilsson & Losberg (1976) recommended that the reinforcement loop fromeach adjacent part of the structure should be taken out into the cornerregion, as far as cover restrictions allow, and should then be brought backinto the same cross-section adjacent to the inclined reinforcement (seeFigures 32(c) and 32(d)). The main reinforcement should be designed on thebasis of the moments in the adjacent sections (Ml & M2) , ignoring theeffect of reinforcement loop curtailment in the compression zone and theinclined reinforcement. The cross-sectional area of the inclinedreinforcement should be approximately one-half the area of the largest mainreinforcement. Bars should never be spliced in the corner region.Normal code requirements regarding the least permissible bendingradius and the spacing of reinforcing bars generally result in the dimensionsof structural elements being limited* The dimensions of the cross-sectionshould also be chosen in such a way that the following restrictions on thereinforcement percentage are satisfied in order to avoid failure in the corner(a)For Swedish deformed bars K S 40 (yield stress 390MPa) , steel* 1.25%.(b)For Swedish deformed bars K S 60 (yield stress 590MPa) , steel*? 0.8%.Note that these steel percentage restrictions, are for right angledcorners; acute or obtuse corners have lower steel percentages. The restrictionare based on tests using concrete with a cube strength of 30MPa.


-73-These values for the maximum steel percentage may be interpolatedor extrapolated with regard to the yield strength of other steel grades. Forexample, for reinforcing steels to BS 4449, which are commonly used in HongKong, the following percentages apply :Characteristic Strength Maximum steelDeformed high yield bars 410 MPa 1.20%Mild steel 250 MPa 1.56%11.9.2 Re/utrfoA^-tng Steel VztcuLLngBased on the recommendations in Section 11.9.1, the corners inretaining walls should be reinforced according to the general solutions givenin the following paragraphs.When the length of the toe is less than the stem thickness, the jointshould be reinforced as a corner subjected to an opening moment. Thereinforcement in the base slab should be taken out into the toe as far as thecover requirement permits (see Figure 32 (c)).When the length of the toe is greater than the stem thickness, andthe length of the toe is sufficient to provide adequate anchorage length,reinforcement can be as in Figure 32 (d). The concrete Code or Regulationrequirements regarding bending radius, spacing of bent bars and cover shouldbe borne in mind. To limit corner crack widths, inclined reinforcement withcross-sectional area approximately one half the area of the largest mainreinforcement should be used. The limitations on steel percentage given inSection 11*9.1 apply only to the main reinforcement, and the diagonal barsshould not be included in this percentage.Haunches in the re-entrant corner, accommodating substantial diagonalflexural bars, force the plastic hinge away from the face of the joint. Thisimproves the anchorage of the main tensile steel where it enters the joint.The increased internal lever-arm within the joint, in turn, reduces theinternal tensile force. Haunching would allow the use of higher steelpercentages, but Nilsson & Losberg (1976). make no specific recommendations onallowable steel percentages for haunched right angled corners.For large joints with up to 0.5% steel, Park & Paulay (1975)recommended the use of diagonal bars across the corner equal in area to 50%of the main reinforcement*


Above 0.5% of steel, they proposed that radial hoops (Figure 32(e))be provided, the area of one radial hoop being given by :asj* . (P-".uu. }|^- /! + (£!.)» I ^ (25)Awhere p = r 2^ in the critical member,b.d en = no. of legs.A s ] = area of steel limiting the magnitude of the moment that canbe applied to the joint 9fyj * yield stress of radial hoops,It should be emphasised that problems of construction may arisebecause of steel congestion at such corners, and it is usually a bettersolution to thicken the concrete sections involved.Where the backfilled faces of a retaining wall meet at an acuteangle in plan, then similar considerations to those above should be given tothe detailing of the reinforcing steel. Additional horizontal reinforcingsteel will be required in the outside face of the wall.11.10 JOINTS11.10.1 VoJutic.aL Jo^intA £01 Long^tudinai Movm&ntVertical joints are required in retaining walls to minimise theeffects of temperature changes and shrinkage* and because of constructionstages. In reinforced concrete walls, vertical construction joints withV-notches at the face should be provided at sections preferably not over 10mapart, together with reinforcement through the joints. Expansion jointswith grooved shear keys should be provided not more than 30m apart» thereinforcement not being carried through such joints. la gravity concretewalls, similar expansion joints should be provided, preferably not morethan 10m apart* Where the water table is high, water stops should beprovided at all construction and expansion joints.Where there are large temperature variations, expansion joints mayrequire resilient jointing material to allow movement.


-75-Sections where there is a substantial change in wall stiffness orwall type (e.g. counterfort to cantilever), or where the nature of thefoundation changes (e.g. from fill to rock), require careful detailing. Atsuch locations, it is usually possible to work out the direction of movementthat may occur and to provide adequate clearance to accommodate the movements.It is usually best to provide a structural separation, rather than to attemptto reinforce the junction to take the bending moments and shears involved.11.10.2 Horizontal ConAtttuc^ion JolntbThe standard of roughness and clean-up on horizontal constructionjoints should be clearly specified and controlled. Keys in such joints shouldbe avoided, and water stops should be provided in joints below the water table.The construction joint at the base of a cantilever stem shouldalways be detailed as being at least 100mm above the heel slab, to enable theconcrete formwork to be held during construction.In the stem of a wall, the position of all construction joints shouldbe carefully considered from the point of view of appearance as well asstructural performance (see Section 11.12).11.11 COMTWL Of CRACKINGTo prevent unacceptable cracking of retaining structures the followingsteps should be taken, in addition to normal good quality concrete practice :(a) Provide shrinkage and temperature reinforcement. This steelshould be in accordance with Chapter 4 of the PWD CivilEngineering Manual to ensure that the crack widths given inthat chapter are not exceeded. Note that there is arelationship between the reinforcing bat size, steel percentageand crack width involved. In no case should the steelpercentage used be less 0.3% of the gross concrete area of thewall both horizontally and vertically. In the stem of thewall exposed to the air-two thirds of this steel should beon the outside face and one third of the steel on the earthface.


-76-(b)(c)(d)(e)(f)Specify that the concrete placing and temperature is to bekept as low as practical, especially in the summer period.Specify successive bay, not alternate bay, construction*Specify early curing for the purpose of cooling, so as tominimise the heat rise.Specify good quality concrete and, where appropriate, limitthe cement content.Additional protection against cracking can be given by paintingthe earth face of a wall with, for instance, two coats ofasbestos filled bituminous or asphaltic paint.11.12 APPEARANCE Of RETAINING WALLSRetaining structures are very dominant forms on the urban and rurallandscape. Careful design can make a considerable improvement to the appearanceof a retaining wall without, in many cases, influencing the cost of the wall.The aspects and features of retaining walls that are important totheir aesthetic impact are :(a)(b)(c)(d)wall height,front face slope of the wall,slope and surface treatment of backfill behind wall,longitudinal elevation of wall in relation to plan (poordesign can give the appearance of the wall having a f kink*in it),(e)(f)concrete surface textures, and the expression and positionof vertical and horizontal construction joints (concretetextures can be cheaper than 'smooth 1 finishes), andthe coping of the wall.


-77-Aggour, M.S. & Brown, C.B. (1974).The prediction of earth pressure onretaining walls due to compaction. Geotechnique 3 Vol. 24, pp 489-502.Bjerrim f L. & Eide, 0. (1956). Stability of strutted excavations in clay.Geotechnique, Vol. 6, pp 32-47.Bowles, J.E. (1977). Foundation Analysis and Design. McGraw Hill, New York750 p.British Standards Institution (1972).Code of Practice for the Structural Useof Concrete,, CP 110:1972. British Standards Institution, London, 54 p.British Standards Institution (1972). Code of Practice for Foundations,CP 2004:1972. British Standards Institution, London, 158 p.British Standards Institution (1978). Steel, concrete and composite bridges -Specification for loads, BS 5400:Part 2:1978. British StandardsInstitution, London, 158 p.Broms, B.B. (1971). Lateral Earth Pressure Due to Compaction of CohesionlessSoils. Proceedings of the 5th Budapest Conference on Soil Mechanicsand Foundation Engineers, Budapest, pp 373-384.Broms, B.B. & Ingelson, I* (1971).Earth pressure against the abutments ofa rigid frame bridge. Geotechnique, Vol. 21, pp 15-28.Caquot, A. & Kerisel, J. (1948). Tables for the Calculation of Passive Pressure,Active Pressure and Bearing Capacity of Foundations.. (Translated fromthe French by M.A. Bee, London) Gauthier - Villars, Paris, 120 p.Canadian Geotechnical Society (1978).Canadian Foundation Engineering Manual,Part 4. Canadian Geotechnical Society, Ottawa, 68 p.Cedegren, H.R. (1977). Seepage, Drainage & Flow Nets. 2nd Ed. Wiley, NewYork, 534 p.CIRIA (1974). A comparison of quay wall design methods. Construction IndustryResearch & Information Association, London, Report No 54, 125 p.


-78-Danish Geotechnical Institute (1978).Code of Practice for FoundationEngineering. Bulletin No. 32^ Danish Geotechnical Institute* 52 p.Davies, R.V. & Henkel, D.J. (1980). Geotechnical problems associated with theconstruction of Chater Station Hong Kong. Proceedings of the Conferenceon Mass Transportation in Asia* Hong Kong* Session J3, pp 1-31.Department of Transport, U.K. (1978).Reinforced earth retaining walls and bridge abutments for embankments*Department of Transport, Technical Memorandum (Bridges) BE 3/78., 80 p.Federal Highway Administration (1978). Standard Specifications for Constructionof Roads and Bridges on Federal Highway Projects. Federal HighwayAdministrations Washington, D.C., 355 p.Geotechnical Control Office (1979).Government Press, Hong Kong, 230 p.Geotechnical Manual for Slopes.Goldberg, D.T., Jaworski, W.E. & Gordon, M.D. (1975). Lateral support systems& underpinning : Vol. 1, Design and construction. Federal HighwayAdministration Report No. FHWA RD~75-128> 312 p.(National Technical Information Service No. PB257210).Gould, J.P. (1970). Lateral pressures on rigid permanent structures.Proceedings of the Speciality Conference on Lateral Stresses in the Groundand Design of Earth Retaining Structures^ Ithica* New York* pp 219-270.Hanna, T.H. (1980). Design and construction of ground anchors. ConstructionIndustry Research and Information Association,, U.K. Report No. 65, 2nded., 67 p.Huntington, W.C. (1961). Earth Pressures and Retaining Walls* Wiley, NewYork 534 p.Ingold,T.S. (1979). The effects of compaction on retaining walls. Geotechnique>Vol. 29, pp 265-284,Jaky, J. (1944) The coefficient of earth pressures at rest. Journal of theSociety of Hungarian Architects and Engineers^ pp 355-358.


-79-James, E.L. & Jack, B.J. (1974). A design study of diaphragm walls,Proceedings of the Conference on Diaphragm Walls and Anchorages, London,pp 41-49.Janbu, N. f Bjerrum, L & Kjaerasli, B. (1956). Veiledning red losning avfounderaenterringsoppgaker (Soil Mechanics applied to some engineeringproblems). Norwegian Geotechnical Institution* Publication No 16, 93 p.Japan Society of Civil Engineers (1977).Guide to Tunnelling by Cut and CoverMethod. Japan Society of Civil Engineers, Tokyo, 203 p.Lambe, T.W. & Whitman, R.V. (1969). Soil Mechanics,, Wiley, New York, 553 p.Lambe, T.W., Wolf skill, L.A. & Wong, I.E. (1970).Measured performance of braced excavations.Journal of the Soil Mechanics and Foundations Division^ American Societyof Civil Engineers ; Vol. 96, pp 817-836.Lumb P. (1975). Slope failures in Hong Kong. Quarterly Journal of EngineeringGeology* Vol. 8, pp 31-65.Ministry of Works and Development, New Zealand (1980). Specification CD 209 -Cribwalling and Notes. Ministry of Works and Development, New Zealand,Wellington, 9 p.Ministry of Works and Development, New Zealand (1973).Notes.Retaining Wall DesignMinistry of Works and Development, Civil Division PublicationCDP 702/C, 43 p.Morgenstern, N.R. and Eisenstein, Z. (1970). Methods of estimating lateralloads and deformations. Proceedings of the Specialty Conference on LateralStresses in the Ground and Design of Earth Retaining Structures. Ithica,New York> pp 51-102.Morton, K., Cater, R.W. & Linney, L. (1980).Observed settlements of buildingsadjacent to stations constructed for the modified initial system of theMass Transit Railway, Hong Kong.Proceedings of the 6th Southeast AsianConference on Soil Engineering* Taipei. Vol. 1, pp 415-430.


NAVFAC (1971), Design Manual - Soil Mechanics* Foundations* and EarthStructures* DM - 7. Department of the Navy, Naval Facilities EngineeringCommand, Washington, B.C., 223 p.Nilsson, I.H.E. & Losberg, A. (1976). Reinforced concrete corners and jointssubjected to bending moment. Journal of the Structural Division* AmericanSociety of Civil Engineers* Vol. 102, pp 1229-1254,O'Rourke, T.D., Cording, E.J, & Boscardin, M. (1976). The ground movementsrelated to braced excavation and their influence on adjacent buildings.US Department of Transportation* Report No. DOT-T&I 76* T-23* 123 p.Park, R. & Paulay, T (1975).769 p.Reinforced Concrete Structures* Wiley, New York,Peck, R.B. (1969). Deep excavations and tunnelling in soft ground.7th Int. Conf. Soil Mechs* and Foundation Engineering* Mexico City.State-of-the-art Volume, pp 225-290.Peck R.B., Hanson W.E. & Thornburn T.H. (1974).2nd Ed. Wiley, New York, 514 p.Foundation Engineering.Public Works Department, Hong Kong (1977). Civil Engineering Manual* Vol. V :Roads. Public Works Department, Hong Kong, 350 p.Rankilor, P.R. (1981). Membranes in Ground Engineering. Wiley, Chichester 377 p.Rowe, P.W. (1952). Anchored sheet pile walls. Proceedings of the Institutionof Civil Engineers. Vol. 1, pp 27-70.Rowe, P.W. (1955). A theoretical and experimental analysis of sheet pile walls.Proceeding of the Institution of Civil Engineers* Vol. 4, pp 32-87.Rowe, P.W. (1957). Sheet pile walls in clay. Proceedings of the Institutionof Civil Engineers* Vol. 7 f pp 629-654.Rowe, P.W. & Peaker, K. (1965). Passive earth pressure measurements.Geotechnique* Vol. 15, pp 57-78.


-81-Schuster, R.L., Jones, W.V., Sack, R.L. & Smart, S.M* (1975). Timber retainingstructures. Transport Research Board, National Research Council.Special Report 160: Low Volume Roads* pp 116-127.Scott, Glanville & Thomas (1965).Explanatory Handbook on the B.S. Code ofPractice for Reinforced Concrete CP 114:1957, 2nd Ed. ConcretePublications, London, 172 p.Seed, H.B. & Whitman, R.V. (1970), Design of earth retaining structures fordynamic loads.Proceedings of the Specialty Conference on LateralStresses in the Ground and Design of Earth Retaining Structures* Ithica.,flew York,, pp 103-147.Swann, R.A. (1969).Flexural strength of corners of reinforced concrete portalframes. Cement and Concrete Association,, U.K.* Technical Report TRA 434.14 p.Teng, W.C. (1962). Foundation Design. Prentice-Hall, New Jersey, 466 p.Terzaghi, K. (1943). Theoretical Soil Mechanics. Wiley, New York, pp 129-130.Terzaghi, K. (1954). Anchored bulkheads.of Civil Engineers,, Vol. 119, pp 1243-1324.Transactions of the American SocietyTerzaghi, K. & Peck R»B. (1967). Soil Mechanics in Engineering Practice,,2nd Ed., Wiley, New York, 729 p.Tschebotarioff, G.P. (1951). Soil Mechanics, Foundations and Earth Structures.McGraw Hill, New York, 1st Ed., 655 p.Vesic, A.S. (1975). Bearing capacity of shallow foundations.Foundation Engineering Handbook, Edited by H.F. Wlnterkojm and H.Y. Fang,pp 121-147. Van Nostrand Reinhole Co., New YorkWhite, R.E. (1974). Anchored walls adjacent to vertical rock cuts. Proceedingsof the Conference on Diaphragm Walls and Anchorages, London, pp 181-188.Wu, T.H. (1975). Retaining walls. 'Foundation Engineering Handbook, Edited byH.F. Winterkorn and H.Y. Fang, pp 402-417. Van Nostrand Reinhold Co.,New York.


APPENDIX ASYFBOLAA!AS* A s l, A S 2, A S 3BB!bccfcc ' efa eccentricity of load on base in the directionsof length and breadth respectivelyF sffyfactor of safetymoment arm of vertical component of earthpressure forcecharacteristic strength of reinforcement8c> gq» 8 Y foundation ground slope factorsH, HI, etc. height of plane on which earth pressure iscalculated (from underside of base or bottomof key to ground surface)Hhh cii wic» iq» iyK 0tangential component of foundation loadingdistance of resultant force from wall toecritical depth of fill where compactionpressures equal active pressurehydraulic gradientwaviness of rock jointbearing capacity inclination factorscoefficient of earth pressure at rest


-#»-KqKpK sKLVLj^L sL tcoefficient of active earth pressurecoefficient of passive earth pressurecoefficient of subgrade reactioncoefficient of permeabilitylength of baseeffective length of baselength of wall heelclear span between counterfortslength of wall toeM» MI, M2» Mj bending moments for reinforcement designM 0M rNfoN c , NQ, NynpPAP OPHPNPpPXPQPVsum of moments causing overturningsum of moments resisting overturningstability factor relating to excavation base failurebearing capacity factorsmoment arm of resultant water force on back of wallequivalent line load due to rolleractive earth pressure force'at rest 1 earth pressure forcehorizontal component of active earth pressure forcenormal component of earth pressure forcepassive earth pressure forcetangential component of earth pressure forcelateral earth pressure due to line or point surcharge load(per unit length of wall)vertical component of earth pressure force*V water force due to water in tension crack», Pmax, P t pressure for structural designQQ Ltotal loadline load


-85-point loadintensity of load on base or surcharge loadallowable bearing capacity^d%ltflow rate through drainultimate bearing capacity» &a» R p» % resultant forcessshear strength of soilStotal shearing resistance at underside of basec* Sq* s foundation shape correction factorsTthickness of wall stemt C 9 tq, tfoundation tilt factorsU, Uj 9 1^2 resultant force due to water pressuresU IVuVVhorizontal and vertical components of resultant water forcepore water pressurenormal component of foundation bearing pressureshear force for reinforcement designW, W b weight of backfillW tXyY 0weight of wallresultant horizontal reactionlateral deformation of retaining wallvertical depth of tension crack in cohesive soilZZdepth below final fill leveldepth below final fill level of maximum residual compactionpressure« angle of inclination of foundation base3 * angle of inclination of the back of the retaining wallyyiY wbulk unit weight of soileffective unit weight of submerged soilunit weight of watery ^'satsaturated unit weight of soil


Asettlement of wall6 angle of wall friction6]3 angle of base friction* ®i* ®2 location angles for failure planeQfra 9 a 1angular rotation of foundation basetotal and effective normal stress0, 0 f angle of shearing resistance in terms of total andeffective stressa) angle of ground slopeTshear stress


-87-LIST OFBTable.Indicative proportions of maximum wall friction developed 17(granular soils, passive case) (Rowe& Peaker* 1965)Insitu permeabilities of Hong Kong residual soils 18Wall displacements required to develop active and 22passive earth pressures (Wu, 1975)Suggested Surcharge loads to be used in the design 31of retaining structures (Public Works Department, 1977)Filter design criteria to be used in Hong Kong 38(Geotechnical Manual for Slopes, 1979)Allowable bearing pressure on jointed rock 49(Peck, Hanson & Ihornbum, 1974)Minimum specification for select backfill for 58reinforced earth retaining walls(Federal Highway Administration, 1978)


-89-LIST OFC1 Loading on a typical retaining wall for 91Ranklne assumption)2 Effect of wall movement on wall pressure 933 Effect of deformation on earth pressures 954 Earth pressure coefficients - sloping ground 975 Earth pressure coefficients - sloping wall 996 Trial wedge method - coheslonless soil 1017 Trial wedge method - cohesive soil 1038 Trial wedge method - layered soil and porewater 105pressure (active case)9 Influence of heel length on analysis method 10710 Approximate method for determination of direction 109of Rankine active earth pressure.11 Point of application of active force 11112 Point of application of resultant force 113pressure distribution13 Passive force by circular arc method - layered 115soil and pore water pressure14 Earth pressure due to compaction 11715 Lateral loads on wall due to point and line 119load surcharges16 Surcharge load cases 12117 Effect of surface Infiltration & drain location 123on water pressures18 Drainage details for retaining walls 12519 Design of inclined drains 12720 Permeability of drainage materials 129


21 in 13122 for 13323 1975) 13524 137excavation25 Factor of safety with to 13926 Crib wall details 14127 Crib wall design curves 14328 Large excavations - settlement guide 14529 Building settlement data - MTE construction, 147Hong Kong30 Design loading on heel slab31 Counterfort walls - detailing at Junction ofcounterfort with heel and stem32 Detailing of cantilever wall reinforcement 153


-91-VIRTUAL BACK CF WALLMAXIMUM DEPTH OFBURED SERVCESPLANE OF SLIDING —*BSLIDING STABILITYNOTES1. Material shaded rs/s/A is included in the total weight forcalculation of sliding stability.2. Earth pressure denoted by * is used for the stem design.(See Section 11 .4.1).3. Water forces not shown.LOADING ON TYPICAL RETAINING WALL(DRAWN FOR RAN KIN E ASSUMPTION)FIGURE 1


UJctGOLUOfCLX I—ITUJsuooLLLL81086540-20.006-H 65yjTERZAGHIooGOUJtrQ.cni S08 LUPRINCETON TESTS0-6 S' 5 fe0-3 fiooJ_ X01om 0.020WALL ROTATION WALL-q-HHPASSIVE CASEACTIVE( after Canadian Society , 1978 )EFFECT OF mil MOVEMENT OMFIGURE 2WAIL PRESSURE


-95-. Y EXPANSIONH\YHEXPANSIONACTIVE STATE HHK a *H.Y, COMPRESSION.y_, COMPRESSIONrlPASSIVESTATEKV LJ I/* V LJDO ** *^p0 "(a) RIGID WALL FREE TO TRANSLATE OR ROTATE ABOUT ITS BASENO DISPLACEMENTH(b) RESTRAINED RIGID WALLEXPANSIONBOTTOM OF WALLDISPLACED OUTWARDMORE THAN TOPOF WALL(c)TOP OF WALL RESTRAINEDEXPANSION(d) STRUTTED FLEXIBLE WALL^^^^^^^^^^^^^^^^^^__UJILJ|J | | | ....giiiin^EFFECT OF DEFORMATION OH EARTH PRESSURES |FIGURE 3


- 97-REDUCTION FACTOR, R 9OF Kp FOR VARIOUSRATIOS OF 6/0.574 520.536 .475 .417Pf =NOTE : CURVES SHOWN ARE FOREXAMPLE : 0 s 25" j tf I® - -— R= 0.711— (K f FORKp =0711* 3.62 =2.58SURFACEARITHMC SPIRALACTIVE PRESSUREANGLE OF SHEARING RESISTANCE, ft> DEGREES{Caquot 1 K^J^ gj^EARTHPRESSURE COEFFICEMTS-SLOPIN6 6ROUND | FIGURE


-99 -REDUCTION FACTOR, R,OF' K FOR VARIOUSRATIOS OF V'07-^/01015202530354045-0.7.978.961.939.812.878.836.783.718-0-6.962.933.901.860.811.752.682.600-0.5.946.907.862.803.746.674.592.500-0.4r .929.881.824.759.696.603.512.414-0.3.912.854.787.711.637.536J.439.339-0.2.898.830.752.b66.574.475.375.276-0. I.881.803.716.620.520.417.316.2210.0.854.775.673.574H4G7.362.262. 174I— NOTE: CURVES SHOWN AREFOR 6/0 = -1EXAMPLE 0 =K p=R(Kp FORR =0.81130 40 45ANGLE OF SHEARING RESISTANCE, $ > DEGREES (Cmguot ft _K«ns€t, 19*8^EARTH PRESSURE COEFFICIENTS-SLOPING WALL FIGURE 5


- 101 -FORCE POLYGONACTIVE ( FULL LINE )PASSIVE (DOTTED LINE )b)COMBINATION OF FORCEPOLYGONS TO OBTAIN MAX. PA(ACTIVE CASE ONLY)NOTESThe lateral earth pressure is obtained by selecting a number of trial failureplanes and determining corresponding values of P^ (or Pp) by drawing aforce polygon— see (a). For the active pressure case, the maximum value ofP^ is required and for the passive case, the minimum Pp is required. Theselimiting values are obtained by interpolating between the values for thewedges selected-see (b) .2. Lateral earth pressure may be calculated on any surface or plane through thesoi 1 .3. See Figures 11 and 12 for the point of application of P^.k. The trial wedge method may also be used for a level or constantly slopingground surface, in which case it should yield the same result as that givenby Rankine's or Coulomb's equations (whichever is applicable).TRIAL WEDGE METHOD - COHESIONLESS SOIL FI6URE 6


- 103 -SURFACE ON WHICH PRESSUREIS CALCULATEDDEPTHTENSIONOFZONETRIAL WEDGES FORACTIVE PRESSUREFORCEPOLYGON FOR TYPICAL WEDGECOMBINATIONOFNOTESFORCE POLYGONS TO OBTAIN1 The above example shows Rankine's conditions but the same principle applies forCoulomb's conditions. (Adhesion on the back of the wall is ignored).2. For direction P A see Figure 10 (Rankine's conditions) or Figure 6 (Coulomb'scondi tions).MAX.3. See Figures 11 and 12 for point of application.4. See Figure 12 for resultant pressure diagram.5. The trial wedge method may be used for a level or constantly sloping groundsurface.TRIAL WEDGE METHOD-COHESIVE SOIL FIGURE 7PA


-105-mAs\/ TRIALWEDGE1T U YERRW, / \/TRIAL WEDGEIV\4X/ISCOMPUTED FROM ANALYSISOF TRIAL FAILURE WEDGESFA,FORCESFORCEPOLYGON (a)-ONLAYER (JLAYER CPRESSURE DISTRIBUTIONIS MADE EQUIVALENTTO RESULTANTS P AH ,AND PAH*Ui AND U* = RESULTANTWATER PRESSURE ONFAILURE WEDGES(c)FORCEPOLYGONSPROCEDURE1. Draw trial wedge I in layer (1) (as shown) and obtainfailure plane and drawing the force polygon (a).maxby varying the2. Draw trial wedge JT (as shown) by choosing failure plane AB in layer (2).3. F*nd by varying the inclination of plane BC from B and drawing theforce polygon (b).k. Using X draw force polygon (c) and find5. Repeat steps 2. to 5. using other trial failure planes AB 1 , etc. until P A2 maxis determined.NOTEWhere layer 2 is rock-like material, such that no earth pressures are exertedagainst the wall, due account should however be taken of water pressures and jointcontrolled failure modes.TRIAL WEDGE METHOD- LAYERED SOILAND PnBFW&TEH PRESSURE (ACTIVE CASE)FIGURE 8


- 107-OUTER FAILURE SURFACEINNER FAILURESURFACEMCVES WITH WALLVERTICAL VIRTUAL BACKOF WALLMOVES WITH WALLaOPING VIRTUAL BACKOF WALL!b) COULOMBNOTE(1) If line AB does not intersect the wall, Rankine ! sconditions apply.If line AB does intersect the wall, Coulomb 1 sconditions apply.(2) p « i(90 - 0 1 ) - i(e - «) where sinsin (risin 0 8INFLUENCE OF HEEL LENGTH ON ANALYSIS iETHOD FIGURE 9


-109-PROCEDUREDraw a line from the point wherethe ground surface intersects theback of the wall (B) to a point onthe ground surface located at adistance equal to 2H' from B.The pressure on A-A 1 may beassumed to act parallel with this line,APPROXIMATE METHOD FOR DETERMINATION OF DIRECTIONOFRANKINE ACTIVE EARTH PRESSUREFIGURE 10


-Ill -CDUcOS —T3oCLC03 4->CD-Ctn Q.CDO -M— u CD0) ooL_ L.•3 CDcn 4-i— cCDCDO OQCQ en"D— 0)CD 203 OL_03 •Q. enX O±j CDC _C— 4-»o CLX:enx: Den OZJ !_O JCU 4-1JZ•M CDCCD —C —0303 O5 *^03 UL. CDO >OcCDJCCD T3^- CDcn j->c__3C UCO —034-» OruO t/)CL (ACD_C L.cn CLDO CDU _CJC 4J4-»JZ'J> O4_» .—U JC< CQ. O(D •C X— O


-113 -SURCHARGEPRESSURE-SURFACETRIAL WEDGES PRESSURE OH A-8Use when the ground surface is very irregular or when a non-uniform surchargeis carried.PROCEDURE1. Subdivide the line A-4 into about 4 equal parts h^ (below the depth y () oftension crack).2. Compute the active earth pressures P 1 , P 2 , PS, etc., as if each of thepoints 1 , 2, 3, etc. , were the base of the wail . The trial wedge method isused for each computation.3. Determine the pressure distribution by working down from point 4. Alinear variation of pressure may be assumed between the points wherepressure has been calculated.4. Determine the elevation of the centroid of the pressure diagram, y. This.is the approximate elevation of the point of application of the resultantearth pressure,NOTE : Water forces must be considered separately.POINT OF APPLICATION OF RESULTANT FORCEAND PRESSURE DISTRIBUTIONFIGURE 12


-us -TRIAL FAILURESURFACELAYER® V"~PLANEFORCEWEDGEPOLYGONSGENERALPROCEDURECIRCULAR ARC WEDGEFORCE POLYGONREPRESENT TOTALuLAYER @ (c)VERTICALYLOADS.i1 Determine the directions of surface of sliding BA 1 and the plane portion A'M ofthe surface of rupture from the following formulae :0 2 = ^(90° + 0) - He + ) where,


-117-qs57kM/n»1111•o05'IH132JQci 10 20 30 m m AND VALUESi \(EARTH PRESSUREDUE TO WEIGHT OFBACKFILL )1 0.2 f*a.i i*t4*400 k§120k§ PLATECOMPACTSCWTICALrepTHZe f«lo.st0-S20.350.4S0*32HOftUQtftALEmm PRESSUREc lioMA^IkN/» 2 I20.019.®12*516.011. SNOTE.XDIAGRAM DRAWN FOR 10.2 4 SMOOTH WHEEL ROLLEROH FILL, = 30* , r = 18 kN /m 3EFFECTIVE WEIGHT OF VIBRATORY ROLLERS ASSUMED TO BETWICE TOTAL STATIC WEIGHT.ti)COMPACTION AGAINST UNYIELDING WALLS (BROMSJ971)MFIBSIKXESSSVECWmyCTEO LAYERS8LOCUS OF *faito)EAHtH PRESSUREOF COMPACTINGSURFACE LAYER OF FILL WHICHPLACED WITHOUT CCMBIOTICN.HCMZOHTAL EARTH«bl SHOWS INFLUENCE OF SUCCESSIVE!*CONFACTiNG LAYERS OF SOIL 8E68H*l!ff6AT EASE OF WALL,\r hm -MAXIMUM VALUE OF HORIZONTAL STRESS SUSTAINEDAFTER COMPACTION.\WHERE p 2 EaiWALENT LINE LOAD DUE TOROLLER. FOR V®RATOfflr ROLLERSCALCULATE p USING ANEQUIVALENT WEIGHT EQUAL TODEADWEIGHT OF ROLLER PLUSCENTRIFUGAL FORCE ifJUCEDBY ROLLER VIBRATINGMECHANISM.HORIZONTAL EARTH PRESSURE1C) SHOWS PROPOSED DESIGN P«SS4*IEDIAGRAM.Hi)COMPACTION PRESSURES - DESIGN DATA !SNGOLD,1979)EARTH PRESSURE DOE TO FIGURE 14


0-i 0.2Nu C 0.4u.ouj 0.6ID _j> 0.81.0qq£~3- 119 -^5^311—. "1m - fl R !Ii*< ^% h ^^^^ ** te **^1'*^^^v^ ~^J^2m=0.7-A ^\])^^in*u.4jX^/*^^ ''V 1tjfir" lii*'"" ^J»%,// / m Y/ // - a L -fin H/ "jr u.** uu n/_// 0.5 '56H7~v7 0.6 -52H/ /// 07 • 48 H0.2 0.4 0.6 0.8 lqVALUE OF P Q (±L)N \/m < 0.4^•\ ~-^--.^N\\m= = 0.6^N^m = 0.5^1 ////^/ *f /I//h< ^ v^pm P(rk Y// 0.4 -78 -59H).5 -60 -54HX6 -46 -48Hf r/II0 0.5 1.0 1.5VALUE OF P Q (^-)I i — |—1 rcuNr^H j(XHr i-or m i 0.4^)""""H•^-ir^jnr ^ Q L (0.16 t n 1 *•— / ^/ - P 0 = 0.55QL— / Y*"/ / ' PQP nn > 0 4HI, ^,mH _iQp~*^~~ m ~~****«>^^ ff&y/wf/•«ffl"'"^VyH*;N _J*j/,,„„« -- L.nrvf 1D^p Qr H )1.28nr?nFor m ^ C.4' 0.64Q L+' u ( m* + 1 1PRESSURES FROM LINE LOAD Q LMODIFIED BOUSSINESQ )— J|OL__— ..J, ;/^r/1 \ \S RESULTANT1 \ P = K aOt1 r£/ ^5ImH .n ,H*, 0.28 n 2u^Op' ~ (0.16 * n*)For m > 0. 4P tf, 1.77nrfn*vi n f rn* ju n 4^SECTION PQ= Fleas' 1-106 )A -ARESULTANT FORCE FROM LINE LOAD Qt( APPRoTMETHOD FOR LOW RETAINING WALL )\ LINE LOAD ( TER2AGHI t PECK 1967 )MM^iHi'iM^«wgBgKs«a«aaaa»B!aiiiij mm iiiiiailiiiii *""T 'M m * | • f|llC Tft-POINT AND LINE LOAD SURCHARGESPRESSURE FROM POINT LCAD OPPOINT LOAD ( MODIFIED BOUSSINESQ \JL«— —•• | —•— «1 r i n n n r 1C


- 121 -uniform. surchargeI—- virtual back of wallr%$;LOADING 1CRITICAL FOR BEARING PRESSURES ANDWALL REINFORCENENTuni formsurchargevi rtual back of wal1177]^:LOADING 2CRITICAL FOR STABILITY


- ]23 -Potential failure planeDrainWater pressuredistribution onpotential failureplane due tosteady seepage.(a) NORMAL STEADY STATE SEEPAGE CONDITION (VERTICAL DRAIN)AhInfiltrationJL JL X Potential failure planeDrainNote increase inwater pressure onpotential failureplane due tosurface infiltration.(b) SURFACE INFILTRATION (VERTICAL DRAIN)InfiltrationI 4Potential failure planeDrainNote water pressureis zero on potentialfailure plane.(c) SURFACE INFILTRATION (INCLINED DRAIN)( FLOW NETS ASSUME HOMOGENEOUS, ISOTROPIC SOIL ]EFFECT OF SURFACE INFILTRATION AND DRAINFIGURE 17LOCATION ON WATER PRESSURES


-125-Note :For ease of construction, wherefilter layers are constructed at asteep incline, filter material maybe placed in hessian bags.Water pressure should be consideredin design (Section 5.3)drainagechannels|weep holes(a)protected surfaceor sgi rialbackfillgroundfiIterdrainage material layer designedinaccordancewi th Section 5 Aconstruct ion;batterdrainage Materiallongitudinal porous pipesubsoipi pe laidto gulleyblinding layer- 1impervious baseto dra i nCANTILEVER / COUNTERFORTweepholesdra i nageterialaced intermaterialdrainagematerialblinding layer(b) CANTILEVER / COUNTERFORTused when (a) is not possibleWater pressure should be consideredin design (Section 5«3)dra i nagechannelapron onsteepslopesdrai nagematerialdetail as (a)i Iterdes i gnedin accordancewith Sectionblinding layer/y filter layer designed'M-lp accordance withSection 5.J*drainage materialplaced in hessianbags(c) GRAVITY TYPE(d) GRAVITY TYPEused when (c) is not possibleDRAINAGE DETAILS FOR RETAINING WALLS FIGURE 18


- 127-PROTECTED SURFACE>:1DRAINAGE MATERIAL \ ASSU MED IMPERMEABLEBASE(a) TYPICAL FLOW NET FOR SEEPAGE INTO INCLINED FILTERJlk sKY = PERMEABIUTY OF FLIERKg = PERMEABILITY OF SOILINCLINATION OF FILTER ( SEE ABOVE)(b) CHART DEVELOPED FROM FAMILY OF FLOW-NETS( after Cedergren, 1977)DESIGN OF INCLINED DRAINS FIGURE 19


-129-occLUiZ68.1 234681 234 6810 23468100GRAINSIZE (mm)COEFFICIENT OF PERMEABILITYFOR CLEAN COARSE- GRAINEDDRAINAGE MATERIALu


IIIFOROF iHFiMfTE 0EPIHMFOROF FINITE DEPTHFACTOfi OF SAFETY AGAINST HEAVING INLOOSE SAHD Oft PfPHi© IH DENSE SAHOFACTOR OF SAFETY A6AIHST PIPING'a) SHEET PENETRATION IN GRANULAR SOILS-"l"2J!dHI3 1£•^v2*


- 133-WMLTYPELOAD DIAGRAMSTABILITY CRITERIASLIDINGS - (W t + P vFs (sliding)tan>1.5a:oi.e. F.S. on any included ultimate passive > 3.0OVERTURNINGmMoments about the toe of the baseFs (overturning) • Homents Desisting overturning m MrMoments causing overturning Mo Hr*io•4->Oa:oLU>LU OLU 'VERTICALSTEMRANKINECASERANKfNECASEMr * W t aMo *Fs »H.8.W t a(Passive ResistancePp ignored)>2.0it is illogical to take vertical componentsof the disturbing forces and usethem as restoring moments in theexpression for F.S. See section 6,3.2Overturning may be ignored if R w lies withinmiddle third (soil), middle half (rock).For gravity type walls, overturning must bechecked at selected horizontal planes, theresultant must remain within the rriddlethird.LOCATION OF RESULTANTPoint where R w intersects base, from toe,W t a + P v f - PHy * U 1V C - U 1H d - U 2 eh * W t + P v + ^1V " U 2(Passive resistance Pp ignored)For soil foundation material, R w shouldwithin middle third of the baselieFor a rock foundation, R w should He withinmiddle half of the baseBEARING PRESSURESee section 6.4 for Calculation of factor ofsafety for bearing Fs (bearing) 3-0Wt = total weight of the wall including soilon toe plus soil above heel (forcantilever walls only)resultant of W tf 1*2SLOPE FAILURE SN SURROUNDING SOILWith shear surfaces passing under the wall,the factors of safety should comply withthe requirements of Table 5*2 of theGeotechnical Manual for Slopes.•z< oOWATER FORCESReference should be made to Chapter 5 forcases other than those shown here*STABILITY CRITERIA FOR RETAINING WALLS [FIGURE 22


-B5-SHAPE FACTORSn-JLJ*LL N eSy1-0-4.1.SqINCLINATION FACTORSTILT FACTORSN eL 1 " V+B'L' cco»0'Jm. m+1'r. iq- 1~frNc ton4/"»*'Bwrw^ m.__gm- 2^ L]^ L^OTE : Hgjj^ - V tOH$* A C®0060®EFFECWE ^AHEA A'» B' LB'= B - 2e§L'- L- 2nCSECTIOM 6,4.31WWIDED TIC i«CLIMAT!«l C^ LOA0 K M THE0|||ECT|OH ^ g300WHEUE oc IS fH RADfAHS20®GROUND SLOPE FACTORS9c - §n - N c tan0'100§0so9 r - 9q=[l - ton to]WHERE of. A SHOULD ALSO K MADEFOR OVERAUL SLOFE STABILITY.3. FOR THE EFFECTS OF MWMOMO©O«WSSOIL AND SOIL COMPRESSIBILm AND SCALEEFFECTS REFEREUCE SHOULD BE TO WSfC.4. WHERE THE FOUNDATION S SET BACK MMTHE CREST OF THE SLOPE , REFER TOSECTION S.i»* *0*4S1 © 1 5 2 ® 2 5 3 0 3 S 4 0 4 S S QAN0LE Of ^CARtlG RESISTAtCE 0'( degrees )BEARING CAPACITY FACTORSBEARING CAPACITY DATA (VESIC, 1975) FIGURE 23


-137 -STRUTSCX25H0.75 HWHEREk - iKQ- I~^cQSBi^tH-\SANDSN.C. CLAYS0.25H0-5 HRANKINE COEFFICIENT OF ACTIVEEARTH PRESSURED.25H1THO.C. CLAYSNOTE: WATER AND SURCHARGE LOADINGSSHOULD BE CONSIDERED.la) after Peck, 1969"^^---^___hKSANDO.AH0-2-0.3H-hHARD CLAY I N>4 )0.4 H0.2^0.4SOFT CLAY ( N«4)0-4 H0.4-0.5KtHK = COEFFICIENT OF EARTH PRESSUREN = STANDARD PENETRATION TEST VALUE(b) after Japan Society of Civil Engineers, 1977PRESSURE ENVELOPES FOR INTERNALLYBRACED EXCAVATIONFIGURE 24


-139-c = AVERAGE UNDRAINED SHEARSTRENGTH OF THE SOIL FROM BASELEVEL TO A DEPTH OF 0.25H BELOWTHE BASENb= STABILITY FACTORL= EXCAVATION LENGTHB10I I>=3CDSTABILITY FACTOR FOR VARIOUSGEOMETRIES OF CUT(After Janbu et alJ956;FACTOR OF SAFETY WITH RESPECT TO BASE HEAVE | FIGURE 25


.NOTES1. Crib wall units to be Infilled with freedraining material, wel1 compacted Inlayers. Care should be taken to avoiddisturbing the units.headersstretchers2. Design criteria for gravity walls applyto crib walls. Wall section resistingoverturning is taken as a rectangle ofd Intension (H x b) .3. Low walls (under 1.5m high) may be madewith a plumb face. Higher walls shouldbe battered as shown.A. For high walls (km high and over) thebatter is Increased or supplementarycribs are added at the back.cast in-srtu reinforcedconcrete base slab( a) TYPICAL SECTION( diagrammatic )long closerfalse headerheader( b) TYPICAL FORM OF CRIB WALLING


- 143 -TRIPLEWALLSINGLEWALLDOUBLEWALLASSUMPTIONS: Soil properties : *' c = 0, y=19.5kN/m 3Wail properties : S=-|-0, Ww = 15.5kK/m 3Wall slope : & =-14* ( 1 in 4 )Water-table below base of wall.Live load surcharge equal to 0.6m ofsoil included.Fs (sliding) = 1.5 min.Fs (overturning) = 2.0 min.5 10BACKFILL SLOPE a) (degrees)CRIBWALL DESIGN CURVES FIGURE 27


- 1/6-Distance from Excav.Max. Depth of Excav.0.4 0-8 1.21.6 2.0Distance from Excav.Max. Depth of Excav .1.0 2.0 3.0 4,0ZONE FOR MEDIUM TODENSE SAND WITHINTERBEDDED STIFFCLAY, AVERAGE TOGOOD WORKMANSHIPDistance from Edge of Excav.Depth of Excav.(a) SETTLEMENT DATA[After Peck, 1969]Zone iSand and soft to hard clay averageworkmanshipb) SETTLEMENT DATANoteL after Q' Rourke et al, 19761May be used for approximateguidance only for residual soils.r -,simplified/ "^settlementprofi lelateral ^^^movementsbase heave — .I / ^\ Struts '' i \ ' i^ —-—-\I \n i f"7^---L• ///**$•/// i**Zone I !b)Vory soft to soft clay1 ) Limited depth of clay belowbottom of excavation.2^Zone 1 I I( c ] GENERAL TRENDSt Exaggerated^^flexibl** side supSignificant depth of claybelow bottom of excavationbut N b < 5.14.Settlements affected byconstruction difficulties .Very soft to soft clay to asignificant depth below bottom ofexcavation and with N| D > 5,14 ,o HwhereLARGE EXCAVATIONS-SETTLEMENT GUIDE FIGURE 28


- 147-StationBuilding Effe^t?^ Depth SettlementNotation ^oundatlon Factor During WaiDepth (d) d/D InstallationmmmSettlement SettlementDue To DuringDewatering Excavationmmmm-r , LateralT ° talWallSettlement * Val1 fMovementmm mmChater(CHA)Prewar IPrewar 1 11960 J1960 I !1960 ! H1960 !V1960 V1970 144810101010150. 160.160. 310. 390. 390. 390. 390. 596336462835154029463220614922153737166209131514610582406933755958_.-____-Waterloo(WAT)1960 }1960 ! 11960 1 1!1970 15133120. 200. 520. 50*0. 48300064061543352113103113623___-Argyle(ARC)1950 11950 1 11950 1 1 I1950 IV1960 11970 I412131016170. 150. 460. SO0. 380. 610.65_____10856365035454418+44113604068398_43_31329;PrinceEdward(PREJPrewar I1960 11960 i i4890. 170. 340. 38__-42311153b1154537109-WongTai Sin(WTS)1950 11950 1 11950 1 II1950 IV988100. 470. 420. 420. 52145101423935392121505856604018/28---* High rock at this location D - b metres+ Also influenced by adjacent sheet pile excavation150 -140 -130 -120 -building withshallow foundation vstationfloorslabs?bu'I'ding onpiled foundation^ 110 -E~ 100 -c0)E 90 -I 80 -ocf 60 -033 50 -\'X \Xf \station walls *Rock!!!!!!!!!!!IIHllliilliimtiiill.. TYPICAL SECTION THROUGH STATION(constructed * top down')40 -30 -20 -10 -„\ X\® CHAX WATA ARGA PRE+ WTSI i B I » '•.1 .2 .3 .4 .5 .6 .Depth factor.8 .9 1.0RELATIONSHIP BETWEEN TOTAL BUILDING SETTLEMENTAND DEPTH FACTORmm nmn CCTTI CUCMT DATABUILDING SETTLEMENT DATACONSTRUCTIONHONG KONG(after Morton, Cater £ Unney, 19&Q)


-149-TOEEFFECT ON HEELThe toe support moment produces aloading on the heel. If it isassumed that no moment is transmittedinto the stem, an equivalentparabolic heel loading is as shownbelow, with the maximum ordinategiven byP t = 2.4where My is the toe support moment.WEIGHT OF BACKFILL ABOVE HEEL! | 1 II ISELF WEIGHT OF HEELLOADING FROM TOE MOMENTqmgxOrrtnASSUMED FOUNDATION BEARINGPRESSURESRESULTANT LOADING ON HEEL+ VE ( MAY BE FULLY POSITIVE)NOTE : PRESSURE DIAGRAMS NOT TO SCALEDESI6N LOADING ON HEEL SLAB FIGURE 30


- 151-OR VCOR 'U'SECTIONB-BCOUNTERFORT{DIAGRAMMATIC)WALL/-COUNTERFORTSTEMTYPE 'B'— CRITICAL SECTION FORSHEAR& FLEXURE-TYPE 'B' BARSSECTIONA-ANOTES1. Type A bars, or 'hangers 1 , must be designed to take the full reaction from the wallspanning between the counterforts.2. Type B bars provide additional mechanical anchorage for the hanger bars.3- Note that the critical section for shear is at the face of the counterfort not at adistance d equal to the effective depth from the face.k. For clarity,only limited steel is shown on the sketches.COUNTERFORT WALLS - DETAILING AT JUNCTIOM OF COUNTERFORT • riminc O1WITH HEEL 4 STEM I FlOUHt Ol


-153 -MMa) UNSATISFACTORY DETAIL b) UNSATISFACTORY DETAILM05 ACRITICAL SECTIONFOR SHEAR )N TOEdWHICHEVER IS THEGREATERCRITICAL SECTIONFOR SHEAR IN HEELc) RECOMMENDED DETAIL FOR L t T &Lt GREAT ENOUGH TO PROVIDEANCHORAGE LENGTHNOTES1. Refer to Sections 11.8 & 11.9 fordiscussion, including limitations onsteel percentage.2. For clarity, not all steel is shown inthese sketches. Additional steel fortoe moment M3 is shown dotted. Noshrinkage, temperature or distributionsteel is shown.3. If desired, a fillet may be included.e) RECOMMENDED DETAIL FORLARGE JOINTS (Asi> 0.5%)DETAILING OF CANTILEVER WALL REINFORCEMENT | FIGURE 32


XDlbTBSIDX O 1 6 9 3810* 24 ' 164 1318698Office!"*'Geotechni cal ControDate Due 1 1318698Binding;_ ( i i /i '•^2 TEE Bl SfMOTFOR LOA^TOC

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