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MINERALOGICAL ANDFABRIC CHARACTERIZATIONAND CLASSIFICATIONOF WEATHERED VOLCANICROCKS IN HONG KONGGEO REPORT No. 66T.Y. IrfanGEOTECHNICAL ENGINEERING OFFICECIVIL ENGINEERING DEPARTMENTTHE GOVERNMENT OF THE HONG KONGSPECIAL ADMINISTRATIVE REGION


MINERALOGICAL ANDFABRIC CHARACTERIZATIONAND CLASSIFICATIONOF WEATHERED VOLCANICROCKS IN HONG KONGGEO REPORT No. 66T.Y. ManThis report was originally produced in February 1997as GEO Spedal Project Report No. SPR 1/97


© The Government of the Hong Kong Special Administrative RegionFirst published, July 1998Prepared by :Geotechnical Engineering Office,Civil Engineering Department,Civil Engineering Building,101 Princess Margaret Road,Homantin, Kowloon,Hong Kong.This publication is available from :Government Publications Centre,Ground Floor, Low Block,Queensway Government Offices,66 Queensway,Hong Kong.Overseas orders should be placed with :Publications Sales Office,Information Services Department,28th Floor, Siu On Centre,188 Lockhart Road, Wan Chai,Hong Kong.Price in Hong Kong : HK$106Price overseas : US$17 (including surface postage)An additional bank charge of HK$50 or US$6.50 is required per cheque made in currenciesother than Hong Kong dollars.Cheques, bank drafts or money orders must be made payable toThe Government of the Hong Kong Special Administrative Region


- 3 -PREFACEIn keeping with our policy of releasing information,we make available some of our internal reports in a series ofpublications termed the GEO Report series. The reports inthis series, of which this is one, are selected from a widerange of reports produced by the staff of the Office and ourconsultants. A charge is made to cover the cost of printing.The Geotechnical Engineering Office also publishesguidance documents and presents the results of researchwork of general interest in GEO Publications. Thesepublications and the GEO Reports may be obtained from theGovernment's Information Services Department.Information on how to purchase these publications is givenon the last page of this report.R.K.S. ChanPrincipal Government Geotechnical EngineerJuly 1998


FOREWORDAs part of its research and development programme, theGeotechnical Engineering Office has been looking into ways ofdetermining and characterising the mineralogical and fabricproperties of weathered rocks, in particular the soil grades, with theobjective of understanding their behaviour in the laboratory and inthe field.The results of the research on weathered granitic rocks havealready been published in a GEO Report by Irfan (1996a). Thisdocument summarizes the study results on the mineralogical andfabric properties of the weathered volcanic rocks, in particular, thetuffs.The results of the studies on weathered granitic and volcanicrocks have already been used by the GEO in drafting local standardsfor soil classification and index tests based on BS1377:1990 version(see Chen, 1993). The results of the study on the rock grades arebeing incorporated into local standards for rock tests being preparedby the GEO.The report has been written by Dr T. Y. Irfan who alsoinitiated the study, conducted the research and carried out most ofthe in-house mineralogical and fabric analyses, and coordinated andarranged for testing at overseas institutions and laboratories. Thechemical and X-ray diffraction analyses were carried out by theBritish Geological Survey and at three commercial laboratories inAustralia.Dr S.D.G. Campbell and Dr R. J. Sewell reviewed the draftdocument.( J. B. Massey )Government Geotechnical Engineer/Development


- 5 -ABSTRACTAs part of its research and development programme, theGeotechnical Engineering Office has been looking into ways ofdetermining and characterising the mineralogical and fabricproperties of weathered rocks, in particular the soil grades, with theobjective of understanding their behaviour in the laboratory and inthe field.The results of the research on weathered granitic rocks havealready been described by Irfan (1996a). This document summarizesthe study results on the weathered volcanic rocks, in particular, thetuffs. It documents the typical chemical, mineralogical and fabricchanges that occur in these rocks due to weathering and otheralteration effects.Based on the results of the study, a model for weathering ofvolcanic rocks is established. The effects of hydrothermal alterationwhich are a major influence in profile development and soilformation in Hong Kong are also considered. Modification, similarto those recently proposed for granites (Irfan 1996a), are proposedto the material and mass weathering schemes commonly adopted inHong Kong for the characterization of volcanic works in engineeringuses.The results of the studies on weathered granitic and volcanicrocks have already been used by the GEO in drafting local standardsfor soil classification and index tests.


- 6 -CONTENTSPageNo.Title Page 1PREFACE 3FOREWORD 4ABSTRACT 5CONTENTS 61. INTRODUCTION 92. GEOLOGICAL SETTING OF VOLCANIC ROCKS IN HONG KONG 92.1 General 92.2 Late-Stage Alteration and Metamorphism 102.3 Structure 103. WEATHERING PROFILE DEVELOPMENT AND ENGINEERING 10CLASSIFICATION OF WEATHERED VOLCANIC ROCKS3.1 Weathering Profile in Volcanic Rocks 103.2 Weathering Grade Classification 113.2.1 Classification of the Weathered Rock Mass 113.2.2 Classification of the Weathered Rock Material 124. METHODS OF CHARACTERIZATION OF WEATHERED AND 12ALTERED ROCKS AND THE WEATHERING INDICES5. PREVIOUS WORK ON HONG KONG WEATHERED VOLCANIC 13ROCKS6. METHODS OF THE PRESENT STUDY 137. DESCRIPTION OF SAMPLING LOCATIONS AND WEATHERING 14CHARACTERISTICS7.1 Fine Ash Tuff, Ap Tsai Wan, Tseung Kwan O 147.2 Fine Ash Tuff, Tseung Kwan O 147.3 Coarse Ash Tuff, Sai Kung 15


- 7 -PageNo.8.7.4 Coarse Ash Tuff, Aberdeen7.5 Coarse Ash Tuff, Yuen LongDESCRIPTION OF MATERIAL WEATHERING GRADES IN TUFFS8.1 General8.2 Fresh Rock8.3 Slightly Decomposed8.4 Moderately Decomposed8.5 Highly Decomposed8.6 Completely Decomposed8.7 Transition Soil8.8 Residual Soil15161616161717171718189. MINERALOGY AND FABRIC CHARACTERIZATION OF 18WEATHERED TUFFS9.1 General 189.2 Fresh Rock 209.3 Slightly Decomposed 209.4 Moderately Decomposed 229.5 Highly Decomposed 239.6 Completely Decomposed 249.7 Transition and Residual Soil 269.8 Iron-oxide Minerals in Volcanic Saprolites and Residual Soils 279.9 Kaolin Veins 2810. CLASSMCATION OF WEATHERED VOLCANIC ROCKS AND 28SOILS IN HONG KONG10.1 Classification of Material Weathering Grades of Volcanic Rocks 2810.2 Classification of Volcanic Saprolitic and Residual Soils 2811. QUANTITATIVE WEATHERING INDICES 2911.1 General 2911.2 Chemical Indices 29


- 8 -PageNo.11.3 Degree of Decomposition Index 3011.4 Micropetrographic Indices 3012. CONCLUSIONS 3013. REFERENCES 32LIST OF TABLES 35LIST OF FIGURES 64LIST OF PLATES 84APPENDIX A : SAMPLE DESCRIPTIONS 97APPENDIX B : METHODS OF PETROGRAPHICAL AND CHEMICAL 106ANALYSIS AND IDENTIFICATION OF MINERALS


- 9 -1. INTRODUCTIONAs part of its research and development programme, the Geotechnical EngineeringOffice has been looking into ways of determining and characterizing the mineralogical andfabric properties of weathered rocks, in particular the soil grades, with the objective ofunderstanding their behaviour in the laboratory and in the field.A number of chemical, petrographical and other methods have been used to documentand quantify the mineralogy and fabric properties of weathered igneous rocks, in particularthe granites, and volcanic rocks at each grade of weathering. These included chemicalanalyses, X-ray diffraction (XRD) analysis, optical microscopy, scanning and electronmicroscopy (SEM, TEM) and differential scanning calorimetry.The results of the study on the weathered granitic rocks have been described by Irfan(1996a, b). This document presents the results of the study on the weathered volcanic rocks.It documents the typical chemical, mineralogical and fabric changes that occur in volcanicrocks due to weathering and other alteration effects, in particularly the tuffs which arewidespread in Hong Kong.The index and engineering properties of weathered volcanic rocks are not discussed here.A review of the mineralogy and structure of saprolitic and residual soils of volcanic andgranitic rock origin is given in Irfan (1996b, c), who also discusses the effects of specificmineralogy and structure developed in these materials on the classification and indexproperties and behaviour.2* GEOLOGICAL SETTING OF VOLCANIC ROCKS IN HONG KONG2.1 GeneralGranites, granodiorites and the associated volcanic rock suite of Mesozoic age underliea greater part of the Territory of Hong Kong (Figure 1). The volcanic rocks of Jurassic toCretaceous age, several thousand metres thick, have been intruded by a number of graniticplutons of slightly younger age.The volcanic rocks, recently grouped by the Hong Kong Geological Survey under theyounger Repulse Bay and the older Tsuen Wan volcanic groups, comprise a series oflithostratigraphic formations, each with its own distinctive geological characteristics (see forexample, Langford et al. 1995). The volcanic rocks consist mainly of tuffs of pyroclasticorigin derived from ash fall and ash flow deposits with some lavas and intercalatedsedimentary horizons. The rocks are generally rhyolitic (acidic) in composition with someandesitic rocks occurring locally. Macroscopic features such as flow-banding, flow-folding,auto-brecciation and columnar jointing are present in the lavas as well as in the tuffs. Therecent classification of pyroclastic rocks has been in terms of grain size and physicalcomponents (Figure 2). Fine ash vitric tuffs and coarse ash crystal tuffs are the dominantrock types occurring in most formations. Other lithologies including tuff breccia, lapilli tuff,andesitic lavas and tuffites (tuffaceous sedimentary rocks), also occur as mappable units.More detailed information on the mineralogy and textural features of the common volcanicrock types is given in Section 9 and in the Geological Survey Memoirs published by GEO.


- 10 -2.2 Late-Stage Alteration and MetamorphismVolcanic rocks show local evidence of contact metamorphism close to granitic plutons.The effect of metamorphism is most intense within a few metres of the contacts, resulting indestruction and recrystallization of the original tuff texture and formation of hornblende-topyroxene-hornfels. However, in most cases, the volcanic rocks near granite contacts arehardly affected. In the Lo Wu-Tuen Mun area, the regional low to medium-grade metaorphismassociated with overthrusting of Palaeozoic basement onto Jurassic volcanic rocksalso affected the volcanic rocks resulting in mylonitization and formation of a schistosity inthe rocks (Langford et al, 1989). Volcanic rocks also show effects of metasomatic andthermal alteration (hydrothermal alteration) particularly in the vicinity of granitic plutons.2.3 StructureThe granites and volcanic rocks are affected by faulting dating back to the Mcsozoic,the faults being rejuvenated during each subsequent orogeny. The most dominant andpersistent fault directions in the Territory are ENE-WSW and the most regionally extensivestructures trend NE-SW and NW-SE.Some large-scale gentle and open folding has been recognised in volcanic rocksoverlying granitic plutons (Strange et al, 1990). These are at least in part related to the riseof the underlying granitic plutons into a relatively competent volcanic cover. Two majorcollapsed caldera-type structures have been postulated in the Sai Kung area (Strange et al,1990), and a further caldera on Lantau (Langford et al, 1995), which resulted in roughlyhorizontal or gently dipping volcanic strata at the centres.Some of the joint sets in the volcanic rocks are clearly related to those formed ingranites. Additional joints sets are also developed in volcanic rocks related to folding andfaulting due to the intrusion of granites, metamorphism, and the cooling and erosional history.Strong foliation or schistosity is also present in some of the volcanic rocks, in particular inthe Northwest New Territories, associated with overthrusting. The stress-release related gentlydipping joints are generally poorly developed or non-existent in volcanic rocks. The jointspacing is usually much closer than in the granitic rocks, except perhaps in some crystal tuffs,being generally between 0.1 m to 0.6 m. However, the mean joint spacing and also thespacing of individual sets may vary from one locality to another, being closer in the vicinityof faults and fault zones.3- WEATHERING PROFILEDEVELOPMENT AND ENGINEERINGCLASSIFICATIONOF WEATHERED VOLCANIC ROCKS3.1 Weathering Profile in Volcanic RocksThe majority of studies on rock weathering and profile descriptions have been confinedto the granitoid rocks in Hong Kong. The volcanic rocks have received less attention andonly brief references have been made to profile development in these rocks (Powell & Irian,1987; Irfan et al, 1987; GCO, 1988; Irfan, 1994). Recently, Irfan (1996a, b) reviewed thecommon weathering processes responsible for deep weathering profiles which have developedover the igneous and volcanic rocks in Hong Kong.


- 11 -In general, soil zones of the weathering profile are comparatively shallow in volcanicrocks, usually less than 15 m thick, in contrast to the deep profiles that occur over thegranitoid rocks. However, thick soil profiles of up to 35 m have also developed, particularlyin the coarser grained volcanic rocks (Figure 3). Deeper soil profiles have been recorded, butthese are considered to be confined to faults and closely fractured zones. Corestonedevelopment, which is characteristic of the medium- and coarse-grained granites, is notcommon in weathering profiles developed over volcanic rocks, except in coarse ash tuffs andlapilli tuffs (Plates 1 and 2).Cataclasis and shearing of the rocks by tectonic processes and granite intrusions and theassociated hydrothermal alteration are a major influence in profile development and soilformation at many locations in Hong Kong. Hydrothermal alteration generally results inweakening of rocks and deeper penetration of weathering (Irfan, 1996a; Irfan & Woods,1997). Some volcanic rocks are more resistant to weathering (e.g. lavas) than others, resultingin well-defined escarpments where resistant rocks cap and protect the softer more easilyweathered tuffs. For example, in the western New Territories, the slightly metamorphosedtuffs form thin weathering profiles with corestones, but strongly metamorphosed tuffsgenerally form resistant ridges (Langford et al, 1990).The general weathering profile in lithologically uniform volcanic rocks is similar to thatof granitoid rocks, and consists of four major layers grading from residual soil at the top tofresh rock at depth. In many localities in Hong Kong, the transitional rock and soil zone isusually relatively thin, except in coarse-grained volcanic rocks which have a similar profiledevelopment to medium to coarse-grained granites (Plate 1). In bedded sequences of varyinglithology, the rock mass may consist of relatively unweathered bands or beds alternating withdecomposed bands or beds. In some cases, the soil layer may be underlain by the fresh rock,with intermediate layers not present (Plate 3).The residual soil layer over the volcanic rocks is not very well developed, as is the casewith granites also. It is generally underlain by the transitional soil layer containing relict rockfabric but no structures. This zone is up to 3 m thick at many locations even on gently tomoderately sloping ground.3.2 Weathering Grade Classification3.2.1 Classification of Weathered Rock MassAs the weathering profiles developed over the volcanic rocks are similar to those of thegranitic rocks, the same weathering grade classification scheme is generally applicable to bothrock types. The six-fold mass grading scheme recommended by BS 5930 (BSI, 1981) hasalso found favour for classifying various components of the weathering profile developed overvolcanic rocks (Powell & Irfan, 1987; GCO, 1990; Irfan, 1994) (see Table 1). The zonalscheme proposed by GCO (1988) is particularly suitable for corestone-forming, generallycoarse-grained volcanic rocks (e.g. coarse ash tuffs). However, it is less well suited forfine-grained rocks which show gradual loss of strength with weathering without forming anysignificant soil material.The schemes proposed by GCO (1988) and BSI (1981) and the most recent classificationschemes proposed by the Geological Society of London (1995) fail to cater adequately in


- 12 -characterizing soil zones of the weathering profile for engineering purposes, A classificationscheme for soils formed from weathered rocks in tropical areas has been proposed by theGeological Society of London (1990). However, this scheme is complex and uses many termsunfamiliar to geotechnical engineers. Also, it does not account for the structuralcharacteristics of saprolitic and residual soil layers, that are incorporated in the simplerclassification scheme proposed by Wesley & Irfan (1996). A brief description of this schemetogether with a critique of various mass weathering classification schemes applicable to rocksin Hong Kong is given in Irfan (1996b).3.2.2 Classification of Weathered Rock MaterialTraditionally, a six-fold decomposition grade scheme has been used in Hong Kong toclassify the weathered state of rock material (GCO, 1988). This scheme is generallyapplicable to most volcanic rocks. Various authors have suggested general characteristicsbased on observation or simple tests for assessment of decomposition grades, in particular ingranitic and volcanic rocks (Hencher & Martin, 1982; GCO, 1988; Irfan, 1988, 1996b).The most accurate method of describing the degree of decomposition, or weathering ingeneral, is to use a quantitative index. Assessment of decomposition grade should besupplemented by description of the state of disintegration of the rock material as disintegrationis also an intrinsic component of weathering. The terms used for description ofdecomposition grades of rock material may be used for hydrothermally altered rocks also asdecomposition is a general term used for chemical changes.Irfan (1996b) subdivided the 'completely decomposed' granite grade into; (i) completelydecomposed, (ii) (completely) disintegrated, and (iii) (completely) altered, based on thedominant type of weathering and alteration process. He also proposed the term 'transitionsoil* for material that shows partial loss of fabric before it transforms into true residual soil.These subgrades also appear to apply to volcanic rocks. Further division of each subgradecan be made, as necessary, based on relative strength (density or consistency) of the soilmaterial, which is a function of its degree of weathering and alteration.4- METHODS OF CHARACTERIZATION OF WEATHERED AND ALTERED ROCKSAND THE WEATHERING INDICESThere have been several attempts to quantify the degree of weathering and the weatheredrock fabrics using chemical, petrographical, X-ray diffraction and electron microscopemethods, or by simple field and laboratory mechanical and physical index tests. Although inmost cases, the quantitative and semi-quantitative weathering indices have been derived forquantifying changes in purely geological properties, some quantitative weathering indices havebeen used to relate and indirectly to determine the engineering properties of weathered rocks(e.g. Dearman & Irfan, 1978). Many of the weathering indices have been specificallydeveloped for characterizing coarse-grained igneous rocks, and particularly granites. Somequantitative weathering indices have also been applied to fine-grained igneous rocks includingvolcanic rocks (Weinert, 1964; Dearman et al, 1987).


- 13 -Irfan (1996a, b) reviewed the various weathering indices proposed to date and theirsuitability for granitic rocks in Hong Kong. The suitability of selective weathering indicesfor volcanic rocks is discussed in Section 10.5. PREVIOUS WORK ON HONG KONG WEATHERED VOLCANIC ROCKSLittle published data exist on the clay mineralogy and fabric properties of weatheredvolcanic rocks in Hong Kong, and these are confined to only a few samples collected froma limited number of rock types (Brock, 1943; Parham, 1969; Lumb & Lee, 1975). There isalso little mineralogical and chemical data from other countries on the weathering aspects ofolder volcanic rocks. In order to understand the laboratory and field behaviour of weatheredrocks, in particular the soil grades, the basic mineralogical and fabric properties of these rocksand soils need to be characterized. However, characterising these properties for engineeringpurposes is not easy because of their great variability and inhomogeneity, particularly in thecase of volcanic rocks, as a result of their geological and weathering history.The earlier studies indicated that the most abundant clay minerals in the near-surfacesoils of volcanic rock origin are the kaolin group of minerals. Both kaolinite and halloysiteare formed mainly from the decomposition of feldspars, which are abundant in these rocks.Halloysite is more common in saprolitic soils, co-existing with a minor quantity of kaolinite.This is to be expected as these soils have been formed on well-drained sideslopes of themountainous terrain in Hong Kong under a prolonged subtropical climate. Halloysite andkaolinite may give way to gibbsite under advanced weathering conditions close to the top ofthe weathering profile, particularly in residual soils (Lumb & Lee, 1975; Irfan, 1996b). Asmall amount of the swelling mineral vermiculite may also be present (Parham, 1969).Allophane has been identified in volcanic rocks, at least in the initial stages of decompositionof feldspars, but it then rapidly changes to halloysite with further decomposition (Lumb &Lee, 1975). Recently, Irfan (1994) published the results of chemical, mineralogical and fabricanalyses of coarse ash tuffs from a landslide site in Hong Kong. The results have beenincorporated into this document.6. METHODS OF THE PRESENT STUDYThe study undertaken on the volcanic rocks was less comprehensive than that in asimilar study of granitic rocks (Irfan, 1996a). The principal aim of the study is to establishthe basic mineralogy and fabric properties of selected weathered rock types, mainly the tuffs.The following methods were used to determine the mineralogy and microfabric of the volcanicrocks:(a)(b)Detailed description of hand specimens.Optical microscopy on thin sections to determine and quantifythe mineralogy and fabric properties (e.g. grain boundaryrelationships, voids, weathering state of the mineralconstituents and their arrangement).


- 14 -(c) Chemical analyses, to determine chemical and normmineralogical composition by X-ray fluorescence (XRF),inductively coupled plasma atomic emission spectrometrv(ICPAES) or atomic absorption spectroscopy (AAS). Loss onignition (LOI) and moisture determinations were also made.(d)X-ray diffraction (XRD) analysis, on bulk samples and clayfractions.Brief description of the methods of analysis used by the four laboratories which carriedout the analyses under items (c) and (d) is given in Appendix B. The comments by these fourlaboratories and the author on the accuracy of identification of various minerals from thetechniques used are also given in Appendix B.7. DESCRIPTION OF SAMPLING LOCATIONS AND WHM1I11IMCHARACTERISTICS7.1 Fine Ash Tuff, Ap Tsai Wan, Tsueng Kwan 0The sampling site is situated on a ridge at Ap Tsai Wan headland, north of the town ofTseung Kwan O (Figure 4). The headland is composed of fine ash tuff, eutaxite and tuffbreccia of the Ap Lei Chau Formation. The site has recently been formed into a series of cutslopes.The saprolitic layer is about 15 m thick on the site but the residual soil layer is missing,at least along the ridge line. There is a regular layering of the mass weathering grades at thetruncated headland with the core of the hill composed of fresh to slightly weathered fine ashtuff and eutaxite. The regular layering is disturbed by fault zones where weatheringpenetrates to greater depth (Plate 4).Hand and block samples were collected from the different material grades at the sitewith the more weathered specimens obtained from higher levels in the cut slopes. Briefdescriptions of hand samples are given in Appendix A.7.2 Fine Ash Tuff, Tseung Kwan 0The sampling site is situated at an elevation of about 130 mPD on the mid-slopes of TaiSheung Tok (elevation 400 m), west of Tseung Kwan 0 (Plate 5). Volcanic soil from thissite was used to assess the effect of matric suction on direct shear strength of Hon« Kongsoils by Gan & Fredlund (1992).The site is underlain by fine ash vitric tuff of the Ap Lei Chau formation. Theweathering profile is variable and relatively thin at the site (Figure 5). Only the upper portionof the weathering profile is exposed along the 5 m high road cutting where the samples weretaken. A layer of mottled brown and yellowish brown residual soil is present in some placesunderneath a thin colluvium layer. The transition from residual soil to saprolitic soil isirregular but generally occurs within a metre from the ground surface. The presence of whitekaolin veins along certain discontinuities within the rock may indicate hydrothermal alteration.


Corestones are present but poorly developed.- 15 -Three block samples were used for the direct shear strength research and mineralogicaland microfabric studies were conducted on hand specimens representing different weatheringgrades. Brief descriptions of the samples are given in Appendix A.7.3 Coarse Ash Tuff, Sai KungThe sampling site is situated in a disused borrow area on top of a small isolated hill(elevation of 60 mPD), on a small peninsula in Sai Kung (Figure 1). The site is underlainby coarse ash crystal tuff, originally mapped as belonging to the Tai Mo Shan Formation byStrange et al (1990), now referred to as Sai Kung Formation.The thickness of saprolitic soil at the site is not known. However, it is considered tobe over 10 m thick, based on the coastal exposures of weathered rock. The residual soil layeris thin, less than 1 m, and irregularly developed. The rock shows typical spheroidal type ofweathering commonly associated with the coarse-grained tuffs in the Territory (Plate 6).Numerous corestones, usually less than 0.5 m in diameter, are present in the saprolitic soil andoccasionally in the residual soil.A limited number of specimens were collected from the saprolitic and residual soilzones. Fresh to moderately decomposed samples were obtained from the corestones in thesaprolitic soil. Brief descriptions of the samples are given in Appendix A.7.4 Coarse Ash Tuff, AberdeenThe sampling site was originally a minor ridge crest on the southern footslopes of anortheast-southwest trending major ridge in Aberdeen. Borrowing activity in the 1920sresulted in removal of the colluvium cover and top few metres of the saprolitic-residual soillayer and the formation of two dish-shaped areas.A shear plane, formed at a depth of about 4 m in the saprolitic soil as a result ofcontinuous creep, acted as the release surface for a landslip in 1988. A petrographical studyof the weathered rocks in the slope was undertaken to determine the causes of the creepingbehaviour (Irfan, 1992, 1994).The coarse ash tuff is deeply weathered on the site, with the saprolitic soil zone 15-20 mthick (Figure 6). Numerous white clayey veins criss-cross the saprolite. The layer above thepre-existing shear plane consisted of soft mottled volcanic soil with a deformed tuff fabric.A thin transition soil layer was observed above the saprolitic soil.Samples for the mineralogical study were collected from various soil materials on thesite as well as the less weathered rock grades recovered as cores from the boreholes (Plate 7).Brief descriptions of the samples are given in Appendix A.


- 16 -7.5 Coarse Ash Tuff, Yuen LongThe sampling site is a newly formed cut slope adjacent to a haul road in I*ai long Eastborrow area, south of Yuen Long (Figure 1). The slope, 30 to 50 ni high with a gradient ofabout 30°, was formed in early 1991 by cutting into the natural hillside. The slope wassubject to instability soon after cutting in June 1991. The instability is confined to a thinlayer of volcanic rocks which overlie a fine-grained granite.The volcanic rock at the site, mapped as undifferentiated tuff and tuffite on the newgeological map, is a lapilli-bearing coarse ash crystal tuff of the Shing Mun Formation. Therock is rich in quartz crystals, up to 5 mm in diameter, and mica (up to 10%) but poor infeldspar crystals. It is probably contact-metamorphosed by the granite.The tuff above the granite contact is completely to highly weathered. The granite siiowsless intense weathering, at least where it is exposed along the cutting. Occasional corestonesare present in the saprolite but these are not well developed unlike other coarse ash tuffs inthe Territory. The topmost layer of the saprolitic zone shows a highly deformed fabric andstructure where it has been affected by recent slope movements.Only five samples were collected from the site, with two each from the highlydecomposed and completely decomposed tuff and one A fresh' sample from inside a corestone.Details of the samples are given in Appendix A.8. DESCRIPTION OF MATERIAL WEATHERING GRADES INjnJFFS8.1 GeneralThe material weathering grade scheme adopted in this document follows Irian (1996a).However, subdivisions based on relative strength values have not been attempted for thesevolcanic rocks due to lack of laboratory and insitu data. Plates 8 to 10 illustrate the variousmaterial grades in fine ash and coarse ash tuffs.8.2 Fresh RockFresh Fine Ash Tuffs are usually medium grey to dark grey but also light grey or black,and extremely strong. These rocks generally consist of a small amount of euhedral tosubhedral broken crystals of feldspars or quartz or both and occasional vitric fragments in avery fine vitric (glassy) matrix. If fiamme are present, then the rock is called 4 eutaxite\ Thematrix may be finely recrystallized, particularly if the rock is metamorphosed.Fresh Coarse Ash Tuffs are usually bluish grey to dark grey and very strong toextremely strong. They generally comprise abundant crystals of feldspars (up to 3-4 mm insize) with some quartz and biotite and occasional lithic fragments in a fine-grained vitric torecrystallized matrix. Welding fabric may be present in the matrix.


- 17 -8.3 Slightly DecomposedSubdivisions can be made within this grade for fine and coarse ash tuffs (and forvolcanic rocks in general) based on the degree of discoloration (Plates 8 to 10), similar to thesubgrades suggested for granitic rocks. Discoloration is usually an indication of weathering.Slightly Discoloured: The rock material is discoloured less than 10% by volume fromjoint and fracture surfaces inwards to yellowish grey to yellowish brown. The interior of thejoint-bounded block retains its original grey to dark grey colour.Moderately Discoloured: The rock material is discoloured 10% to 50% by volume toyellowish grey to light grey with dark brown in the vicinity of fractures and joints. Thefresher centre may show very light grey discoloration.Highly Discoloured: The rock material is discoloured more than 50% by volume.Colour banding may be present with medium grey fresher centres surrounded, in turn, by lightyellowish grey, yellowish grey and dark brown rims. The same concentric colour bandingmay also be present in corestones which are commonly present in the saprolitic zone in coarseash tuffs, with the outermost layers being highly to completely decomposed. In thecorestones, feldspars become progressively softer (more decomposed) towards the exterior.There is also development of concentric microfractures in the discoloured rim in both coarseash and fine ash tuffs, which increases in intensity towards the exterior of a corestone or ajoint-bounded block in this subgrade.8.4 Moderately DecomposedThe rock material is completely discoloured, light yellowish to yellowish greenish grey.The joint surfaces may be stained reddish brown to dark brown. The rock is very strongwhen least weathered to moderately weak when most weathered within this grade. Somefeldspar crystals are gritty, particularly in the most weathered tuff specimens. Near jointsurfaces they may be gritty to soft. Biotite, if present, is dull greenish grey and soft. Therock fabric is fractured throughout by a network of tight but iron-oxide stained microcracks.8.5 Highly DecomposedThe rock material is completely discoloured, light yellowish grey or greyish yellow withyellowish brown near joints and fractures. The strength is very much reduced due toextensive microcracks. Feldspar crystals are largely gritty, some hard (in the weak rockrange) to powdery and occasionally soft (in the extremely weak rock range).8.6 Completely DecomposedThis grade of weathered tuff could be divided into subgrades similar to those for graniticrocks, based on the type and degree of alteration and weathering and the relative density(strength), but this has not been attempted here.


- 18 -The rock material is discoloured throughout to yellowish grey, yellowish greenish greyor reddish brown, and it can also be mottled. Pinkish grey and white patches can be presentlocally if it is affected by hydrothermal alteration, usually in the vicinity of kaolin orkaolin-quartz veins. Differential weathering may be present between the lithic fragments andthe matrix in lapilli-bearing tuffs. Feldspar crystals are usually powdery to soft with degreeof decomposition being more intense than in the hydrothermally altered areas. Quartz, ifpresent, is dull and biotite is transformed into colourless to silvery grey flakes. Thecompletely decomposed specimens can be easily disaggregated into soil particles by fingerpressure.8.7 Transition SoilTransition soil specimens are usually dark reddish brown, but may also he mottled withpatches of yellowish grey. The soil shows partial loss of the tuff fabric, the proportion ofwhich increases up the weathering profile. Relict discontinuities are largely absent in thetransition soil zone. Occasional partially decomposed feldspar crystals may be present in areasstill retaining the original tuff fabric.8.8 Residual SoilThe specimens are usually yellowish brown, but may also be yellowish grey whenbleached of its iron minerals. The soil comprises some sand-size quartz grains in astructureless clayey silt to silty clay matrix. Nodules or pods of iron-oxide cemented soilaggregations may be locally present. In extreme cases, cementation by iron-oxides andhydroxides may produce a weak rock (i.e. a laterite).9. MINERALOGY AND FABRIC CHARACTERIZATION OF WEATHERED TUFFS9.1 GeneralThe textural and mineralogical properties of volcanic rocks in Hong Kong are veryvariable depending upon the type of volcanic activity and subsequent processes which formedthese rocks, the mode and history of solidification and later metamorphism, hydrothermal andother alteration processes. As a result, wide variations in texture and mineralogy can evenoccur in the same rock type at each location (Table 2).Coarse ash tuffs occur in many formations. In general, they consist of crystals or crystalfragments of feldspar and quartz, up to 6 mm in size, with small biotite and occasionalhornblende and secondary rnuscovite set in a fine-grained matrix of the same minerals. Clastsof sedimentary or igneous rocks of various sizes may also be present. The matrix may bepartly or wholly recrystallized, particularly in the vicinity of granite intrusions. Some crystalalignment and weak to strong welding fabric with elongate recrystallized siliceous shards andfiamme may be present. Crystal tuffs of the Shing Mun Formation are particularly variablein grain size and texture and can show rapid variation both laterally and vertically from coarseash to fine ash to lapilli tuffs. The total crystal content can vary from 20% to over 50%.


- 19 -Feldspar crystals are particularly abundant in Tai Mo Shan and Yim Tin Tsai tuffs.Quartz grains may be rounded to broken and angular, occasionally resorbed with no clearlydefined grain boundaries. Biotite occurs as individual large grains as well as smallaggregations scattered throughout the groundmass. It is usually partially chloritised. Alkalifeldspars may be cloudy and partly replaced by epidote. Plagioclases may be slightlysericitized even in the fresh rock. These mineralogical changes are probably the result of subaerialprocesses during the formation and solidification of the rock as well as the laterhydrothermal activity from intruding granites.Fine ash tuffs including eutaxites are particularly common on Hong Kong Island and inthe vicinity of Junk Bay and Sai Kung. They generally consist of a small amount of scatteredcrystals (up to 20% of the rock), dominantly quartz with some feldspars and occasional lithicfragments, in a very fine vitric (glassy) matrix (Strange & Shaw, 1986). The matrix may befinely recrystallized, particularly in the vicinity of igneous intrusions. In the eutaxites, awelding compaction structure arising out of alignment of flattened fiamme, stretched pumiceand lithic fragments is present. Majority of crystals are usually broken and often resorbedwith in distinct grain boundaries.The crystal-bearing fine ash welded tuffs which occur in the vicinity of High Islanddisplay well-developed columnar jointing (Strange et al, 1990). The quartz and feldsparcrystals comprise up to 20% of the rock in a very fine-grained or vitric groundmass. Quartzcrystals are invariably corroded and resorbed. Feldspars occur as euhedral grains or brokencrystal fragments. The fine ash tuffs of the Silverstrand Formation have prominent eutaxiticfabric. Fiamme is usually abundant comprising up to 30% of the rock. Lenses ofsedimentary rocks and chert fragments may be abundant at some localities.In the following sections, a brief summary is given of the mineralogical and fabricproperties of the volcanic rocks weathered to different degrees, with particular emphasis onthe fine ash and coarse ash tuffs as these were the only rocks studied in detail in this study.The mineralogical compositions of the freshest specimens from the five rock types aregiven in Table 3, with the results of quantitative petrographical determinations shown inTables 4 and 5. The results of chemical analyses, arranged in order of weathering grade, aregiven in Tables 6 to 8. The results of XRD analyses on bulk and clay fractions are presentedin Tables 9 to 14 in terms of the relative abundances of minerals identified from the traces.Figures 7 to 10 show the typical XRD traces for selected specimens. The bulk normmineralogies calculated from the results of chemical analysis using the mineral types identifiedby XRD are given in Tables 15 to 17. Schematic representations of the mineralogical changesoccurring with weathering in each of the five tuffs are given in Figures 11 to 15. Plates 11and 12 are micrographs illustrating the typical mineralogical and fabric properties of fine andcoarse ash tuffs weathered to various degrees.Quantitative petrographic analyses were not carried out for the finer tuffs due to thedifficulty in recognising the minerals and voids, particularly in the weathered specimens.Comments on the identification of minerals from the XRD analysis and on the accuracy ofthe chemical analyses results are contained in Appendix B.


- 20 -9.2 Fresh RockThe fine and coarse ash tuffs included in the study are all rhyolitic in composition. Themajor constituents are feldspars and quartz with some mica occurring as large euhedralcrystals or crystal fragments in a devitrified or recrystallized groundmass of the sameminerals. Occasionally, lithic fragments are also present. The crystal content reaches over50% in the case of coarse ash tuff from Aberdeen, but in the fine ash tuffs it is usually lessthan 25% of the rock (Table 4). Feldspars are the dominant constituents with plagioclasebeing more abundant than alkali feldspar. Overall, quartz contents vary between 25% and45%. However, the quartz crystal contents are usually low, less than a few percent, exceptin the coarse ash tuff from Aberdeen it reaches 15% of the rock. The chemical analysis andoptical microscopy revealed that the specimen ATWl from Ap Tsai Wan has a significantlydifferent chemical and mineralogical composition than the other specimens collected from thesame site (Table 8).Biotite, when present, is mostly partially to completely altered to a mixture of hydrousmica, chlorite, iron oxides and sometimes to vermiculite. The mica content of the coarse ashtuffs from Yuen Long is rather high (about 10%). Small amounts of secondary minerals suchas muscovite, hydrous mica, chlorite, occasionally calcite, and in sonic cases, kaolinite areinvariably present in addition to minor amounts of iron-oxide minerals, mainly magnetite.The majority of secondary products is attributed to probable effects of epigenetic processesduring or subsequent to volcanism and solidification. Later effects of contact metamorphismand hydrothermal alteration by the intruding granitic magmas might also have contributed tothe formation of micaceous minerals in these rocks.In thin section, most plagioclases and some alkali feldspars show slight to moderatealteration to micaceous products (commonly termed sericite) in all five tuffs. Calcite alsooccurs in some crystals and also in the groundmass, possibly as a byproduct of breakdown ofNa-Ca plagioclase to micaceous minerals and kaolinite. Fine-grained micaceous minerals andchlorite also occur in the groundmass. The fresh rock is unfractured except for occasionalshort and tight microcracks in quartz and feldspar crystals.The tuff from Aberdeen has the coarsest grain size, with quartz and feldspar crystals,up to 4 mm in diameter. The fine ash tuff from Ap Tsai Wan has a strong eutaxitic foliation.A faint flow fabric is also present in the tuff from Tseung Kwan 0.9.3 Slightly DecomposedNoticeable decreases occur in Na 4 * and Ca 2+ contents which indicate the commencementof decomposition of Na - Ca feldspars (plagioclases) in the slightly decomposed tuff grade.The behaviour of K + , Si 4+ and Al 3+ are not so predictable and, in some cases, these cationsshow increase or decrease against the expected trend with increased weathering. For example,fC and Si 4+ contents are higher than those of the fresh rock in some of the slightlydecomposed specimens (Table 8). These variations are attributed to variations in the chemicaland mineralogical composition of the original rock within each sampling site including thosearising from hydrothermal and other alteration processes.The carbonate minerals, which may be present in small amounts in fresh tuffs, are


- 21 -largely removed by solution in this grade as indicated by significant reduction in CO 2 andCa 2+ contents, although most of the reduction in the latter is probably due to decompositionof Ca-bearing feldspars. The optical microscopy revealed void formation within somefeldspars and in the groundmass at the locations of calcite, especially in the stained portionsof the specimens.In thin section, a small amount of feldspar decomposition, mainly to micaceous minerals,is present even in the fresher portions of the specimens. The degree of decomposition offeldspars increases in the stained rims, especially in plagioclase megacrysts. There arevariations in the type and intensity of feldspar weathering amongst the tuffs. For example,all feldspar megacrysts show slight to moderate degrees of decomposition in the tuffs fromSai Kung, Tseung Kwan O and Ap Tsai Wan as reflected in their calculated mineralogies(Tables 16 and 17), but the plagioclases are only slightly decomposed and the alkali feldsparsare not generally affected by weathering in the coarse ash tuffs from Aberdeen. Themoderately and highly decomposed specimens show an increase in the calculated alkalifeldspar content (Table 15). This is considered to be mainly due to the higher alkali feldsparcontents of these specimens.In the stained rims, some areas within the partially altered feldspars appear 'cloudy' or'speckled'. These could be poorly crystalline alumina-silicate minerals transitional tokaolinite or halloysite. They might also be the locations of solution pits indicating the onsetof solution weathering (Dearman & Baynes, 1978; Irfan, 1996b). Slight to noticeable solutionof feldspars was observed by Kwong (1988) in slightly to moderately decomposed volcanicrocks from Hong Kong Island using electron microscopy. Reddish brown staining of the rockfabric by iron-oxides without any apparent microfracturing might also be taken as evidenceof solution in the feldspar megacrysts and also the groundmass.Biotite, shows partial to complete alteration to a mixture of secondary products includinghydrous mica, chlorite, iron-oxides and sometimes vermiculite. The trace amounts of chlorite,hydrobiotite and vermiculite or intermixed chlorite-vermiculite identified by XRD in sometuffs are considered to be formed from the alteration of biotite. The presence of chlorite andvermiculite also occurring in the unweathered rock suggests that these minerals are largelyformed by hydrothermal processes although vermiculite and hydrobiotite can also form fromweathering of biotite in well-drained conditions in humid climates (Gilkes & Suddhiprakaran,1981).The feldspars in the devitrified groundmass also show increased decomposition withincreased staining where the groundmass assumes a semi-opaque appearance under themicroscope. In the case of tuffs with eutaxitic texture, staining is more intense along bandsthat also show more intense alteration.Trace to small amounts of kaolin minerals were identified by XRD in the discolouredportions of specimens in some tuffs (Tables 10 and 11). Kaolin minerals are also present intbe-feesh rock or unstained portion of some of the specimens, which suggests that the majorityof kaolin minerals present in the fresh and slightly decomposed tuffs are formed bypre-weathering processes such as hydrothermal alteration. The predominant kaolin mineralis kaolinite with only trace amounts of halloysite in the more weathered specimens of theslightly decomposed grade. The halloysite content increases in the moderately and highlydecomposed grades, suggesting that this mineral is formed directly from the decomposition


- 22 -of feldspars in intermediate stages of weathering. The increase in kaolin content in thediscoloured portions and in the moderately decomposed specimens is mostly derived from thealteration of plagioclases as the alkali feldspars do not show any significant petrographicchange.A trace amount of a possible smectite mineral was indicated in the coarse ash tuff fromAberdeen in addition to minor amounts of chlorite and vemiiculite or an intermediate mineralbetween the two (Table 9). The hydrothermal alteration of the rock might have resulted inthe formation of metastable smectite in the initial stages of feldspar alteration, as well asformation of chlorite and vermiculite from alteration of biotite (Man, 1994).The total amount of secondary minerals varies, up to 12% or even more in the slightlydecomposed tuffs. The proportion of microcracks and pores is very low, usually less than 1% by volume.A network of simple-branched and tight but mostly iron-oxide infilled or coatedmicrocracks is developed in the stained portions. The frequency of microfracturing increasestowards the joint surfaces. The boundaries of quartz and feldspar megacrysts are tight in thefresher cores but they may be coated with iron-oxides in the stained rims. The staining orcoating along grain contacts may indicate the development of intergranular fracturing or theformation of dissolution pits and channels around the exterior of the larger feldspar grains,as identified by Kwong (1985).9.4 Moderately DecomposedRapid reductions occur in Na + and Ca 2+ contents with the exception of the coarse ashtuff from Yuen Long (Tables 6 to 8), indicating a significant decomposition of plagioclase.The associated increases in clay mineral contents are reflected in LOI values which are almostdouble those of the slightly decomposed tuff. These changes are confirmed by XRD results,which show that the kaolin mineral content in this grade is increased significantly from a fewpercent to up to 20% or more (Tables 15 to 17). No consistent change is apparent in K +content indicating that the alkali feldspars are not yet significantly affected by weathering inthis grade.Although there is a sharp drop in Na + content as the rock weathers from the slightlydecomposed to the moderately decomposed state, the Ca" content is unaffected in coarse ashtuff from Yuen Long (Table 7). This might be due to the presence of calcite as indicated bythe high CO 2 content or absence of Ca- bearing feldspars in this rock type.Optical microscopy revealed that the majority of large plagioclase grains are decomposedto kaolin and micaceous minerals, with alkali feldspars only slightly affected but generallyhighly microfractured. The feldspars in the fine-grained to cryptocrystalline groundmass alsoappear to be affected by weathering in this grade. Numerous isotropic speckles were observedin many large feldspars under the microscope, similar to those identified in granites (Man,1996b). These are probably either the locations of solution pits and trenches observed byKwong (1985) and Dearman & Baynes (1978), or some intermediate alumina-silicate growthstransitional to kaolin minerals.


- 23 -The main kaolin mineral is a poorly crystalline kaolinite with a possible trace amountof halloysite only detected in the tuff from Tseung Kwan O. Parham (1969) also reported thegrowth of flame - shaped films and halloysite tubes on feldspars in some slightly tomoderately decomposed rhyolite- porphry specimens (probably a coarse ash tuff).In addition to kaolin minerals, small amounts of sesquioxides, vermiculite and chloritewere identified in some rocks from the XRD patterns. A trace amount of a smectite mineral,or an interstratified clay mineral of possible illite-smectite composition, was also detected inthe tuff from Aberdeen. The quantitative modal and mineralogical analyses show that the clayand micaceous mineral content can be up to 25% in this grade (Tables 15 to 17).The most significant change in the rock is the formation of an extensive network of tightto slightly open microcracks in this grade. As a result, a fairly rapid reduction in intactstrength occurs across the grade. Most large feldspars and some quartz grain boundaries arestained which indicate that the grain boundaries are opening up. Apart from the pores formedat the locations of dissolved calcite grains in some specimens, solution features have not beenobserved under the microscope.9.5 Highly DecomposedFairly rapid changes occur in the major oxides contents within the highly decomposedgrade (Tables 6 to 8), indicating more intense decomposition and more rapid variation of theclay mineral content of the volcanic rocks in this grade than in the moderately decomposedgrade. Both Na + and Ca 2+ contents are reduced to very small values in all rock types exceptthe coarse tuff from Yuen Long, with complete disappearance of plagioclase in the moreintensely weathered specimens as confirmed by XRD and optical microscopy. A noticeablereduction in K + content in all rock types except the tuff from Aberdeen indicates that alkalifeldspars are now being affected by weathering within this grade. The Si 4+ content also showsa noticeable reduction except in some hydrothermally altered specimens (Tables 7 and 8).Both feldspar types show variations in their degree and type of alteration. Plagioclasecontents are less than 3% in the fine ash tuffs, but can be up to 17% in the coarse ash tufffrom Yuen Long. Although plagioclases show near-complete alteration, the type of clayfabrics formed can be different in each rock. For example, the decomposed feldsparpseudomorphs are very porous in the coarse ash tuff from Aberdeen in comparison to thedense clay fabrics produced in the coarse ash tuff from Sai Kung. The altered feldspars inthe specimens affected by hydrothermal alteration have, in general, more clayey, densermicro fabrics than those affected only by weathering.Alkali feldspars are little altered in the tuff from Aberdeen, but they are intenselymicrofractured. In contrast, the tuffs from Sai Kung and Tseung Kwan 0 are highlydecomposed to clay minerals. The intensive solution of alkali felspars generally occurs in thecompletely decomposed grade (Plate 11). The decomposition of feldspars in the groundmassis more advanced in this grade than in moderately decomposed tuff, with specimensunaffected by hydrothermal alteration having more porous groundmass than those affected.The groundmass is particularly porous in the tuff from Aberdeen.Kaolin minerals are the dominant clay minerals, with small to accessory amounts of


- 24 -micas and trace amounts of chlorite and vermiculite or intermixed ehlorite-vermieulite in somerocks (Tables 15 to 18). Kaolin minerals in the fine ash tuffs form up to 40% o( the rock andeven more in the altered specimens, but generally less than 25% in the coarse ash tuffs. Thecoarse ash tuffs have similar kaolin contents to granites (Irian, 19%a). The halloysite contentgenerally increases in this grade but not in all rock types. It forms up lo half of the kaolincontent in tuffs from Aberdeen and Ap Tsai (Tables 12 and 14). Halloysite was not detectedin tuffs from Sai Kung and Yuen Long. Due to the lack of detailed electron microscopestudies it is not known whether the halloysite in the coarse ash tuff from Aberdeen is formeddirectly from the decomposition of feldspars or through an intermediate mineral. Kaolinitemay form (i) by hydrothermal alteration of feldspars, (ii) by direct transformation o( feldspars,or (iii) through an intermediate alumina-silicate mineral. It is possible that all three sourceshave contributed to kaolinite formation in the weathered volcanic rocks. A small amount ofkaolinite might also have formed from the decomposition of biotite. Trace amounts ofvermiculite or an intermediate vermiculite-chlorite identified by XRI) in the tuff fromAberdeen. It is highly likely that it is formed from alteration o( biotite. Small amounts ofsesquioxides occur in all rocks, generally as poorly crystalline aggregations.Thermogravimetric analysis indicates that these are mostly goethite.The rock fabric is extensively micro fractured by tight to slightly open mieroemeks,which together with voids comprise up to 20% of the rock in tuffs from Aberdeen {Table 5).Some disturbances of the fine-grainedgroundmass in the vicinity of the micro fractured quartzand feldspar grains indicate a slight expansion of the rock at this stage ol weathering.9.6 Completely DecomposedIn spite of the differences in their chemical and mineralogical compositions and textures,the completely decomposed fine and coarse ash tuffs show many similar mineralogical andfabric characteristics in the completely decomposed grade. However, significant differencescan also occur, even in the same rock type, if the rock has been subjected to hydrothermal andother alteration processes prior to weathering. Variations in the present-day weatheringenvironment can lead to significant differences in void characteristics and clay mineralogy ofthe soil formed from the same rock at different locations.Plagioclases are completely decomposed in this grade in coarse ash tuffs from Aberdeenand Ap Tsai, whilst complete decomposition of plagioclases occur in the highly decomposedgrade in all other tuffs studied. The Ca 2+ and Na f contents are further reduced to very smallamounts compared with highly decomposed tuffs, indicating the removal of cations from thesoil, except in the tuff from Yuen Long where the Ca 2+ content is still appreciable (Table 7).This is attributed to the accumulation of Ca-bearing minerals (e.g. calcite) in the soil orabsorption of freed Ca 2+ by other mineral species rather than the presence of unaltered Cabearingplagioclases.Significant reduction in K + content in all completely decomposed tuffs compared withhighly decomposed tuffs indicates intense weathering of alkali feldspars within this grade(Tables 6 to 8). Nevertheless, there are variations in the way the alkali feldspars decompose(e.g. solution versus alteration to clay minerals), depending on the environment of weatheringand the degree of hydrothermal alteration or contact metamorphism. For example, the tuffsunderlying gentle hillslope environments where active removal of cations by migrating


- 25 -groundwater occurs (e.g. Aberdeen coarse ash tuff, Tseung Kwan O fine ash tuff), showwidespread solution type weathering with advanced honeycomb fabric development in thecoarse ash tuffs (Irfan, 1994). The groundmass in these rocks also assumes a highly porousappearance.Although the Yuen Long site has also a relatively gentle hillside topography, thecompletely decomposed tuff specimens from this site do not have the highly porous fabricsdisplayed by Aberdeen tuffs. This might have been the result of contact metamorphism ofthe tuff by the underlying granite at this site, which had resulted in the formation of kaoliniteand micaceous minerals in the fresh rock. Kaolinite, being a stable mineral would not besubject to solution type weathering, which usually results in the removal of cations,particularly from the feldspars. The tuffs from Aberdeen also show the effects ofkaolinisation and sericitization due to hydrothermal alteration, but these appear to be restrictedto the vicinity of kaolin or kaolin-quartz veins (Irfan, 1994).The results of quantitative mineralogical analysis are in general agreement with theobservations made on thin sections. For example, the unaltered alkali feldspar content isreduced from about 13 to 19% and 27% in the highly decomposed tuffs from Ap Tsai andAberdeen respectively, to less than 8% in the completely decomposed tuffs from both sites.The alkali feldspars in the completely decomposed specimens from Sai Kung, which has acoastal low hilltop environment, show chemical decomposition to clay mineral products(mainly kaolinite).Biotite usually occurs in a completely altered state comprising a mixture of fineaggregations of hydrous mica, chlorite, kaolin minerals and iron-oxides. The altered mineralstructure shows evidence of expansion and, in some cases, complete disaggregation intosmaller grains.The kaolin mineral content increases sharply to up to 50% in this grade and particularlyin the more intensely weathered specimens (Table 17). Halloysite becomes almost asabundant as kaolinite in the least weathered specimens in this grade, being particularlyabundant in the coarse ash tuff from Aberdeen (Table 12), concentrations being similar togranites (Irfan, 1996a, b). However, in the completely decomposed tuffs from Sai Kung andYuen Long, only small amounts of halloysite were detected in the clay portions (Table 13).There may be a number of reasons for the dominance of kaolinite in these two rock types,some of which are given below :(i)(ii)(iii)The contribution from direct alteration of alkali feldspars tokaolinite. The alkali feldspars show widespread decompositionto kaolinite rather than solution type weathering in this grade.Direct formation of kaolinite from pre-weathering processessuch as contact metamorphism and hydrothermal alteration.The tuffs at both sites show evidence of contactmetamorphism, with the granite immediately underlying tuffat Yuen Long.Poor drainage in a low-lying coastal topography at Sai Kungmight also have inhibited the formation of halloysite.


- 26 -(iv)Most halloysite may be of silt size, as is the case with granites(Irfan, 1996b), and hence is not present in the clay portionsanalysed.In the more intensely weathered completely decomposed specimens and in the transitionsoil grade, kaolinite becomes dominant with only small amounts of halloysite (Tables 12 to14).The other alteration products include trace to small amounts of tine-grained micaceousminerals, sesquioxides and, in some rocks, small amounts of vermiculite or chlorite. Thechlorite content appears to decrease with increased weathering. Chlorite was not detected inthe completely decomposed and residual soil specimens from some sites. Gibbsite was alsonot identified in the XRD traces of any of the specimens in this grade, The micaceousproducts were identified by XRD as mostly illite in tuffs from Aberdeen based on the linebroadening and relative peak heights (Appendix B).The weathered rock shows extreme fabric variations in the completely decomposedgrade. In general, very fractured and porous fabric is developed in the coarse-grained tuffsfrom hillside environments. Generally, denser clayey fabrics are formed in the fine-grainedtuffs and those rocks previously affected by contact metamorphism or hydrothermal alteration.Poorly drained conditions, for example, in valley bottom type environments, may also leadto generally clayey fabrics. The cementation by iron-oxides may locally be very intense andso obliterates the soil fabrics. Organic matter may also be present in some specimens, asreflected in their higher than usual carbon contents.9.7 Transition and Residual SoilThe most significant chemical change is in K f content where it decreases appreciablywithin the residual soil stage, indicating intense decomposition of alkali feldspars. However,the K + content is still significant in the residual soil specimens of the fine ash tuff fromTseung Kwan 0 and the coarse ash tuff from Sai Kung (Tables 7 and 8). As the alkalifeldspars are completely decomposed, the major contribution to IC content is from kaolinite,which now forms over 50% of the total soil weight. The increased decomposition of alkalifeldspars to clay minerals indicates that chemical alteration of these feldspars takes place ata very advanced stage of weathering, subsequent to solution weathering in the initial andintermediate stages. The other important chemical change is in Si 44 content which shows anoticeable reduction for the first time in this grade indicating a noticeable solution of quartz.The XRD traces and semiquantitative calculations confirm some reduction in quartz contentin agreement with the microscopical observations (Tables 16 and 17). The reduction in silicaor quartz content may also be due to hydrothermal effects.Halloysite content decreases significantly in the transition soil and disappears completelyin the residual soil derived from tuffs. The halloysite in this advanced stage of weatheringwas identified as the dehydrated type. Lumb & Lee (1975) attributed the formation ofdehydrated halloysite in near-surface volcanic soils to considerable seasonal desaturation,together with a much greater chance of oxidation resulting eventually in complete dehydrationto kaolinite.


- 27 -In addition to predominant kaolinite and trace amounts of halloysite, small amounts ofmicaceous minerals (illite and muscovite), trace chlorite and, in some soils, vermiculite andpossibly interstratified clay minerals of illite-smectite composition are present. Smallquantities of gibbsite was reported by Lumb & Lee (1975) in near-surface volcanic soils andby Irfan (1996b) in residual granitic soils. Gibbsite has not been identified in any of thevolcanic soils analysed here. In some soils, small amounts of CO 2 were detected. This isattributed to the presence of organic matter in the specimens. There is a small increase in thetotal iron-oxide content, which forms up to 8% of the soil. The dominant iron mineralpresent in the volcanic soils was identified by thermogravimetric analysis as poorly crystallinegoethite with haematite in some specimens.The original rock fabric gradually disappears within the transition soil grade and theresidual soil comprises some silt-to sand-size quartz grains (usually less than 20% of the soil)in a structureless clay mineral-iron oxide mineral matrix. The coating and cementation byiron minerals are so intense, locally, that the details of soil fabric may be completelyobliterated. Occasional large pores, some of which are ant holes and decayed root locations,are present. Microcracks are much reduced in amount and most are secondary features, e.g.desiccation cracks. There may be small aggregations or 'pebbles 4 of reddish brown iron-oxideminerals throughout the soil fabric and these may be taken as an indication of commencementof laterization at some sites.9.8 Iron-oxide Minerals in Volcanic Saprolites and Residual SoilsThere are variations in the iron-oxide contents of the saprolitic and residual specimensanalysed, as reflected in their Fe 2 O 3 contents, depending on the degree of weathering and thelocation of the specimen within the weathering profile. Generally, the iron-oxide content isless than 10%. However, it may be higher in areas of iron-oxide concentration, e.g. in thegroundwater fluctuation zone, as was the case with the Aberdeen landslide site (Irfan, 1994).In thin section, iron minerals generally appear as poorly crystalline aggregationsscattered throughout the groundmass, but also as coatings to partially to completelydecomposed minerals, or as microcrack infillings. Locally, weak cementation is also present.Detailed studies on granitic rocks indicated that goethite is the main iron-oxide mineralformed via ferrihydrite or directly from the decomposition of biotite in the intermediate andadvanced stages of weathering (Irfan, 1996a). This is probably the case for the coarse ashtuffs and most other volcanic rocks in Hong Kong. However, the lack of any weight lossbetween 200° and 400°C in the thermogravimetric analyses of specimens from Sai Kung andother volcanic rock sites (except Aberdeen) were interpreted as these rocks not containing anydetectable goethite or gibbsite by one of the laboratories (British Geological Survey).Haematite was considered to be the main iron mineral present in the weathered tuffs fromthese sites. The tuffs from Sai Kung contain some magnetite. Diffuse magnetite boundariesmay indicate the commencement of alteration to haematite in the moderately to highlydecomposed stage. Thermogravimetric analyses carried out for Aberdeen specimens indicatedthe presence of poorly crystalline goethite. In granites, Irfan (1996a) reported that thecrystallinity of goethite increased with weathering. This might also be the case for volcanicrocks.


- 28 -9.9 Kaolin VeinsThe XRD analysis of a number of kaolin veins taken from the landslide sites on HongKong Island (Irfan & Woods, 1997) show that these veins contain dominantly kaolinite withsome illite and quartz and, in some cases, a smectite mineral (Table 18). Optical microscopyindicates that there may be an increase in the micaceous alteration products (mainly formedwithin the feldspars) adjacent to these veins and in the "pink feldspar' areas. This mayrepresent the situation described by Sales & Meyer (1940) that in the vicinity of hydrothermalveins in igneous rocks, a zonal arrangement of clay minerals may occur, with a sericitic micazone immediately adjacent to the vein followed by kaolinite and smectite - chlorite zones.Small amounts of swelling minerals (smectite) were identified in the weathered rock fromAberdeen, in addition to up to 15% smectite identified in some of the veins. It is alsopossible that smectite may have formed from weathering processes in a poorly drainedenvironment, but this is considered unlikely for the Aberdeen site.10. CLASSIFICATION OF WEATHERED VOLCANIC ROCKS AND SOILSJN^HONGKONG10.1 Classification of Material Weathering Grades of Volcanic RocksA characterization of the material grades of volcanic rocks (primarily the tuffs) in HongKong is given in Table 19 in terms of their mineralogical and microfabric properties.10.2 Classification of Volcanic Saprolitic and Residual SoilsA classification of various soil types formed from volcanic and granitic rocks is givenin Table 20, together with a summary of their structural and mineralogical properties, inaccordance with the classification scheme proposed by Wesley & Irfan (1996). The soils ofboth granitic and volcanic origin fall into Group A, as structural (both micro and macrostructure)influences are dominant in controlling their behaviour. Thin layers or lenses ofGroup C soils rich in sesquioxides, may also form near ground surfaces, particularly in poorlydrained areas. The formation of the Group (C c ) soils in Hong Kong is considered to be ofminor occurrence in comparison to those developed in tropical regions : these soils are looselyreferred to as lateritic or laterite.In volcanic saprolitic soils, the kaolinite content may be more dominant than halloysite;they may also contain appreciably more mica, some chlorite and small amounts of swellingminerals of vermiculite, smectite and layered minerals of illite-smectite composition.Kaolinite may also be more dominant in volcanic soils if the original rock had been affectedby hydrothermal and other alteration process prior to weathering.Almost all soil zones including the highly weathered volcanic rock zone in Hong Kongwould be classified as 'ferruginous (sensu stricto) soil* under the classification schemeproposed by the Geological Society (1990), except perhaps the uppermost residual soil zone,which would be described as 'ferralitic (kaolinitic)\


- 29 -11. QUANTITATIVE WEATHERING INDICES11.1 GeneralA brief review of the weathering indices proposed to assess the degree of weatheringof rocks is given in Irfan (1996a, b).A large number of these weathering indices are derived from the results of chemicalanalyses. The selected chemical indices used to quantity the degree of weathering in volcanicrocks is given in Irfan (1996a). A number of petrographic indices were also determined fromthe results of the modal analysis, such as the percentages of altered and unaltered minerals,the micropetrographic index (Ip) following Irfan & Dearman (1978).As the physical and mechanical properties of the volcanic rocks studied were notavailable, the indices derived from these properties were not determined. Also no attempt wasmade to classify the weathered volcanic rocks in terms of their dry density, void ratio,Schmidt hammer values or SPT N values, similar to that done for the granites (Irfan, 1996b).11.2 Chemical IndicesThe chemical weathering indices calculated for the volcanic rocks are presented inTables 21 to 24. The mobiles index (I mob ) proposed by Irfan (1996a) serves as an index ofthe degree of decomposition of the feldspar-bearing rocks, particularly in reasonably welldrained(leaching) conditions, such as those found in most hillsides and hilltops in HongKong. The silica-to-alumina (SA) ratio has also been found to be a good index of chemicalweathering in most free draining, usually acid, weathering environments in humid climates,especially on acid and intermediate rocks that weather to kaolinite or illite group of minerals(Ruxton, 1968). Both indices are not considered to be suitable for weathering conditionswhich result in smectite or vermiculite minerals, especially for basic and ultrabasic rocks.These two indices and the others calculated from chemical analyses do not account for thephysical and fabric changes occurring with progressive weathering. It is the fabric propertieswhich influence the engineering properties most in weathered rocks (Massey et al, 1988; Irfan,1996b, c).The results in Tables 21 to 24 indicate that the weathering potential index (WPI),alumina-to-potassium-sodium (AKN) ratio, the loss-on-ignition (LOI) value and the mobilesindex are generally good indicators of the degree of weathering of volcanic rocks. Theseindices appear to be more suitable for characterizing the weathered materials formed over aparticular rock type at a particular weathering environment. The chemical composition offresh volcanic rocks can vary significantly not only from formation to formation but alsowithin the same formation. In addition to chemical variation in the original rock, themineralogy and chemical composition of the weathered product are affected by theenvironment and history of weathering at each site as well as other alteration processes asdiscussed in Section 9. This is illustrated in Figure 16, which is a plot of silica-to-aluminaratio against mobiles index for the weathered coarse ash tuffs from Aberdeen and Sai Kung.Similarly, Figure 17 shows an almost linear relationship between WPI and I mob for the coarseash tuff from Aberdeen in contrast to a wide scatter of values shown in Figure 18 when thedata from all rocks are included.


- 30 -The alumina-to-calcium-sodium (ACN) ratio appears to be useful only for indexing therock grades of weathering and not sensitive to chemical changes occurring in the soil grades.The lixiviation index (jS) is not a good indicator of weathering even for the soil grades, whichit was originally proposed for by Rocha Filho et al (1985).11.3 Degree of Decomposition IndexThe degree of decomposition index (Lumb, 1965) nor the modified degree ofdecomposition index (Irfan, 1988) were determined for volcanic rocks as these indices wereconsidered inappropriate for fine-grained volcanic rocks, especially those with a high contentof cryptocrystalline or microcrystalline feldspar and quartz. Also, as discussed by Irfan(1996b), the degree of decomposition index is not a very reliable indicator of feldspardecomposition and it does not take into account the structural fabric elements such as voidsand microcracks.11-4 Micropetrographic IndicesPetrographical methods using an optical microscope and good quality thin sections arevery useful in studying weathering effects. Not only do they allow cheap and rapid mineralidentification but also the microfabric properties of the materials can be studied andcharacterized for the whole range of the weathering spectrum.The point counting technique described by Irfan and Dearman (1978) was employed todetermine the modal compositions of the mineral constituents, voids and microcracks for theweathered coarse-grained crystal tuffs only. Because of the extremely fine-grained nature ofthe matrix minerals, no individual mineral determinations were made and these were lumpedtogether as the 'fresh matrix' or 'altered matrix' minerals. The results of quantitativedeterminations are presented in Table 5. It was not possible to differentiate between variousfine-grained alteration products and small pores in the more weathered specimens under thenormal resolution of an optical microscope. The results are, however, still useful as anappropriate means of quantifying the observed fabric changes occurring in each material.The technique was found inappropriate for fine ash tuffs and the coarse ash tuffs witha large cryptocrystalline mineral contents, especially when they were weathered beyond theslightly decomposed grade. Also, general iron-staining or cementation of the fabric inmoderately and more intensely decomposed volcanic rocks made the identification of mineralconstituents very difficult.12. CONCLUSIONSAlthough chemical weathering has been the dominant process in profile development andsoil formation in volcanic rocks in Hong Kong, cataclasis and shearing by tectonic processesand granite intrusions and the associated hydrothermal alteration have also been important.The general weathering profile in lithologically uniform volcanic rocks is similar to that ofgranitoid rocks, although the soil zones are generally thinner and the transition from 'all soillayer' to 'all rock layer* is generally more gradual and less distinct.


- 31 -The recent classification scheme proposed by Wesley & Irfan (1996), which groups thesoils on the basis of their mineralogical and structural characteristics, appears to be suitablefor classifying the saprolitic and residual soils of volcanic rock origin.The six-fold material weathering grade scheme, based on that adopted by GeotechnicalEngineering Office (1988) but with further subdivisions and refinements that are proposedhere can be used to describe and classify the weathered volcanic rock materials encounteredwithin each broad soil group or rock mass zone.A large number of mechanical, physical, chemical and petrographical indices areproposed to quantify the degree of weathering and weathered rock fabrics and some of thesehave been used to relate and indirectly determine engineering properties of weathered rocks.Only the chemical and petrographical indices have been determined for weathered volcanicrocks due to lack of data on physical and mechanical properties of the rocks studied here.Most proposed chemical indices, in particular the mobiles index, the weathering potentialindex and the alumina-to potassium-ratio, have been found to be good indicators of the degreeof chemical weathering in a particular volcanic rock type, but direct comparison of indexvalues at a particular weathering grade is not generally possible even for the same rockoccurring in different formations. This is due to variations in the chemical composition andmineralogy of the weathering products arising from variations in the fresh rock and theenvironment and history of weathering at each site. The micropetrographic indices are onlyappropriate for the coarse-grained crystal tuffs due to the difficulty in recognising various finemineral and fabric components under the microscope in fine-grained rocks, particularly thosewith vitrified and cryptocrystalline textures.Although only the mineralogy and fabric properties of the fine and coarse ash tuffs werestudied in detail, the results of the study and the general mineralogy and fabric characteristicsobserved in these rocks are considered to apply to other volcanic rocks in the Territory, exceptperhaps the more basic types.In spite of great variation in their mineralogy and chemical composition and textures,the volcanic rocks show many similar mineralogy and fabric characteristics in the way theyweather. However, significant differences can also occur, even in the same rock, if the rockhas been affected by metamorphism and hydrothermal or other alteration processes prior toweathering. The unweathered rock may contain small amounts of secondary alterationproducts, mostly micaceous minerals, but also in some cases, kaolinite, calcite, chlorite andvermiculite. These secondary minerals are attributed to effects of epigenetic processes duringvolcanism and solidification, as well as the effects of contact metamorphism and hydrothermalalteration by the intruding magmas. Variations in the present day weathering andtopographical environment can also lead to significant differences in void characteristics andclay mineralogy of the soils formed from the same rock at different locations.In general, similar to granites, halloysite tends to form first, mainly from thedecomposition of feldspars, unless the rock has been affected by other alteration processes,in which case, kaolinite is dominant, in the early stages of weathering. Halloysite becomesthe dominant clay mineral with some muscovite/illite in the intermediate stages of weathering,which is then replaced by kaolinite in the near-surface saprolitic and residual soils. Smallamounts of gibbsite may also be present in near-surface residual soils of volcanic rock origin.


- 32 -Similar to granites, alkali feldspars in the tuffs are generally affected b\ weathering,initially by solution, much later than plagioclases, usually in the completely decomposedgrade. Alteration may commence earlier in the highly decomposed grade m some tuffs. Thetuffs underlying gentle hilisiope and hilltop environments, where ieachinti i»t cations bygroundwater occurs actively, show widespread solution type weathering ot' both the alkalifeldspars and the fine-grained matrix with advanced honeycomb fabric de\ el* »pmcnt Solutionweathering seems to be more common in the coarse ash crystal tu(t* than the fine ash tuffs.The kaolin veins of both weathering and hydrothermal or mixed origin arc common inthe saprolitic soil zone. Those of hydrothermal origin, as well as the weathered rock itself,may contain some smectite minerals.The presence of halloysite and sesquioxide minerals and the specific microstructuredeveloped in the soil grades, require that special precautions are taken in the preparation ofvolcanic soil specimens for conventional laboratory testing and in the interpretation of the testresults as discussed by Irfan (1996b, c).13. REFERENCESBaynes, F.J. & Dearman, W.R. (1978). The microfabric of a chemically weathered granite.Bulletin of the International Association n of Engineering Geology, No. IS, pp 91-100.Brock, R.W. (1943). Weathering of igneous rocks near Hong Kong. Bulletin_of_theGeological Society of America, vol. 54, pp 717-738.BSI (1981). Code of Practice for Site Investigations, BS.,,S_93Q ; 1 981,Institution, London, 149 p.British StandardsDearman, W.R. & Irfan, T.Y. (1978). Assessment of the degree of weathering in graniteusing petrographic and physical index tests. Intenia^[letcrkiTationand Protection of Stone Monuments, Unesco, Paris, Paper 2.3.Dearman, W.R., Turk, N., Irfan, T.Y. & Rowshanei, H. (l l )87). Detection of' rock materialvariation by sonic velocity zoning. Bulletin of the International Association jrfEngineering Geology, No. 35, pp 3-8.Gan, J.K. & Fredlund, D.G. (1992). Direct Shear Testing of a Hong Kong SoiJJLJnderVarious Applied Matric Suctions. Geotechnical Engineering Office, Hong Kong, 217 p.(GEO Report No. 11)GCO (1988). Guide to Rock and Soil Descriptions (Geoguide 3).Office, 189 p.Geotechnical ControlGCO (1990). Foundation Properties of Marble and Other Rocks in the Yuen Lonn-Tuen MunArea. Geotechnical Control Office, Hong Kong, 117 p, (GCO Publication No. 2/90).Geological Society of London (1990). Tropical residual soils. Geological Society WorkingParty Report. Quarterly Journal of Engineering Geology, vol. 23, pp 1-102.


- 33 -Geological Society of London (1995). The description and classification of weathered rocksfor engineering purposes. Geological Society Working Party Report. Quarterly Journalof Engineering Geology, vol. 28, pp 207-242.Gilkes, RJ. & Suddhiprakaran, A. (1981). Mineralogical and chemical aspects oflateritisation in Southwestern Australia. Laterisation Processes. Proceedings of theInternational Seminar on Laterisation Processes. Balkema, pp 34-44.Hencher, S.R. & Martin, R.P. (1982). The description and classification of weathered rocksin Hong Kong for engineering purposes. Proceedings of the Seventh South-east AsianGeotechnical Conference, Hong Kong, vol. 1, pp 125-142.Irfan, T.Y. (1988). Fabric variability and index testing of a granitic saprolite. Proceedingsof the Second International Conference on Geomechanics in Tropical Soils, Singapore,vol. 1, pp 25-35.Irfan, T.Y. (1992). Mineralogical Assessment of Creep-Type Instability at Two LandslipSites. Geotechnical Engineering Office, Hong Kong, 143 p. (GEO Report No. 13.)Irfan, T.Y. (1994). Mechanism of creep in a volcanic saprolite. Quarterly Journal ofEngineering Geology, vol. 27, pp 211-230.Irfan . T.Y. (1996a). Mineralogy and Fabric Characterization and Classification of WeatheredGranitic Rocks in Hong Kong. Geotechnical Engineering Office, Hong Kong, 158 p.(GEO Report No. 41)Irfan . T.Y. (1996b). Mineralogy, fabric properties and classification of weathered graniticin Hong Kong. Quarterly Journal of Engineering Geology, vol. 20, pp 5-35.Irfan . T.Y. (1996c). Suitability of standard classification and index tests for saprolitic andresidual soils in Hong Kong. Proceedings of the Twelfth Southeast Asian GeotechnicalConference and the Fourth Conference on Tropical Soils, Kuala Lumpur, vol. 1, pp 559-568.Irfan, T.Y. & Dearman, W.R. (1978). The engineering petrography of a weathered granitein Cornwall, England. Quarterly Journal of Engineering Geology, vol. 11, pp 233-244.Irfan, T.Y., Koirala, N. & Tang, K.Y. (1987). A complex slope failure in a highlyweathered rock mass. Proceedings of the Sixth International Congress on RockMechanics, Montreal, pp 397-402.Irfan, T.Y. & Woods, N. (1997). Discontinuity controlled landslides in weathered rocks inHong Kong. Transactions of the Hong Kong Institution of Engineers (in press).Kwong, J.K.P. (1985). Effects of Geological and Environmental Factors on the EngineeringProperties of Weathered Igneous Rocks in Hong Kong. Ph.D. Thesis, University ofLeeds, UK, 291 p, unpublished.


. 34 -Langford, R.L., Lai, K.W., Arthurton, R.S. & Shim, R. ilWK iieoli^ouh^^New Territories. Geotechnical Control Office, Hong Kong, 140 p. (Hong KongGeological Survey Memoir No. 3).Langford, R.L., James, J.W.C., Shaw, R., Campbell, S.D.(L Kirk, P.A.


- 35 -LIST OF TABLESTableNo.PageNo.1 Comparison of Various Engineering and Geological 37Schemes Proposed for Classifying Weathering Profile inIgneous and Volcanic Rocks (after Irfan, 1996b)2 Chemical Composition of Some Fine Ash and Coarse Ash 38Tuffs in Hong Kong3 Mineralogical Composition of 'Fresh' Volcanic Rocks 39(Based on Chemical Analysis and XRD)4 Composition of 'Fresh' Volcanic Rocks Determined from 40Modal Analysis5 Results of Modal Mineralogy and Quantitative Microfabric 41Determination, Weathered Coarse Ash Tuff, Aberdeen6 Results of Chemical Analysis, Coarse Ash Tuff, Aberdeen 427 Results of Chemical Analysis, Coarse Ash Tuffs, Sai Kung 43and Yuen Long8 Results of Chemical Analysis, Fine Ash Tuff and Eutaxite, 44Tseung Kwan O9 Results of Semi-quantitative Bulk XRD Analysis, Coarse 45Ash Tuff, Aberdeen10 Results of Semi-quantitative Bulk XRD Analysis, Coarse 46Ash Tuffs, Sai Kung and Yuen Long11 Results of Semi-quantitative Bulk XRD Analysis, Fine Ash 47Tuff and Eutaxite, Tseung Kwan O12 Results of Semi-quantitative Clay Mineralogy Determined 48from XRD, Coarse Ash Tuff, Aberdeen13 Results of Semi-quantitative Clay Mineralogy Determined 49from XRD, Coarse Ash Tuffs, Sai Kung and Yuen Long14 Results of Semi-quantitative Clay Mineralogy Determined 50from XRD, Fine Ash Tuff and Eutaxite, Tseung Kwan O


36TableNo.PageNo.15 Results of Quantitative Bulk Norm Mineralogy Coarse Ash 51Tuff, Aberdeen (Based on Chemical Analysis)16 Results of Quantitative Bulk Norm Mineralogy, Coarse Ash 52Tuffs, Sai Kung and Yuen Long (Based on ChemicalAnalysis)17 Results of Quantitative Bulk Norm Mineralogy, Fine Ash 53Tuff and Eutaxite, Tseung Kwan O (Based on ChemicalAnalysis)18 Mineralogy of Kaolin Veins in Hong Kong (after Irfan & 54Woods, 1997)19 Classification of Material Weathering Grades of Volcanic 55Rocks in Terms of Petrological Change20 Classification of Granitic and Volcanic Soils in Hong Kong 58(Wan, 1996c)21 Chemical Weathering Indices, Weathered Coarse Ash Tuff, 59Aberbeen22 Chemical Weathering Indices, Weathered Coarse Ash Tuff, 60Sai Kung23 Chemical Weathering Indices, Weathered Fine Ash Tuff, 61Tseung Kwan O24 Chemical Weathering Indices, Weathered Fine Ash Tuff; Ap 62Tsai, Tseung Kwan O25 Micropetrographic Indices for Weathered Coarse Ash Tuff, 63Aberdeen


- 36 -TableNo.PageNo.15 Results of Quantitative Bulk Norm Mineralogy, Coarse Ash 51Tuff, Aberdeen (Based on Chemical Analysis)16 Results of Quantitative Bulk Norm Mineralogy, Coarse Ash 52Tuffs, Sai Kung and Yuen Long (Based on ChemicalAnalysis)17 Results of Quantitative Bulk Norm Mineralogy, Fine Ash 53Tuff and Eutaxite, Tseung Kwan O (Based on ChemicalAnalysis)18 Mineralogy of Kaolin Veins in Hong Kong (after Irfan & 54Woods, 1997)19 Classification of Material Weathering Grades of Volcanic 55Rocks in Terms of Petrological Change20 Classification of Granitic and Volcanic Soils in Hong Kong 58(Irfan, 1996c)21 Chemical Weathering Indices, Weathered Coarse Ash Tuff, 59Aberbeen22 Chemical Weathering Indices, Weathered Coarse Ash Tuff, 60Sai Kung23 Chemical Weathering Indices, Weathered Fine Ash Tuff, 61Tseung Kwan O24 Chemical Weathering Indices, Weathered Fine Ash Tuff, Ap 62Tsai, Tseung Kwan O25 Micropetrographic Indices for Weathered Coarse Ash Tuff, 63Aberdeen


- 63 -Table 25 - Micropetrographic Indices for Weathered Coarse Ash Tuff, Aberdeenr_ . . SampleDescription ,. rSlightlyDecomposedModeratelyDecomposedHighlyDecomposedCompletelyDecomposedTransitionSoilNotes : *IR1IR2IR4IR5IR6Percentage of poresareas and pores ismicroscope.AlteredMinerals%11.824.943.058.164.5SoundMinerals%88.173.734.929.619.8Microcracksand Voids%0.11.322.012.2*15.6*MicropetrographicIndexIp7.872.810.540.420.25could be higher. Differentiation between minute alterednot possible under the normal resolution power of


- 64 -FigureNo.PageNo.1 Geological Map of Hong Kong and Sampling Locations 662 Classification of Pyroclastic Volcanic Rocks 673 Weathering Profile in Coarse Ash Tuff, The Peak 684 Sketch of Weathering Profile in Fine Ash Tuff, Tseung 69Kwan O5 Sketch of Weathering Profile in Fine Ash Tuff, Ap Tsai, 70Tsueng Kwan 06 Weathering Profile in Coarse Ash Tuff, Aberdeen (Prior to 71the Landslide)7 Whole Rock XRD Scans of Slightly, Moderately and Highly 72Decomposed Coarse Ash Tuff8 Whole Rock XRD Scans of Completely Decomposed and 73Residual Soil of Coarse Ash Tuff9 Clay Portion XRD Scans of Weathered Coarse Ash Tuff, 74Yuen Long10 Clay Portion XRD Scans of Weathered Fine Ash Tuff, 75Ap Tsai, Tseung Kwan 011 Variation in Mineralogical Composition with Weathering, 76Coarse Ash Tuff, Aberdeen12 Variation in Mineralogical Composition with Weathering, 77Coarse Ash Tuff, Sai Kung13 Variation in Mineralogical Composition with Weathering 78Coarse Ash Tuff, Yuen Long14 Variation in Mineralogical Composition with Weathering, 79Fine Ash Tuff, Tseung Kwan O15 Variation in Mineralogical Composition with Weathering 80Fine Ash Tuff, Ap Tsai, Tseung Kwan 0


- 65 -FigureNo.PageNo.16 Variation in Chemical Indices for Weathered Coarse Ash 81Tuffs from Different Localities17 Weathering Potential Index Versus Mobiles Index, Coarse 82Ash Tuff, Aberdeen18 Weathering Potential Index Versus Mobiles Index, All 83Rocks


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- 84 -LIST OF PLATESPlateNo.No.1 Weathering Profile in Coarse Ash Tuff, Mount Davis, Hong 85Kong Island2 Complex Weathering Profile in Fine Ash Tuff, Chai Wan, 86Hong Kong Island3 Soil Layer Overlying Rock Layer in Fine Ash Tuff, Chai 87Wan, Hong Kong Island4 Weathering Profile in Fine Ash Tuff, Ap Tsai, Tseung 88Kwan O5 Sampling Site at Tseung Kwan O 896 Sampling Site at Sai Kung 907 Weathered Coarse Ash Tuff Samples, Abedeen 918 Material Weathering Grades in Fine Ash Tuff, Tsueng 92Kwan O9 Material Weathering Grades in Fine Ash Tuff (Eutaxite),Ap 93Tsai, Tseung Kwan 010 Material Weathering Grades in Coarse Ash Tuff, Sai Kung 9411 Micrographs Illustrating Mineralogical and Fabric Properties 95of Coarse Ash Tuff Weathered to Different Degrees12 Micrographs Dlustrating Mineralogical and Fabric Properties 96of Fine Ash Tuffs Weathered to Different Degrees


- 91 -(Negative No. SP8907803 & 04).Plate 7 - Weathered Coarse Ash Tuff Samples, Aberdeen


- 97 -APPENDIX ASAMPLE DESCRIPTIONS


- 98 -APPENDIX A : SAMPLE DESCRIPTIONSLocality : New Slope Cutting at Ap Tsai Wan, Tseung Kwan OSample No.ATW1(JB8)ATW2(JB5)ATW3(JB5)ATW3FATW3WATW4(JB3)ATW5(JB3)LocationSample taken from freshrock zone near base of50-60 m high cut.Sample taken from lowerpart of moderatelyweathered zone in cutslope, half way up the hill.Adjacent to sample ATW2.Sample taken from closelyjointed, highly weatheredrock zone, in cut slope neartop of hill (about 5-7 mbelow original groundsurface before cutting).Adjacent to ATW4.Description of Hand SpecimenFresh. Very strong, dark grey, veryfine grained matrix with up to 20%feldspar and occasional quartzmegacrysts (max. 2 mm), occasionallapilli, fine ash TUFF.Slightly decomposed (slightlydiscoloured). Very strong, yellowishbrown discoloured rim, up to 10 mmwide with very strong, medium greyfresh core, fine ash TUFF.Slightly decomposed (moderatelydiscoloured). Strong, yellowish brownto dark brown discoloured rim, up to20 mm wide with very strong,medium grey, fresher core, fine ashTUFF, occasional feldspars are grittyin discoloured rim; a series of tightmicrocracks parallel to discolouredzone.Fresh Core.Discoloured rim (ModeratelyDecomposed).Highly Decomposed. Weak to veryweak, completely discolouredyellowish grey with brown and patchyblack staining along fractures, fine ashTUFF; large feldspars are grittyoccasionally powdery; white kaolin(?)infill (vein?), macro-fractures at 5 to20 mm spacing.Highly Decomposed (SlightlyAltered). Very weak, completelydiscoloured, light yellowish grey withpatchy brown and black on fracturesurfaces, fine ash TUFF; feldspars aregritty to powdery; kaolinized adjacentto one face; macro-fractures at 3 to40 mm spacing.


- 99 -Sample No.ATW6(JB1)LocationSample taken fromcompletely weathered rockzone in cut slope (approx.2 m below original groundsurface before cutting).Description of Hand SpecimenCompletely Decomposed. Extremelyweak, yellowish brown with pinkishorange and olive green patches, fineash TUFF (eutaxitic?); feldsparsclayey to powdery, loss of tuff fabricat places, particularly around rootlets(Clayey sandy silt).


100 -Locality : Access Road Cutting, above Tseung Kwan 0 New TownSample No,,TKO1TKO2FTKO2WTKO3TKO4TKO5LocationSample taken from freshcore of a small corestone inmoderately weathered tuffzone cutting; along accessroad.Sample taken from partiallydiscoloured portion ofabove corestone.Same as TKO2F(discoloured rim ofcorestone).Adjacent to TKO1, smallcorestone.Sample taken from highlyweathered rock zone alongaccess road cutting,adjacent to TKO4.Sample taken fromcompletely weathered zone.DgscriptionFresh core of slightly discolouredblock. Very strong to extremelystrong, dark grey, crystal-bearing fineash TUFF.Fresher core of moderately discolouredblock. Very strong, light to mediumgrey (faintly discoloured) core withyellowish grey to yellowish brown rim(TKO2W\ 20 mm wide, crystalbearing fine ash TUFF; black oxidecoating on joint surface.Completely Discoloured (moderatelydecomposed) rim of TKO2. Lightgrey with yellowish grey to yellowishbrown (on joint surface), moderatelystrong to strong, fine ash TUFF.Moderately Decomposed. Moderatelyweak, light yellowish grey withreddish brown and black on jointsurface, completely discoloured fineash TUFF; some feldspars are gritty,irregular fractures at 10 to 80 mmspacing near joint surface.Highly Decomposed. Weak, yellowishgrey with yellowish brown on jointsurfaces, patches of black, fine ashTUFF; micro-fractured throughout at 5to 40 mm spacing. Light pinkfeldspars on one side of block(altered?); most large feldspars arepowdery.Highly Decomposed. Very weak,yellowish grey with reddish brown andpatches of black, fine ash TUFF;micro-fractured at 10 -20 mm spacing,irregular pattern; veinlets of kaolin(altered?)(Can be crumbled to sandysilt with strong finger pressure).


- 101 -Sample No.TKO6TKO7(S2)*TKO8LocationSample taken fromcompletely weathered zone,a few metres West ofTKO7.Sample taken from trial pitin transition soil zone.Sample taken from residualsoil zone, about 1.5 mbelow ground surface.Description of Hand SpecimenCompletely Decomposed (very dense).Extremely weak, mottled yellowishgrey and reddish brown, fine ashTUFF; all large feldspars are powderyto soft; easily crumbled to individualgrains (Sandy silt with some clay).Transition soil with partial tuff fabric(about 30%). Loose to dense, reddishbrown and yellowish brown, fine ashTUFF (Clayey silt).Residual soil. Reddish brown withpatches of yellowish brown, smallpatches of tuff fabric (Clayey silt).


- 102 -Locality : Landslip Site behind Island Road Government School AberdeenNote : For details, see Irfan (1994)Sample No.IR1IR7IR2IR8IR4IR9LocationCore sample from boreholeBH1, at 9.6 m depth inhighly weathered rock zone.Core sample from boreholeBH1, at 6.5 m depth, inhighly weathered rock zone.Core sample from boreholeBH1, at 8.6 m depth inhighly weathered rock zone.Core sample from boreholeBH1, at 5.3 m depth inhighly weathered rock zone.Core sample from boreholeBH1, at 8.8 m depth.Core sample from boreholeBH1, at 7.3 m depth inhighly weathered rock zone.Description of Hand SpecimenSlightly Decomposed (SlightlyDiscoloured). Strong, medium greywith light yellowish grey in thevicinity of joint surfaces, coarse ashTUFF; Megacrysts (up to 4 mm) ofquartz and feldspars, in a finelycrystalline matrix; biotite rich, 5 to10%; feldspars gritty in stained rim.Slightly Decomposed (HighlyDiscoloured). Moderately strong tostrong, light grey core with yellowishgrey rim (up to 4 mm wide), coarseash TUFF; most large feldspars grittyin the discoloured rim; very closelyspaced fractures parallel to jointsurface.Moderately Decomposed. Moderatelyweak to strong, light yellowish greywith yellowish brown secondarystaining along joint surface; coarse ashTUFF; feldspars with varying degreesof decomposition but mostly hard togritty, except near joint surfaces theyare powdery.Moderately Decomposed. Moderatelyweak, light yellowish greenish grey,coarse ash TUFF; feldspars powderyto gritty; micro-fractured by a networkof irregular, branched microcracks.Highly Decomposed. Weak, lightyellowish grey, coarse ash TUFF;feldspars powdery to gritty, some soft.Highly Decomposed. Very weak,light yellowish grey, coarse ash TUFF;feldspars powdery, some soft.


- 103 -Sample No.IRSIR3IR6LocationCore sample from blocksample IRS in a trial pit(TP3) at 0.3 m belowlandslip surface incompletely weathered zone.Sample contains up to30 mm thick kaolin veinsand relict joints.Block sample taken acrossshear plane comprising boththe undisturbed saproliticzone containing kaolinveins and creeping zone.Block sample taken from atrial pit, at 1.3 m depth, inresidual (transition) soilzone.Description of Hand SpecimenCompletely Decomposed andhydrothermaily altered. Extremelyweak, light yellowish grey to pinkishgrey (altered portion), coarse ashTUFF with kaolin veins; feldsparspowdery to soft (in the vicinity ofkaolin veins (clayey sandy silt).Specimens IR51 and IR55 are kaolinveins.Completely Decomposed. Extremelyweak, yellowish grey with pinkishgrey and white patches, coarse ashTUFF with relict joints and kaolinveins; feldspars powdery to soft andbiotite greenish grey to silvery grey(Clayey sandy silt).Transition soil. Mottled reddishbrown and yellowish grey with thindiscontinuous kaolin veinlets but norelict discontinuities; partial loss oftuff fabric throughout block sample(Clayey sandy silt).


104Locality : Sai Kung, Old Borrow AreaSample No.SK7(CT5)SK6SK5(CT4)SK4(CT4)SK2 & SK3(CT3)SKI(CT1)LocationSample taken from acorestone, about 3 m beloworiginal ground surface, incompletely weathered rockzone.Sample taken from acorestone in completelyweathered rock zone.Sample taken from core ofa corestone, about 3 mbelow original groundsurface, in completelyweathered rock zone, 1 mto the west of SK7.Sample taken from outerrim of the same corestoneas SK5.Sample taken in completelyweathered rock zone, about2 m below original groundsurface.Sample taken from residualsoil zone, about 0.8 mbelow ground surface.n of Hand SpecimenSlightly Decomposed. Very strong,medium to dark grey fresher core withlight yellowish grey rim, up to 10 mmwide, coarse ash TUFF withoccasional lithic fragments andabundant quart/, and feldspar crystals,up to 4 mm in diameter; feldsparsgritty.Slightly Decomposed, Strong, darkgrey core surrounded by light greyinner rim and light yellowish greyouter core; flow-banded porphyriticLAVA with cluster of quartz andfeldspars (The sample is possibly froma lava band within coarse ash TUFF).Moderately Decomposed, Weak, lightyellowish grey with brown and blackpatches, completely discoloured,coarse ash TUFF; most feldsparsgritty, some hard; occasional quartz upto 6 mm.Highly Decomposed. Very weak,light yellowish grey, coarse ash TUFFwith very closely spaced fractures; thinclay infill along some fractures;feldspars powdery to gritty, some soft.Completely Decomposed. Extremelyweak, yellowish brown with yellowishgrey, coarse ash TUFF; with patchyfabric loss; most feldspars powdery tosoft (Clayey sandy silt).Residual soil. Reddish brown, sandyclayey silt with small patches of tuffmatrix; numerous pores, up to 2 mmin diameter and desiccation cracks.


- 105 -Locality : Tai Tong East Borrow Area, Yuen LongSample No.YL1YL6YL3YL5YL4LocationSample taken from freshcore of a large corestone, incompletely weathered rockzone, within the landsliparea.Sample taken from a highlydecomposed corestone,adjacent to trial pit TP1 inthe landslip area.Sample taken at 1 m depthin trial pit TP1 in thelandslip area, in completelyweathered rock zone.Sample taken at 1.5 mdepth in trial pit TP4 in thelandslip area, in completelyweathered rock zone.Sample taken fromdeformed saprolite layer at0.5 m depth in trial pitTP1, containing polishedshears surfaces.Description of Hand SpecimenFresh. Very strong, medium grey,lapilli bearing coarse ash TUFF withoccasional sedimentary rockfragments, epidote-chlorite veined(altered), abundant feldspars(dominantly plagioclase) with quartzcrystals.Highly Decomposed. Extremely weakto very weak, dark greenish yellowishgrey, coarse ash TUFF containing aseries of parallel micro-fracturesspaced at 5 mm, feldspars powdery togritty; quartz dull, brownish stained;biotite greenish grey, powdery.Completely decomposed. Extremelyweak, light greenish yellowish grey,coarse ash TUFF; feldspars gritty topowdery, occasionally soft; quartzvery dull (Sandy silt with little clay).Completely Decomposed. Very dense,greenish grey with patches ofyellowish grey, coarse ash TUFF;feldspars powdery, occasionally soft;quartz very dull; biotite powdery,yellowish greenish grey; groundmasspowdery to soft (Clayey sandy silt).Deformed completely decomposedtuff. Light greenish grey with patchesof yellowish brown; no relict jointsbut with deformed disturbed tufffabric.


106 -APPENDIX BMETHODS OF PETROGRAPHICAL AND CHEMICAL ANALYSISAND IDENTIFICATION OF MINERALS


- 107 -B.IGeneralThe chemical and XRD analyses of the coarse ash tuff specimens from the landslip siteat Aberdeen were carried out on duplicate specimens at Amdei and Analabs laboratories inAustralia in 1989. The mineralogical interpretations from XRD traces were carried out byDr R. Brown of Amdei and Dr R. J. Gilkes and Mr G.D. Moore of Analabs.The chemical analyses by XRF were carried out by Sietronics Pty in Australia on thetuff specimens from Tseung Kwan 0 (TK series), Sai Kung (SK series) and Ap Tsai Headlead(ATW series), which also carried out the XRD analyses on the same specimens. The XRDanalyses on these rocks using duplicate specimens were repeated by Messrs S. V. Prior andS. J. Kemp of the British Geological Survey (BGS) in the U.K. due to uncertainty with someof the interpretations made by Sietronics Pty. The BGS determined the quantitative normativebulk mineralogies of these rocks using the chemical analyses results of Sietronics Pty.The BGS also carried out the chemical and XRD analyses of the specimens from YuenLong (YL series). The mineralogical and fabric description of all rocks were carried out bythe author (T. Y. Irfan), using thin sections and hand specimens.B.2 Methods of Analyses Used by AmdeiB.2.1Clay Mineralogy bv XRDPortion of each sample was powdered finely and used to prepare an X-ray diffractometertrace which was interpreted by standard procedures.Further, weighed, lightly pre-ground subsamples were taken and dispersed in water withthe aid of deflocculants and an electric blender, and allowed to sediment to produce-2 /xm e.s.d size fractions by the pipette method. The resulting dispersions were examinedby plummet balance to determine their solids contents, and were then used to produce orientedclay preparations on ceramic plates. Two plates were prepared per sample, both beingsaturated with Mg ++ ions, and one in addition being treated with glycerol. When air-dry, thesewere examined in the X-ray diffractometer. Additional diagnostic examinations carried outconsisted of examination of the glycerol-free plate after heating for one hour at 110°C, aftertreatment with formamide or glycol, and after heating for one hour at 550°C (varioustreatments according to individual requirements). The radiation used was CoKa.B.2.2Chemical AnalysesWhole rock chemical analyses were carried out by Inductively Coupled Plasma (ICP)Atomic Absorption Spectroscopy.The loss on ignition (LOI) was determined by heating representative finely groundspecimens to 13G0°C and relating back weight losses to original sample weights.


- 108 -B.2.3Mineral Calculations from Chemical AnalysesA suitable computer program was used for the calculation of mineral proportions usingthe chemical analysis results. Appropriate theoretical, or quoted practical compositions forthe component minerals were used in the calculations, and in some instances these were variedslightly when the calculated fit for minor minerals (e.g. biotite, vermictilite) was poor.Minerals which were not observed by XRD were not introduced into the calculations, althoughin some cases this would have resulted in a considerably improved agreement. In any case,the effect on the calculated percentages of the major minerals is usually negligible. Thefigures given in the tables are quoted to the nearest percent, but the actual accuracy, althoughunknown, is not expected that level. It is possible that appreciable amorphous material(amorphous clay or volcanic glass) is present. If that is the case, the calculated mineralpercentages could be considerably in error.Iron was calculated to goethite on the basis of the observation of goethite in somesamples, especially in the clay fractions.B.2.3Comments on Mineral Determinations by XRDAll three kaolin minerals (kaolinite, hydrated halloysite and halloysite or dehydratedhalloysite) are present in many samples. There is no known method to estimate theproportion of hydrated and dehydrated halloysite by XRD. The presence of hydratedhalloysite was detected from peak shift with glycerol. The approximate proportion ofhalloysite (including both types) in the total kaolin was estimated by formamide treatment ofthe clay fractions. Whether the relative proportions are maintained in the bulk sample is ofcourse uncertain as this would depend on the grain size of kaolinite and halloysite particlesin the bulk material.Smectite was only identified in kaolin veins. In the instances where interstratified claywas identified, the clay layers making up the material were not able to be determined withcertainty. An illite-vermiculite interstratification has been suggested but the evidence is notgood.Muscovite and illite merge into one another, the dividing line being indefinite and thedifferences being essentially those of crystallinity, potassium content, hydration and probablyparticle size. Because of their close relationship, the XRD results are usually reported asmica/illite or muscovite/illite. Based on diffraction peak breadths, which were generally weak,the micaceous minerals in the tuffs from Aberdeen are probably illite.B - 3Methods of Analyses Used by AnalabsB.3.1 Bulk Mineralogy bv XRDFinely pulverised portions of each rock sample were placed in the diffractometer asloose packed randomly orientated samples using the back loading technique to reduceorientation problems. A diffractrogram was run each sample covering the angular range of4 to 65 degrees (d spacings of 22A to 1.43A) and using Cu radiation.


- 109 -Each scan was then examined using transparent overlays to identify the presence orabsence of mineral species. Quantification is based on typical analyses for each type ofmineral. Many minerals such as the clays and feldspars do not have set stoichiometry andtherefore the data must be regarded as semi-quantitative.B.3.2.Clav Mineralogy by XRD on Clay FractionSoil specimens were dispersed in a dilute alkali solution using on ultrasonic probe. Theclay fraction (less than 2 microns) was collected by sedimentation and the clay coated ontothe porous ceramic plates under suction. The clay was saturated with :a. Mg solutionb. Mg-glycerolc. K-550°C prior to XRD analysis using a Philips goniometerwith diffracted beam monochromator.B.3.3Whole Rock Analyses by XRFX-ray Fluorescent Spectroscopy (XRF) was used for the determination of all oxidesexcept for Na 2 O which was determined by Atomic Absorption Spectroscopy (AAS).For the XRF analysis, the samples were prepared as glass fusion discs which reduces oreliminates the inherent effects; the glass eliminates particle size effects and partiallynormalises absorption effects through dilution. An accurately weighed portion of thepulverised sample was added to a preweighed amount of lithium borate flux which containedthe heavy X-Ray absorber lanthanum oxide. These were carefully mixed and then fused inplatinum gold alloy crucibles to produce a homogeneous melt. The molten glass was thenpoured onto a graphite platten and pressed out to form a solid glass disc which after coolingwas presented to the XRF for analysis.The calibration for the XRF is based on a synthetic master standard but for "fine tuning"of the data a series of certified reference materials were also prepared and run with thesamples. Appropriate corrections were applied to correct for backgrounds, peak overlaps andabsorption and enhancement effects.Because sodium is not a particularly sensitive element by XRF this cation wasdetermined by AAS using a mixed acid digest. An accurately weighed portion of the samplewas digested in heated teflon beakers using perchloric, nitric, hydrochloric and hydrofluoricacids. Following dissolution the solution was made up to volume in volumetric flasks usingdeionised water. Analysis by AAS with calibration was based on "Volucon" standardsolutions and certified reference materials used as checks.Moisture and loss on ignition (LOI) were determined by gravimetric methods wherebyweight losses at specific temperatures, 105°C for moisture and 1050°C for LOI, were recordedand related back to the original sample weights. Two of the samples TLC15 and IR39 were


110extremely wet and before any analyses or preparation were carried out, w~ere dried. Theresults reported for these two samples are on the "dried sample' basis. All other samples arereported on the 'as-received' basis.B.4 Methods of Analyses Used_byjkitij^B.4.1Sample PreparationA representative 20 g portion of each sample was dried at 55°C crushed and hammermilledto pass a 200 /xm screen. This material w r as used for thermogravimetric (TG) analyses.To provide a finer and uniform particle size for whole-rock X-ray diffraction (XRD) analysis,approximately 3 g of the less than 200 fim material was subsampled, wet micronised underacetone for 5 minutes and dried at 55°C.Oriented XRD mounts were prepared from weathered samples to provide a more detailedanalysis of the clay minerals present. 20 g of each sample was dispersed in distilled waterusing an ultrasonic probe and laboratory stirrer. The resulting suspension was then passedthrough a 63 /xm sieve and the


- Ill -Approximately 5 g of tema-ground sample was dried for 24 hours at 105°C. Loss onignition was calculated from any further weight loss from 1 g of sample heated at 1050°C for1 hour. Glass beads were then prepared by fusing 0.9 g of each sample with 9 g (correctedfor loss on fusion) of Spectroflux-100 (Li 2 B 4 O 7 ) at approximately 1200°C (in a PhilipsPerl'X-2 microprocessor-controlled automatic fused bead maker) and pouring the melt intoa platinum casting dish. Lithium iodide (Lil) was added to all samples before fusion to actas a releasing agent.Samples were then analysed using a Philips PW1480/10 sequential, fully microprocessorcontrolled wavelength-dispersive XRF spectrometer equipped with a scandium anode tube.Analyte angles were calibrated from international and in-house standard reference materials,prepared as fused glass beads. Drift correction was catered for by an external ratio monitorand background correction was applied where necessary. Ten major elements were calibratedas oxides using the de Jongh calibration algorithm and the alpha coefficients generatedempirically with one regression based line overlap (Ca on Ng). Oxide concentrations weremerged with loss on ignition figures and the results totalled. No lower limits of detection arequoted for major elements but reporting limits are 0.01% for all oxides with the exception ofMnO which is reported to 0.001%.B.4.5Mineral Quantification from Chemical AnalysesBoth K-feldspar and plagioclase contents were calculated from the XRF K 2 O and Na 2 Ofigures assuming a stoichiometry of KAlSi 3 O 8 respectively. The plagioclase concentrationcalculation necessarily assumes the absence of anorthite (Ca-rich) or mixed cation (Ca/Na)species. The K-feldspar concentration calculation also assumes the absence of otherpotassium-bearing phases such as muscovite or illite. Therefore where both K-feldspar andsignificant 'mica* were detected by XRD, the K 2 O figure was apportioned on the basis ofK-feldspar XRD peak height data. A calibration curve was constructed from the 3.30 AK-feldspar peak data from those samples where only trace 'mica* content could then bedetermined. The K 2 O contribution from the K-feldspar was then determined, and the 'mica*concentration calculated from the residual K 2 O figure.However, it was not possible to apply this XRD peak-height method of quantificationto the 'mica* phase(s) as broadening of the 10 A peak indicates a change in crystallinityand/or phase change during weathering.Quartz concentrations were estimated from the residual wt% SiO 2 figure obtained aftersubtraction of the SiO 2 due to 'kaolin*, plagioclase and K-feldspar, and 'mica* assuming thatthese were the only silica-bearing phases present. The hematite concentration was taken asthe wt% Fe 2 G 3 .The total wt% figures for quantitative phases are generally 100+/-10%. Where totalsexceed this limit the sources of error are probably due to the increased concentration of nonquantifiedminerals, such as 'mica', chlorite, vermiculite, hydrobiotite, ?carbonates and?titanium oxides as well as calculation errors due to the stoichiometries assumed.


- 112 -B.4.6Comments on Mineral Identification from XRDDiagnostic testing indicates that the


- 113 -consistent,(b)X-Rav DiffractionUnoriented powder mounts of the total sample were preparedfor bulk mineralogy. Oriented sedimented mounts were madefor examination of clay minerals (


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(Superseded by Geospec 1)GCO PublicationNo. 3/84Review of Superficial Deposits and Weathering in Hong Kong, GCO Publication HK$40by J.D. Bennett (1984), 58 p. (Reprinted, 1993). No. 4/84 (US$8)Review of Hong Kong Stratigraphy, by J.D. Bennett (1984), GCO Publication HK$2586 p. No. 5/84 (US$5.5)Review of Tectonic History, Structure and Metamorphism of GCO Publication HK$20Hong Kong, by J.D. Bennett (1984), 63 p. No. 6/84 (US$5)(Superseded by GCO Publication No. 1/88)GCO PublicationNo. 1/85Groundwater Lowering by Horizontal Drains, by D.J. Craig & GCO Publication HK$74I. Gray (1985), 123 p. (Reprinted, 1990). No. 2/85 (US$12)(Superseded by GEO Report No. 9)GCO PublicationNo. 1/88Review of Design Methods for Excavations (1990), 187 p. GCO Publication HK$40(Reprinted, 1996). No. 1/90 (US$10)Foundation Properties of Marble and Other Rocks in the Yuen GCO Publication HK$58Long - Tuen Mun Area (1990), 117 p. No. 2/90 (US$10)Review of Earthquake Data for the Hong Kong Region (1991), GCO Publication HK$42115 p. No. 1/91 (US$11.5)Review of Granular and Geotextile Filters (1993), 141 p. GEO Publication HK$32No. 1/93 (US$19)Pile Design and Construction (1996), 348 p. (Reprinted, 1997). GEO Publication HK$62No. 1/96 (US$13.5)Report on the Kwun Lung Lau Landslide of 23 July 1994, 2 - FreeVolumes, 400 p. (English Version), (Reprinted, 1996).Report on the Fei Tsui Road Landslide of 13 August 1995, - Free2 Volumes, 81 p. (Bilingual). %>MH a $£mmmm\


Report on the Shum Wan Road Landslide of 13 August 1995,2 Volumes, 63 p. (Bilingual).FreeWhat to Do When You Receive a Dangerous Hillside Order(1996), 16 p. (Bilingual).Free(Hong Kong) Rainfall and Landslides in 1984, by J. Premchitt(1991), 91 p. plus 1 drg. (Reprinted, 1995).(Hong Kong) Rainfall and Landslides in 1985, by J. Premchitt(1991), 108 p. plus 1 drg. (Reprinted, 1995).(Hong Kong) Rainfall and Landslides in 1986, by J. Premchitt(1991), 113 p. plus 1 drg. (Reprinted, 1995).Hong Kong Rainfall and Landslides in 1987, by J. Premchitt(1991), 101 p. plus 1 drg. (Reprinted, 1995).Hong Kong Rainfall and Landslides in 1988, by J. Premchitt(1991), 64 p. plus 1 drg. (Reprinted, 1995).Hong Kong Rainfall and Landslides in 1989, by K.L. Siu(1991), 114 p. plus 1 drg. (Reprinted, 1995).Aggregate Properties of Some Hong Kong Rocks, by T.Y.Man, A. Cipullo, A.D. Burnett & J.M. Nash (1992), 212 p.(Reprinted, 1995).Foundation Design of Caissons on Granitic and VolcanicRocks, by T.Y. Man & G.E. Powell (1991), 85 p. (Reprinted,1995).GEO ReportNo. 1GEO ReportNo. 2GEO ReportNo. 3GEO ReportNo. 4GEO ReportNo. 5GEO ReportNo. 6GEO ReportNo. 7GEO ReportNo. 8HK$118(US$17.5)HKS126(US$20)HKS126(US$20)HKS122(US$19.5)HKSI06(US$16)HK$126(US$20)HK$120(US$19.5)HK$62(US$10.5)Bibliography on the Geology and Geotechnical Engineering ofHong Kong to December 1991, by E.W. Brand (1992), 186 p.(Superseded by GEO Report No.39)GEO ReportNo. 9Bibliography on Settlements Caused by Tunnelling, by E.W.Brand (1992), 50 p. (Reprinted, 1995). (Superseded by GEOReport No.51)GEO ReportNo. 10HK$48(US$8.5)Direct Shear Testing of a Hong Kong Soil under VariousApplied Matric Suctions, by J.K. Gan & D.G. Fredlund (1992),241 p. (Reprinted, 1995).GEO ReportNo. 11HK$136(US$21.5)


Rainstorm Runoff on Slopes, by J. Premchitt, T.S.K. Lam, J.M. GEO Report HK$121Shen and H.F. Lam (1992), 211 p. (Reprinted, 1995). No. 12 (US$19.5)Mineralogical Assessment of Creep-type Instability at Two GEO Report HK$87Landslip Sites, by T.Y. Irfan (1992), 136 p. (Reprinted, 1995). No. 13 (US$15)Hong Kong Rainfall and Landslides in 1990, by K.Y. Tang GEO Report HK$112(1992), 78 p. plus 1 drg. (Reprinted, 1995). No. 14 (US$17)Assessment of Stability of Slopes Subjected to Blasting GEO Report HK$75Vibration, by H.N. Wong & P.L.R. Pang (1992), 112 p. No. 15 (US$12)(Reprinted, 1995).Earthquake Resistance of Buildings and Marine Reclamation GEO Report HK$48Fills in Hong Kong, by W.K. Pun (1992), 48 p. (Reprinted, No. 16 (US$8.5)1995).Review of Dredging Practice in the Netherlands, by S.T. GEO Report HK$76Gilbert & P.W.T. To (1992), 112 p. (Reprinted, 1995). No. 17 (US$12)Backfilled Mud Anchor Trials Feasibility Study, by C.K. Wong GEO Report HK$50& C.B.B. Thorley (1992), 55 p. (Reprinted, 1995). No. 18 (US$9)A Review of the Phenomenon of Stress Rupture in HDPE GEO Report HK$56Geogrids, by G.D. Small & J.H. Greenwood (1993), 68 p. No. 19 (US$9.5)(Reprinted, 1995).Hong Kong Rainfall and Landslides in 1991, by N.C. Evans GEO Report HK$111(1992), 76 p. plus 1 drg. (Reprinted, 1995). No. 20 (US$ 16.5)Horizontal Subgrade Reaction for Cantilevered Retaining Wall GEO Report HK$50Analysis, by W.K. Pun & P.L.R. Pang (1993), 41 p. (Reprinted, No. 21 (US$9.5)1998).Report on the Rainstorm of 8 May 1992, by N.C. Evans (1993), GEO Report HK$126109 p. plus 2 drgs. (Reprinted, 1995). No. 22 (US$20)Effect of the Coarse Fractions on the Shear Strength of GEO Report HK$126Colluvium, by T.Y. Irfan & K.Y. Tang (1993), 223 p. No. 23 (US$20)(Reprinted, 1995).The Use of PFA in Reclamation, by J. Premchitt & N.C. Evans GEO Report HK$52(1993), 59 p. (Reprinted, 1995). No. 24 (US$9)Report on the Rainstorm of May 1982, by M.C. Tang (1993), GEO Report HK$ 135129 p. plus 1 drg. (Reprinted, 1995). No. 25 (US$21)


Report on the Rainstorm of August 1982, by R.R. Hudson(1993), 93 p. plus 1 drg. (Reprinted, 1995).Landslips Caused by the June 1983 Rainstorm, by E.B. Choot(1993), 124 p. (Reprinted, 1995).Factors Affecting Sinkhole Formation, by Y.C. Chan (1994),37 p. (Reprinted, 1995).Classification and Zoning of Marble Sites, by Y.C. Chan(1994), 37 p. (Reprinted, 1995).Hong Kong Seawall Design Study, by P.M. Aas & A. Engen(1993), 94 p. (Reprinted, 1995).Study of Old Masonry Retaining Walls in Hong Kong, by Y.C.Chan (1996), 225 p.Karst Morphology for Foundation Design, by Y.C. Chan &W.K. Pun (1994), 90 p. plus 1 drg. (Reprinted, 1995).An Evaluation of the Suitability of Decomposed Granite asFoundation Backfill for Gravity Seawalls in Hong Kong, byE.B. Choot (1993), 34 p. (Reprinted, 1995).A Partial Factor Method for Reinforced Fill Slope Design, byH.N. Wong (1993), 55 p. (Reprinted, 1995).Hong Kong Rainfall and Landslides in 1992, by P.K.H. Chen(1993), 201 p. plus 2 drgs. (Reprinted, 1995).Methods of Test for Soils in Hong Kong for Civil EngineeringPurposes (Phase I Tests), by P.Y.M. Chen, 1996 Edition, 90 p.Creep, Stress Rupture and Hydrolysis of Polyester ReinforcedGeogrids, by J.H. Greenwood (1995), 67 p.Skin Friction on Piles at the New Public Works CentralLaboratory, by J. Premchitt, I. Gray & K.K.S. Ho (1994), 158 p.(Reprinted, 1995).GEO ReportNo. 26GEO ReportNo. 27GEO ReportNo. 28GEO ReportNo. 29GEO ReportNo. 30GEO ReportNo. 31GEO ReportNo. 32GEO ReportNo. 33GEO ReportNo. 34GEO ReportNo. 35GEO ReportNo. 36GEO ReportNo. 37GEO ReportNo. 38HKS118(US$17.5)HKS83(US$13)HKS40(US$7.5)HKS40(US$7.5)HKS68(US$11)HKS130(US$21)HK$118(US$17.5)HK$38(US$7)HK$50(US$9)HK$I67(US$25.5)1IK$22(US$5.5)HK$38(US$7.5)HK$97(US$16.5)Bibliography on the Geology and Geotechnical Engineering ofHong Kong to May 1994, by E.W. Brand (1994), 202 p.(Reprinted, 1995). (Superseded by GEO Report No.50)Hydraulic Fill Performance in Hong Kong, by C.K. Shen &K.M. Lee (1995), 199 p.GEO ReportNo. 39GEO ReportNo. 40HK$I18(US$19)HK$90(US$16)


Mineralogy and Fabric Characterization and Classification ofWeathered Granitic Rocks in Hong Kong, by T.Y. frfan (1996),158 p.Performance of Horizontal Drains in Hong Kong, by R.P.Martin, K.L. Siu & J. Premchitt (1995), 109 p.Hong Kong Rainfall and Landslides in 1993, by W.L. Chan(1995), 214 p. plus 1 drg.General Report on Landslips on 5 November 1993 atMan-made Features in Lantau, by H.N. Wong & K.K.S. Ho(1995), 78 p. plus 1 drg.Gravity Retaining Walls Subject to Seismic Loading, by Y.S.Au-Yeung & K.K.S. Ho (1995), 63 p.Direct Shear and Triaxial Testing of a Hong Kong Soil underSaturated and Unsaturated Conditions, by J.K.M. Gan & D.G.Fredlund(1996),217p.Stability of Submarine Slopes, by N.C. Evans (1995), 51 p.Strength Development of High PFA Content Concrete, byW.C.^Leung & W.L. Tse (1995), 84 p.AAR Potential of Volcanic Rocks from Anderson RoadQuarries, by W.C. Leung, W.L. Tse, C.S. Mok & S.T. Gilbert(1995), 78 p.Bibliography on the Geology and Geotechnical Engineering ofHong Kong to March 1996, by E.W. Brand (1996), 111 p.Bibliography on Settlements Caused by Tunnelling to March1996, by E.W. Brand (1996), 70 p.Investigation of Some Major Slope Failures between 1992 and1995, by Y.C. Chan, W.K. Pun, H.N. Wong, A.C.O. Li & K.C.Yeo(l996),97p.Environmental Aspects of Using Fresh PFA as Fill inReclamation, by K.S. Ho & P.Y.M. Chen (1996), 46 p.Hong Kong Rainfall and Landslides in 1994, by W.L. Chan(1996), 161 p. plus 1 drg.GEO ReportNo. 41GEO ReportNo. 42GEO ReportNo. 43GEO ReportNo. 44GEO ReportNo. 45GEO ReportNo. 46GEO ReportNo. 47GEO ReportNo. 48GEO ReportNo. 49GEO ReportNo. 50GEO ReportNo. 51GEO ReportNo. 52GEO ReportNo. 53GEO ReportNo. 54HK$70(US$13.5)HK$65(US$17.2)HK$110(US$18.5)HK$64(US$17)HK$40(US$8)HK$65(US$12.5)HK$46(US$8.5)HK$60(US$10.5)HK$58(US$10)HK$45(US$9)HK$31(US$6.5)HK$44(US$8.5)HK$30(US$5.5)HK$70(US$13.5)


Conventional and CRS Rowe Cell Consolidation Test on SomeHong Kong Clays, by J. Premchitt, K.S. Ho & N.C. Evans(1996), 93 p.Application of Prescriptive Measures to Soil Cut Slopes, byH.N. Wong & L.S. Pang (1996), 52 p.Study of Rainfall Induced Landslides on Natural Slopes in theVicinity of Tung Chung New Town, Lantau Island, by C.A.M.Franks (1998), 102 p. plus 3 drgs. under preparation.Tsing Shan Debris Flow and Debris Flood, by J.P. King(1998), 215 p. plus 9 drgs. under preparation.Hong Kong Rainfall and Landslides in 1995, by C.K.L. Wong(1997), 125 p. plus 1 drg.Assessment of Geological Features Related to RecentLandslides in Volcanic Rocks of Hong Kong Phase 2 A - ChaiWan Study Area, by S.D.G. Campbell & N.P. Koor (1998),78 p. plus 6 drgs. under preparation.Factual Report on the November 1993 Natural TerrainLandslides in Three Study Areas on Lantau Island, by H.N.Wong, Y.M. Chen & K.C. Lam (1997), 42 p.Areal Extent of Intense Rainfall in Hong Kong 1979 to 1995,byA.W.Malone(1997),85p.A Review of Some Drained Reclamation Works in HongKong, by J.S.M. Kwong (1997), 53 p.A Study of Hydraulic Fill Performance in Hong Kong -Phase 2, by C.K. Shen, K.M. Lee & X.S. Li (1997)^265 p."Seismic Hazard Analysis of the Hong Kong Region, by C.F.Lee, Y.Z. Ding, R.H. Huang, Y.B. Yu, G.A. Guo, P.L. Chen &X.H. Huang (1998), 145 p. (Bilingual).GEO ReportNo. 55GEO ReportNo. 56GEO ReportNo. 57GEO ReportNo. 58GEO ReportNo. 59GEO ReportNo. 60GEO ReportNo. 61GEO ReportNo. 62GEO ReportNo. 63GEO ReportNo. 64GEO ReportNo. 65HK$35(US$7)HKS12(US$3.5)HK$264(US$39.5)HK$__(US$_JHKS70(US$14.5)HK.S296(US$43.5)HK$92(US$13.5)HK$43(US$8.5)HK$36(US$6.5)HK$150(US$25)HK$«0(US$15.5)Mineralogical and Fabric Characterization and Classification of GEO Report H K$ 106Weathered Volcanic Rocks in Hong Kong, by T. Y. Man (1998), No. 66 (LJS$ 17)113 p.


Assessment of Geological Features Related to RecentLandslides in Volcanic Rocks of Hong Kong Phase 2B~Aberdeen Study Area, by C.A.M. Franks, S.D.G. Campbell &W.W.L. Shum (1998), under preparation.The New Priority Classification Systems for Slopes andRetaining Walls, by C.K.L. Wong (1998), 117 p.Diagnostic Report on the November 1993 Natural TerrainLandslides on Lantau Island, by H.N. Wong, K.C. Lam &K.K.S. Ho (1998), 98 p. plus 1 drg. under preparation.Hong Kong Rainfall and Landslides in 1996, by C.K.L. Wong(1998), 84 p. plus 1 drg. under preparation.Site Characterisation Study - Phases 1 and 2, by N.P. Koor(1998), under preparation.Long-term Consolidation Tests on Clays from the Chek LapKok Formation, by D.O.K. Lo & J. Premchitt (1998), underpreparation.The Natural Terrain Landslide Study Phases I and n, by N.C.Evans, S.W. Huang & J.P. King (1998), under preparation.Natural Terrain Landslide Study the Natural Terrain LandslideInventory, by J.P. King (1998), under preparation.Geotechnical Area Studies Programme - Hong Kong andKowloon (1987), 170 p. plus 4 maps.Geotechnical Area Studies Programme - Central NewTerritories (1987), 165 p. plus 4 maps.Geotechnical Area Studies Programme - West New Territories(1987), 155 p. plus 4 maps.Geotechnical Area Studies Programme - North West NewTerritories (1987), 120 p. plus 3 maps.Geotechnical Area Studies Programme - North New Territories(1988), 134 p. plus 3 maps.Geotechnical Area Studies Programme - North Lantau (1988),124 p. plus 3 maps.GEO ReportNo. 67GEO ReportNo. 68GEO ReportNo. 69GEO ReportNo. 70GEO ReportNo. 71GEO ReportNo. 72GEO ReportNo. 73GEO ReportNo. 74GASP IGASPHGASPIEGASP IVGASPVGASP VIHK$66(US$11.5)HK$240(US$40)HK$150(US$25)HK$150(US$25)HK$150(US$25)HK$150(US$25)HK$150(US$25)


Geotechnical Area Studies Programme - Clear Water Bay(1988), 144 p. plus 4 maps.GASP VIIHKS150(US$25)Geotechnical Area Studies Programme - North East NewTerritories (1988), 144 p. plus 4 maps.GASP VIIIHKS150(US$25)Geotechnical Area Studies Programme - East New Territories(1988), 141 p. plus 4 maps.GASP IXHK5150(US$25)Geotechnical Area Studies Programme - Islands (1988), 142 p.plus 4 maps.GASP XHK$15()(US$25)Geotechnical Area Studies Programme - South Lantau (1988),148 p. plus 4 maps.GASP XII IKS 150(US$25)Geotechnical Area Studies Programme - Territory of HongKong (1989), 346 p. plus 14 maps.GASP XIIHKS150(US$25)Geology of Sha Tin, by R. Addison (1986), 85 p.Geology of Hong Kong Island and Kowloon, by P.J. Strange &R.Shaw (1986), 134 p.GeologicalMemoir No. 1GeologicalMemoir No. 2HKS50(US$9)HKS78(US$12.5)Geology of the Western New Territories, by R.L. Langford,K.W. Lai, R.S. Arthurton & R. Shaw (1989), 140 p.GeologicalMemoir No. 3HK$


Geology of North Lantau Island and Ma Wan by R. J. Sewell Sheet Report Free&J.W.C. James (1995), 46p. No. 4Geology of Ma On Shan by RJ. Sewell (1996), 45p. Sheet Report FreeNo. 5Geological Landscapes of Hong Kong (1998), 61p. (Bilingual). - HK$13(San Tin : Solid and Superficial Geology (1:20 000 map) MapHGM20, HK$80(1989), 1 map. Sheet 2Sheung Shui : Solid and Superficial Geology (1:20 000 map) Map HGM 20, HK$80(1992), 1 map. Sheet 3Kat O Chau : Solid and Superficial Geology (1:20 000 map) Map HGM 20, HK$80(1993), 1 map. Sheet 4Tsing Shan (Castle Peak) : Solid and Superficial Geology Map HGM 20, HK$80(1:20 000 map) (1988), 1 map. Sheet 5Yuen Long : Solid and Superficial Geology (1:20 000 map) Map HGM 20, HK$80(1988), 1 map. Sheet 6Sha Tin : Solid and Superficial Geology (1:20 000 map) Map HGM 20, HK$80(1986), 1 map. Sheet 7Sai Kung Peninsula : Solid and Superficial Geology (1:20 000 Map HGM 20, HK$80map) (1989), 1 map. Sheet 8Tung Chung : Solid and Superficial Geology (1:20 000 map) Map HGM 20, HK$80(1994), 1 map. Sheet 9Silver Mine Bay : Solid and Superficial Geology (1:20 000 Map HGM 20, HK$80map) (1992), 1 map. Sheet 10Hong Kong and Kowloon : Solid and Superficial Geology Map HGM 20, HK$80(1:20 000 map) (1986), 1 map. Sheet 11Clear Water Bay : Solid and Superficial Geology (1:20 000 Map HGM 20, HK$80map) (1989), 1 map. Sheet 12Shek Pik : Solid and Superficial Geology (1:20 000 map) Map HGM 20, HKS80(1995), 1 map. Sheet 13Cheung Chau : Solid and Superficial Geology (1:20 000 map) Map HGM 20, HKS80(1995)"l map. Sheet 14


Hong Kong South and Lamma Island : Solid and Superficial Map HGM 20, HK$80Geology (1:20 000 map) (1987), 1 map. Sheet 15Waglan Island : Solid and Superficial Geology (1:20 000 map) Map HGM 20, HK$80(1989), 1 map. Sheet 16San Tin: Solid Geology (1 : 20 000 map) (1994), 1 map. Map HGM20S HK$80Lo Wu : Superficial Geology (1:5 000 map) (1990). 1 map. Map HGP 5A. HKSI00Sheet 2-NE-DLo Wu : Solid Geology (1:5 000 map) (1990), 1 map. Map HGP 5B. HK$10()Sheet 2-NE-DDeep Bay: Superficial Geology (1:5 000 map) (1989), 1 map. Map HGP 5A. HK$! 00Sheet 2-SW-CDeep Bay : Solid Geology (1:5 000 map) (1989), 1 map. Map HGP 5B, HK$100Sheet 2-SW-CShan Pui: Superficial Geology (1:5 000 map) (1989), 1 map. Map HGP 5A, HK$ 100Sheet 2-SW-DShan Pui: Solid Geology (1:5 000 map) (1989), 1 map. Map HGP 5B, HK$100Sheet 2-SW-DMai Po: Superficial Geology (1:5 000 map) (1990), 1 map. Map HGP 5A, HK$100Sheet 2-SE-AMai Po : Solid Geology (1:5 000 map) (1990), 1 map. Map HGP 5B. HK$10()Sheet 2-SE-ALok Ma Chau : Superficial Geology (1:5 000 map) (1990), Map HGP 5 A, HK$1001 map. Sheet 2-SE-BLok Ma Chau: Solid Geology (1:5 000 map) (1990), 1 map. Map HGP 5B, HK$1()()Sheet 2-SE-BMan Kam To : Superficial Geology (1:5 000 map) (1990), Map HGP 5A, HK$l()01 map. Sheet 3-NW-CMan Kam To: Solid Geology (1:5 000 map) (1990), 1 map. Map HGP 5B, HK$I00Sheet 3-NW-CTin Shui Wai : Superficial Geology (1:5 000 map) (1989), Map HGP 5A, HK$1001 map. Sheet 6-NW-A


Tin Shui Wai: Solid Geology (1:5 000 map) (1989), 1 map. Map HGP 5B, HK$100Sheet 6-NW-AYuen Long : Superficial Geology (1:5 000 map) (1989), 1 map. Map HGP 5A, HK$ 100Sheet 6-NW-BYuen Long : Solid Geology (1:5 000 map) (1989), 1 map. Map HGP 5B, HK$ 100Sheet 6-NW-BHung Shui Kiu : Superficial Geology (1:5 000 map) (1989), Map HGP 5A, HK$1001 map. Sheet 6-NW-CHung Shui Kiu : Solid Geology (1:5 000 map) (1989), 1 map. Map HGP 5B, HK$100Sheet 6-NW-CMuk Kiu Tau : Superficial Geology (1:5 000 map) (1990), Map HGP 5A, HK$1001 map. Sheet 6-NW-DMuk Kiu Tau : Solid Geology (1:5 000 map) (1990), 1 map. Map HGP 5B, HK$100Sheet 6-NW-DTsuen Wan (Part): Solid & Superficial Geology (1:5 000 map) Map HGP 5, HK$100(1995), 1 map. Sheet 6-SE-DMa On Shan : Solid Geology (1:5 000 map) (1996), 1 map. Map HGP 5B, HK$100Sheet 7-NE-D,C (part)Click Lap Kok : Solid & Superficial Geology (1:5 000 map) Map HGP 5, HK$100(1993), 1 map. Sheet 9-NE-C/D'lung Chung Wan : Solid & Superficial Geology (1:5 000 map) Map HGP 5, HK$100(1995), 1 map Sheet 9-SE-APok To Yan : Solid & Superficial Geology (1:5 000 map) Map HGP 5, HK$100(1W7). 1 map.Sheet 9-SE-BLantau Peak : Solid & Superficial Geology (1:5 000 map) Map HGP 5, HK$100(19%), 1 map. Sheet 9-SE-CSunset Peak : Solid & Superficial Geology (1:5 000 map) Map HGP 5, HK$100( 1996), 1 map. Sheet 9-SE-DYam O Wan : Solid & Superficial Geology (1:5 000 map) Map HGP 5, HK$100(1995), 1 map. Sheet 10-NW-B


Siu Ho : Solid & Superficial Geology (1:5 000 map) (1994), Map HGP 5, HK$1001 map. Sheet 10-NW-CChok Ko Wan (Penny's Bay) : Solid & Superficial Geology Map HGP 5, HK$100(1:5 000 map) (1994), 1 map. Sheet 10-NW-DMa Wan : Solid and Superficial Geology (1:5 000 map) (1994), Map HGP 5, HK$ 1001 map. Sheet 10-NE-ATsing Yi : Solid & Superfical Geology (1:5 000 map) (1995). Map HGP 5. HK$ 1001 map. Sheet 10-NE-B/DPa Tau Kwu : Solid and Superficial Geology (1:5 000 map) Map HGP 5. HK$l00(1994), 1 map. Sheet 10-NE-CTai Ho : Solid and Superficial Geology (1:5 000 map) (1995), Map HGP 5. H K$ 1001 map. Sheet 10-SW-AORDERING INFORMATION IS GIVEN ON THE NEXT PAGE

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