Riprap, Flow-Through Rockfill, and Reinforced Rockfill

Riprap, Flow-Through Rockfill, and Reinforced Rockfill

Manual Chapters• Chapter 7: Flow-through rockfill andreinforced rockfill– Lead author: Bob DeweyEmbankment dam design• Chapter 8: Riprap– Lead author: Tony WahlHydraulics laboratory

Rockfill Caveats• Reinforcement is common for rockfill dams• Reinforced rockfill is used to stabilize concrete structures• There are no examples of using reinforced rockfill to protect anembankment dam against overtopping flow or through-flow• There are no examples of reinforcement being added toearthfills with existing downstream shells considered to berockfill• There is too much chance of dam failure to consider such asystem in the design phase of a new or modified high orsignificant hazard earthfill embankment dam.

Simplifications• Existing equations do not take into account the nonhomogeneousand anisotropic nature of a rockfill placement• Rockfill placed in lifts often has variable gradation and density,even within one lift• Vibratory compaction produces a thin, fine-grained layer at topof each lift• Rock materials tend to break down the closer they are to thecompaction machinery• Vertical permeability is less than horizontal, often much less• Some rockfills, particularly the shells of embankments damsthat are called rockfill, have layers or entire zones withexcessive small stones that cause earthfill-type behavior

Design of Rockfill• Size of rock and slope of fill will be limited byeither:– Flow ThroughFlow Over• Hard to know which will control, so bothshould be analyzed– If more than 30% smaller than 1-inch, then itbehaves more like earthfill and Flow Over willprobably control

Allowable Through-FlowDownstream Slope(H:V)Dominant size ofrock in slope(in.)Permissible Flow Through Rockfill (cfs/ft)LooseDense1.5:1 24 4 101.5:1 48 15 401.5:1 60 20 555:1 12 5 155:1 24 20 155:1 36 35 955:1 48 55 1505:1 60 75 20010:1 12 15 4010:1 24 45 12010:1 36 80 22010:1 48 120 33010:1 60 170 470

Allowable overflow• Hartung &Scheuerlein (1970)• Max q versusdownstream slopefor three rock sizes

Mass Slope Stability• Mass or global slope stability must be considered• A slope stability analysis of deep seated failure surfaces isnecessary• Seepage forces must be included in static slope stabilityanalysis to evaluate stability of a flow-through rockfillembankment• Overtopping flow forces should be added to a slope stabilitymodel• Most computer stability tools are set up to solve these types ofproblems, but the challenge to the analyst is to accuratelyestimate the seepage forces for turbulent flow– Flow nets from a laminar seepage analysis are not applicable– More research is needed on forces induced by turbulent flow

Effect of Reinforcement onGlobal Slope Stability• Most reinforcement is intended to protectonly the surface of the downstream slope– Unless reinforcement is designed for a dualpurpose, it is advised to conservatively ignorereinforcement during slope stability analysis

Filter Compatibility• Filter compatibility is required between theouter layers of a rockfill zone, (the armorprotection), and the inner zones of anembankment dam• Filter compatibility must be satisfied by allmaterials in the embankment• This may require multiple layers of graduallysmaller particles from outside to inside• e.g. D 15,coarse < 5*D 85,finer

Reinforcement• Rocks kept in place on slope with a steel reinforcement meshon the surface.• Mesh size related to smallest rock that could be dislodged fromthe downstream outer face of the embankment slope• The mesh should have sufficient strength to resist the tractiveand seepage forces acting on the surface particles• If overtopping occurs, the mesh needs to also withstand theimpact forces of debris carried by the overflow• Materials:– Usually steel reinforcement bars tied together (no. 7 bars or larger)– Chain link fencing and welded wire are weaker alternatives,vulnerable to debris impacts

Reinforcement• To best prevent debris from catching on meshduring overtopping, horizontal bars are placedagainst the fill and the vertical bars are attachedabove the horizontal steel• Large rockfill reduces the cost of reinforcement byallowing more widely spaced bars• Equal horizontal and vertical spacing not needed– Example is Pit 7 afterbay where no. 7 bars were spaced at 10-footcenters on the horizontal and at 1-foot centers on the vertical• Horizontal bars connected to vertical bars wherethey cross with clamps or other devices to maintainthe shape of the mesh

Anchor Bars• Reinforcing mesh is attached to embankment slopewith anchor bars• Anchor bars are embedded into the embankmentbeyond the critical shear surface to a depthsufficient to transfer the design loads in the bars tothe surrounding rockfill and eliminate the possibilityof premature pullout• Parkin suggested an “unsafe” zone between the faceof the downstream slope and a line parallel to thatslope at a distance back into the fill of 2/3 of theembankment height

Anchor Bars• Alternatives to embed the anchors into the rockfillinclude crank-shaped anchors, anchors fixed togrouted dowels in the fill and inclined anchors• Vertical spacing of anchor bars is not an exactscience• Spacing should be close enough to prevent criticalshear surfaces from exiting between the layers ofreinforcement• Along edges, connect reinforcement system tofoundation and abutments with rock bolts or othersolid means

Anchorage Options

Reinforcement - Extent• To resist through-flow, reinforcement should extendwell above the height of the seepage exit elevation• To resist flow over an embankment, thereinforcement should extend over the entiredownstream face, abutment to abutment• Designs should also ensure crest stability duringovertopping• Rockfill is largest and reinforcement would beheaviest at the downstream toe of an embankmentsubject to overtopping

Reinforcement References• Many uncertainties mentioned already mean that methodologyfor the design of rockfill reinforcement is rather empirical• Designs are copied from previous successful dams performingsimilar functions• Good source of info is 1982 report prepared by the AustralianNational Committee on Large Dams (ANCOLD)– 50 reinforced rockfill dams and cofferdams– 18 overtopped and 5 failed• Australian National Committee on Large Dams (ANCOLD),“Report on Mesh Protection of Rockfill Dams and CofferDams,” March 1982.

Corrosion• Corrosion shortens the life of steel reinforcement• Carbonaceous rockfill materials should be avoideddue to their galvanic effect and because of their highelectrical conductivity• If reinforcement becomes buried by saturated soils,corrosion will be influenced by the quality and pH ofthe water, soluble salt content of the overlying soiland aeration

Fighting Corrosion• Substitute nonmetals for metal reinforcement• Use corrosion resistant metal alloys• Protective coatings• Corrosion monitoring systems• Cathodic protection

Fighting Corrosion - Coatings• First line of defense• Zinc, (galvanized coatings), has a limitedlife…sacrifices itself to protect the steel• Zinc is conductive and would require more electricalenergy if cathodic protection were ever added to theprotection scheme• Epoxy has small discontinuities that leave somesmall areas exposed to corrosion• A corrosion monitoring system can tell whencathodic protection might be necessary if all othertypes of protection do not work.

Vulnerabilities and Risk• There is no evidence in the literature of rockfill beingused as a veneer of armor to protect an existingsignificant hazard or high hazard embankment damfrom overtopping or through flow• The introduction of reinforcement into rockfillincreases safety and compensates, to some degree,for analysis uncertainties• Unreinforced rockfill for such a function is notadvised• If reinforced rockfill is to be used for protection of anexisting dam, it should be rather massive

Vulnerabilities and Risk• Groins and areas of flow concentrationshould be most heavily protected• Ensuring stability of the dam crest shouldnot be forgotten• Reinforcement can degrade over time• Surface meshes can be damaged by rocksand logs in overtopping flows• Chemical attack can corrode steelreinforcement

Examples• The reinforcement of rockfill dams isordinarily designed empirically, by copyingdesigns of older dams performingsuccessfully• Two examples that have successfullywithstood overtopping many times:– Pit 7– Des Arc Bayou Site No. 3.

Pit 7

Pit 7 Afterbay Dam – Calif.• Early design for a rockfill with reinforcementthat has been used as a basis of manysubsequent dams (Leps 1973)• Afterbay for Pacific Gas & Electric’s Pit No. 7Powerhouse• Subjected to continuous through-flow andfrequent overflow, with normal flows rangingfrom 2,000 to 6,650 cfs and maximum flowsup to an estimated 85,000 cfs

Pit 7• 36-ft-high rockfill, about 555 ft long• Crest width is 20 ft• Upstream slope 2:1• Downstream slope 2¼:1• Toe berm of reinforced rock about 20 ft wide• Downstream slope reinforced with a surface grid of No. 7 andNo. 8 steel bars, tied back at 3-ft vertical intervals with hooked,37-ft-long, No. 7 bars• Pullout resistance is mobilized along the entire 37-ft-longanchor bars to hold the surface mesh in place. All rock within 4feet of the surface is at least 12 inches in size and the rock inthe toe-berm has a minimum size of 24 in.

Pit 7 - Performance• Leps (1973) - “After a little over 3½ years ofsuccessful operation of the dam, there was somewear and dislocation of the bars, and about 1400 yd 3of rock had been washed away from the downstreamface. In addition, there was a slight bulging of thelower part of the downstream slope and somesagging of the upper part, neither of which hadexceeded 3 ft. The lost rock was replaced in 1968,and additional No. 8 bars were incorporated in thegrid on the downstream face to inhibit further loss ofrock.”

Des Arc Bayou No. 3• NRCS – Arkansas• Rockfill dam with central clay core• Note the short and uniform anchor lengthscompared to the height of the dam indicatingthat the reinforcement is primarily there tohold the surface mesh, not to enhance globalslope stability

Des Arc Bayou Site No. 3.

Des Arc Bayou - Details

Des Arc Bayou – Construction Sequence

Riprap• Subject of Chapter 8• Riprap added as a protective layer over anexisting embankment

Riprap – How and Why• Riprap prevents erosion during overtopping flow byconveying overflow through and above the ripraplayer, reducing velocities and shear stresses againstthe underlying embankment• Often cheaper than other alternatives if rock isavailable nearby• Commonly used in arid zones and on steep slopeswhere vegetation is difficult to maintain• May not be cheaper for steep slopes or high flowrates, where very large rock sizes may be needed

Recent Research• Most has focused on sizing rock anddetermining allowable flow rates• Some research on the hydraulic roughnessand energy dissipation produced by flowdown a riprap slope• General trend has been for riprap testing tomove from flatter to steeper slopes over time

What Type of Rock?• Granite or limestone• Angular• Uniformly sized rock– Gap graded and well graded mixes fail at lowerflow rates

Sizing of Rock• Manual gives design guidance based onthree research studies• Abt and Johnson (1991)– Slopes of 2% to 10%• Robinson et al. (1998)– Slopes of 17% to 40%• Frizell et al. (1998)– Slopes of 40% to 50%

Design Guidance• Similarities– Angular riprap– D 50 < 24 inches (limits of testing)– Riprap layer thickness 2*D50 or greater– Bedding material satisfying filter criteria• Differences– Abt and Robinson relate q allowable to D 50 and slope– Frizell method incorporates variable specificgravity, porosity, and coefficient of uniformity(D 60 /D 10 )

Basic Design Concept• Much of the overtopping discharge isconveyed through the riprap layer –interstitial flow• On slopes flatter than 25%, some flow abovethe rock may be tolerable• On steep slopes, all flow is interstitial

Example: Fixed D 501000D50 = 1.5 ftAbt & Johnson (1991)Robinson (1998)100Frizell, design eqnq a, cfs/ft1010% 10% 20% 30% 40% 50%Slope

Example:4.5q = 30 cfs/ftFixed q allowable043.5Abt & Johnson (1991)Robinson (1998)Frizell, design eqn3D 50 (ft)2.521.510.50% 10% 20% 30% 40% 50%Slope

• Upper StonevilleReservoir Dam –Auburn MA• Riprap overtoppingprotection detailed in2002 ASDSO paper“Throwing Rocks atthe ½-PMF” byWooten and Wood(GEI Consultants)• High hazard(downstream homes,roads)

Upper Stoneville20-ft high, 400-ft long2:1 downstream slopeQ ovtop = 14 cfs/ftD 50 =1.5 ftThickness= 4*D 50$650,000


More magazines by this user
Similar magazines