A solution was provided by Terzaghi in his design of theVigario dam in 1947 by using a central vertical core of filtermaterial to drain leakage and prevent the phreatic surfacereaching the downstream slope. Examples of this design areshown in Figure 18.15.

where necessary, the strength was usually reduced sufficiently(cu= 10 to 15 kN/m2) to enable the puddled clay to be com-pacted down to an air void of about 5% by the heeling of thepuddling gang.

Increasing cost of labour and unsuitability of compactingmachinery of the time for working with puddled clay caused thechange to rolled clay cores. The era of British puddled clay corescame to an end during the 1950s, although there were still damsoccasionally built with these cores until 1970.

The use of earth rather than rockfill for the shoulders posesthe problem of construction pore pressures. In areas of highrainfall and/or when the borrow material is wet, compression ofthe fill under self-weight as height is increased can produceundesirable pore pressure which, in the extreme, may endangerstability by preventing the required increase of effective stresseswithin the fill.

This problem is overcome by use of drainage layers placed inthe fill during construction. These reduce the length of thedrainage path which the pore water must traverse to escape. Thetime required for dissipation of pore pressure is proportional tothe square of the length of the drainage path, so drainage layersare particularly effective in reducing pore pressures in shoulderfill during construction.

Granular material for the drainage layers is often graded sothat it acts as a filter to prevent loss of fines from the fill, but thisaspect is not of prime importance in the downstream shoulder,where volume of escaping pore water is unlikely to causesignificant particle migration. In the upstream shoulder, how-ever, there is a danger that fluctuations of reservoir level couldproduce damaging flows if the layers do not act as effectivefilters.

Example 18.4

The Backwater dam (43 m) was constructed during the period1964-69 (Figure 18.16). At the site, boulder clay overlies aschistose grit and micaceous schist. The embankment was builtfrom compacted boulder clay placed in layers with a slightoutward fall to help shed rainwater. Granular drainage layerswere placed at about 7 m vertical intervals.

The core was also constructed of boulder clay, carefullyselected in the borrow pits to contain least stones and most clay.It was placed over a grout curtain cutoff on to a preparedsurface. It was separated from the downstream shoulder by adrainage filter which connected to the drainage layers and themain underdrainage layer separating shoulder fill from founda-tion.

18.4 Concrete dams

18.4.1 Introduction

Use of certain volcanic ashes by the Romans to cement togethersand and gravel into a reconstituted rock is often regarded as thefirst concrete. The aggregates used for modern concrete, such aswell-graded sands and gravels, and hard rock that has beencrushed, sieved and graded to a designed grading, would formexcellent fill, even without the addition of cement. The ad-ditional strength imparted to it by the cement enables lessvolume to be used, so redressing the high cost of production.

18.4.2 Gravity dams

The simplest type of concrete dam has a gravity section, i.e. it isheavy enough not to be overturned by horizontal thrust fromthe reservoir water. The foundations have to be relatively strongto support the large weight and they should not be subject to

Figure 18.15 Homogeneous earth dams with Brazilian section,(a) Vigario Dike; (b) Santa Branca; (c) Ponte Coberta; (d) Euclidesda Cunha; (e) Limoeira; (f) Graminha

18.3.7 Earthfill dam with central clay core

Central cores of puddled clay were used in the traditional Britishdam in the nineteenth century. It had an upstream slope of 1 in 3and a downstream slope of 1 in 2.5. The puddled clay core wasusually taken down in trench to form a below-ground water-stop.

The fissures, bedding planes, silt layers, etc. found in depositsof clay can give it a relatively high in-situ permeability. Theaction of puddling destroys this fabric and, by addition of water

DrainsGneissResidual soil

Gneiss soil, Drain curtain

Gneiss Grout curtainResidual soil

Gneiss soil Draincurtain

Grout curtainTalus1

Drain curtainGneiss soil

DrainsGroutcurtain

Gneiss soil Draincurtain.

Gneiss soil Draincurtain

Drains

398.0 WL normal

Gneiss soil Draincurtain

Drains

Figure 18.16 Backwater dam: section showing drainage layers,(a) Glacial till core; (b) glacial till shoulder; (c) chimney drain;(d) drainage layers; (e) rubble toe; (f) spoil; (g) concrete block;(h) riprap

long-term settlements that could be caused by consolidation ofan underlying clay.

The heat liberated by hydration of thfc cement in concrete canproduce damaging temperature rises in large masses of concrete.As with the problem of dissipating construction pore pressuresin earthfill shoulders, so consideration has to be given tolimiting the temperature rise which will occur in a gravity damas it is being built. The counterparts of drainage layers are layersof cooling pipes placed in the mass concrete as it is being cast. Arefrigeration plant is used to lower the temperature of brinecirculated through the cooling pipes.

It is usual to construct a gravity dam as several separatesections or, sometimes, as large monoliths. When temperatureshave fallen sufficiently so that little more cooling shrinkage islikely to occur, the joints between the sections are grouted or themonoliths are connected with infilling sections to form thecomplete dam.

Temperature rise can be reduced by slow rates of construc-tion, use of slow-setting, coarse-ground cement and use ofrelatively inert cement replacements such as power station flyash.

Drainage is essential in a gravity dam to limit uplift pressures.In effect, the upstream face forms the waterproof element,supported by the remainder of the massive dam. Reservoirwater percolating along the interface between dam and founda-tion or along any of the horizontal lift joints could developdestabilizing uplift pressures if not safely drained away. Drain-age galleries and shafts are formed in the body of the dam. Agallery is usually provided close to the foundation on theupstream side from which either additional drainage or groutingholes can be drilled into the foundation if found to be necessaryduring reservoir operation.

Compressive stress in the concrete forming the upstream faceshould always exceed reservoir water pressure to avoid tensilecracking.

Example 18.5

The Grand Dixence dam (285 m) was constructed during theperiod 1953-62 (Figures 18.17 and 18.18). This remained theworld's highest dam for 18yr until it was exceeded by theRussian Nurek (300 m) embankment dam.

The Grand Dixence dam was built in the Swiss Alps acrossthe River Dix on sound bedrock. Its 695m crest length wasdivided into 16 m blocks connected by two copper sealing stripsto form a continuous, watertight upstream face. The blocks,

particularly in the lower part of the dam where it was very wide(201 m maximum) were further divided by joints to allow forsome shrinkage movements on cooling. The joints were groutedup only after the concrete temperature had fallen below 6° C.

Cooling was effected with layers of pipes placed on every3.2 m lift. About 1000 km of 20 mm bore pipes were installed tocirculate the cooling water.

The waterproof, upstream face was made with concretecontaining 250kg of cement per cubic metre so as to bewatertight and frost-resistant. Concrete in other parts of thedam contained 140 to 300kg/m3 of cement according to calcu-lated stresses. Air-entraining agent was used to produce 3 to 4%air void in order to improve workability. Maximum aggregatesize was 120mm.

The concrete was placed with the aid of four cablewaysspanning the valley over the dam. On the right bank they wereattached to mobile carriages anchored to rails so that the cablescould be brought over almost every part of the dam. Thecableway buckets could carry 6 m3 of concrete. After dischargefrom the buckets, the concrete was spread by small bulldozers.It was compacted with vibrating pokers: frames of five pokerswere attached to the blades of other bulldozers so that theycould be moved about and lowered into the concrete. At thetime, this was considered to be the first use of earthmovingmachinery on a concrete dam.

18.4.3 Rollcrete dams

The term 'rollcrete' was coined by Lowe10 to name a newapproach to concrete placement. Increasing labour costs weremaking the labour-intensive concrete dams less competitivethan the embankment dam, even on sites suitable for a concretedam. To help redress the balance, Lowe proposed using arelatively dry, lean concrete placed by earthmoving machineryand compacted by smooth vibrating rollers. His first use of thematerial was to construct the core of the cofferdam for Shihmendam, Taiwan in 1961-62.

At the same time, a similar approach was being used in Italy.Gentile11 designed the 175 m high Alpe Gera dam for placementby earthmoving machines. He used a blastfurnace cement toreduce heat of hydration, but also provided an adequatenumber of open contraction joints to allow for shrinkage. Thegravity section, shown in Figure 18.19, was designed only tosupport the forces imposed by the reservoir; it was not intendedto be watertight. A 3 mm thick steel plate was used to form thewaterproof element on the upstream face of the dam. Maximum

Grouted cutoffGrout curtain

Figure 18.17 Grand Dixence dam: cross-section

Crest

Figure 18.19 Alpe Gera dam; rollcrete with sheet steel waterproofelement

compressive stresses in the concrete were calculated to be in therange 3.25 to 5.60 MN/m2.

The concrete was made from an alluvial sand and gravel witha cement content of 115 to 300kg/m3 according to expectedstresses in the dam. It was transported from the batching plantby funicular railway to placement level and carried across thesurface of the new concrete in 6m3 dumpers, spread to 0.8mthickness by angle dozers and compacted by tractor-mountedvibrating pokers. Contraction joints were cut through thefreshly placed concrete by a manganese steel blade 3 m long and1 m high carried on a wheeled chassis and pressed down with aforce of 2.51 by two hydraulic jacks while being vibrated at afrequency of 50 Hz.

No cooling was employed and maximum measured tempera-ture in concrete with a cement content of 150kg/m3 reached350C. Concrete containing 115kg/m3 cement reached a maxi-mum of 3O0C. The dam was built during three-and-a-half6-month summer periods from August 1961 to the end ofSeptember 1964.

Wallingford12 also proposed the use of rolled concrete toreduce the cost of a gravity section. This concept was followedby research studies, described by Moffat,13 at the University ofNewcastle into the properties of dry lean concrete as a potentialmaterial for dam construction by earthmoving machinery. Thedam section proposed by Moffat (Figure 18.20) used a poured-bitumen sheet as the waterproof element in the upstream facingmade of precast units. Following a study of temperature rises inthe Upper Tamar dam, Dunstan14'15 outlined a dam sectioncontaining low cement content concrete placed in continuous0.6 m layers between upstream and downstream facings. These

Figure 18.18 Grand Dixence dam

facings were to be laid as horizontally slipformed kerbs, with asheet of reinforced butyl rubber inside the upstream facing asthe waterproof element. Dunstan's further work16 led him to usealmost an excess of fly ash to ensure that voids between largerparticles were completely filled, and in order to improve worka-bility, reduce water content to very small amounts and producea concrete of low permeability.

Dunstan supervised construction of a trial bank at the site ofWimbleball dam in 1979. The bank was contained betweenslipformed kerbs (Figure 18.21). The cementitious content ofthe concrete was 0.75 fly ash and 0.25 cement. Each cubic metreof concrete contained 85 kg cement. It was compacted in 0.3 mlayers by a 71 duplex vibrating roller. The very cohesive mix didnot segregate, but flowed under the vibrating action of theroller. This produced a hard surface on which the laser-guided,offset slipformer could immediately run to lay a further lift ofkerb. Several different time intervals were used* between lifts,and subsequent tests on cores which were axially drilled forwater-pressure tests showed no leakage at the lift joints. Thework was in preparation for the Milton Brook dam but finan-cial restrictions caused postponement of construction.

In the US, roller compacted concrete has been used to buildseveral dams (Table 18.5). One of the first was the 37m highWillow Creek dam, completed in 1982. The construction jointsproved to be fairly porous, causing the downstream face to befairly wet, giving a clear indication of reservoir level. The jointshave since been grouted to reduce leakage.

Rapid construction is one of the attractive features of roller-compacted concrete dams: some times of construction are givenin Table 18.5. As an example of the concrete mix, that used forthe Upper Stillwater dam contained only 77 kg cement per cubicmetre, with 170 kg fly ash and 107 kg water.

In Japan, roller compaction was used to build the 89 m highShimajigawa dam completed in 1980. Previously a base padover the fractured rock foundation for the concrete gravityOkawa dam (75 m) used the roller-compacted dam method in1978. The mat had an average thickness of 25 m and its volumeof 300 000 m3 was placed in 9 months.

The use of roller compaction for these dams has beenfollowed by its use for the construction of the 10Om highTamagawa dam which has the fairly large (for a gravity dam)volume of 1.14 x 106m3. The maximum size of aggregate,previously limited to 80mm, has been increased to 150mm. Asection of the dam is shown in Figure 18.22 and the mixes usedin the various parts of the dam are given in Table 18.6. Therolled concrete was placed in 0.75 m thick layers and compactedby twelve passes of a Bomag BW-200 vibrating smooth rollerweighing 71.

Other Japanese roller compacted dams are listed in Table18.7.

In South Africa, rollcrete was used for the first time toconstruct the lower half of the concrete section of the 52 m highBraam Raubenheimer dam in eastern Transvaal. The concretegravity section is 33 m high and contains 20 000 m3 of rollcreteplaced in 1984.

It has also been used to construct the Zaaihoek dam and the30 m high De Mist Kraal weir on the Little Fish River in theEastern Cape. Figure 18.23 shows a section of this weir. Rapidconstruction was achieved, with 34000m3 of rollcrete beingplaced in 26 days during the winter of 1986. The contents ofeach cubic metre are given in Table 18.8.

The 36 m high gravity section of the Arabic dam on theOlifants River, 27 km north of Marble Hall was constructed ofrollcrete, completed in 1987.

18.4.4 Buttress dams

The weight and, therefore, volume (and cost) of a gravity damcan be greatly reduced by retaining only the upstream face andsupporting it by buttresses instead of the whole mass of thegravity section. To utilize some of the reservoir water pressureto resist overturning moment, the upstream face is usuallysloped. The water pressure acting on it has a vertical componentthat replaces some of the weight of a gravity section as well asthe horizontal component producing the overturning moment.

Bitumen SectionFigure 18.20 Proposed dry lean concrete dam with bituminouswaterproof element. (After: Moffat (1973) 'A study of dry leanconcrete applied to the construction of gravity dams'. Transactions,11th international congress on large dams, Madrid)

BitumenUpstream Downstream

Details

A plane upstream face, spanning between buttresses, wouldhave to resist bending moments. Some early milldams in timberhad a plane deck at a relatively flat slope, e.g. 1 vertical:3horizontal supported on wedge-shaped timber frames. Theintroduction of reinforced concrete at the beginning of thetwentieth century enabled larger, plane-faced dams to be built.Ambursen designed many dams of this type in the US, includingthe record-breaking 41 m high La Prele dam in Wyoming in1909. Schnitter17 reports that the dam contained only 43% of theconcrete needed for an equivalent gravity section. The dam is atan elevation of 1600 m and the severe climate had disintegratedmore than 20% of face thickness when a new slab was built in1977-79. The highest flat slab buttress dam (83m) was com-pleted in 1948 at Escaba in Argentina.

A more usual approach is to use an arch between buttresses,producing the multiarch buttress dam (Example 18.6).

Downstream

Figure 18.22 Tamagawa roller-compacted dam

Table 18.5 Roller-compacted concrete dams in the USA

Name of dam

Upper StillwaterPamo ValleyElk CreekWillow CreekGalesvilleMonksvilleMiddle Fork

Height(m)

87807652514638

State

UtahCaliforniaOregonOregonOrgeonNorth EastWestern Colorado

Construction time(weeks)

18

216

7

Date completed

1987c. 1987Designed 198319821985

1984

Figure 18.21 Milton Brook trial bank: roller-compacted concretebetween slipformed kerbs

Horizontally slipformed facings .

Rolled concrete heartingin 300-mm thick layers

Roller compacted core:

Table 18.6 Tamagawa dam - concrete mix designs

Position

Upstream faceDownstream faceFoundation contactMain body - rolledCrest sectionReinforced openings

Grade

ABCDEF

Aggregate size(max.) (mm)

15015015015015080

Cement(kg/m3)

16815412691

112189

Fly ash(kg/m3)

726654394881

Water(kg/m3)

11511210895

106138

Table 18.7 Roller-compacted dams in Japan

Name of dam

SakaigawaTamagawaShimajigawaAsahi Ogawa*Shin-NakanoManoShiromizugawatPirica

Height(m)

115.0100.0

90.084.074.969.054.540.0

Volume(Hl3XlO3)

6261140324350201212312360

River

SakaiTamaShimajiOgawaKamedaManoShiroizuShiribeshi

Construction

1988-921983-871977-801986-891979-821985-88Under construction1982-88

* Dam raised with roller-compacted concretef Composite with rockfill

Conventional exterior concrete

Rollcrete

Conventionalexteriorconcrete

Conventionalconcretearoundgallery

Blinding layer ofconventional concrete

Apron and end sill

No-fines concrete2500 X 3000 gallery

Figure 18.23 De Mist Kraal weir. Typical section through spillway

Example 18.6

The Meicende dam northwest Spain (2Om) completed 1961(Figures 18.24 and 18.25). Bedrock is granite and valley floorcontained outcrops of sound granite but some decomposed rockand a lode of feldspar aplite. Use of the multiarch designenabled buttresses to be positioned off areas of poorest quality.

The final design provided a maximum height of 20 m with: 11circular arches, each of 11 m internal radius and 1 m thick; 12buttresses, 2.5 m thick at 22 m centres, with arch springing faceat a slope of 1 vertical:0.577 horizontal.

In contiguous buttress dams, the upstream face may beformed by widening the upstream edges of each buttress so thatthey touch and are sealed by suitable waterstops, instead ofusing a flat slab or multiple arches.

Figure 18.25 Meicende dam: cross-section, where (1) is the archthickness, and (2) is the abutment between arches

Example 18.7

The Itaipu dam, Parana river (196m) constructed 1975-82(Figure 18.26, showing the double buttress of hollow gravityblock in river channel, and Figure 18.27, showing the diamond-headed buttress of the main dam).

This is an outstanding example of a contiguous buttressconcrete dam. It crosses the frontier between Brazil and Para-guay, storing water and providing head for the world's largesthydro-electric installation, designed for an output of12 600 M W.

The river section hollow-gravity dam, composed of double-buttress monoliths and the diamond-headed buttresses, isfounded on basaltic flows 15 to 50m thick. There are sub-horizontal discontinuities at different levels and special treat-ment was required to increase shear strength and reducecompressibility. Treatment included consolidation and contactgrouting, special drainage systems, as well as concrete keying atsome weak points.

Figure 18.24 Meicende dam. (a) Elevation; (b) plan, where (1) isan arch, (2) are spillways, and (3) is the intake

Table 18.8 Rollcrete mix for de Mist Kraal

Material Quantity(kg)

Water 105Portland cement 58Fly ash 58Sand 736Aggregate (75-37.5 mm) 805Aggregate (37.5-19 mm) 537Aggregate (19-9.5 mm) 268Aggregate (9.5^*.75 mm) 121

Conplast air-entraining agent 99 cm3

Compressive strengths (MN/m3)

7 days 8.128 days 13.31 yr 25.0

A preloading test was made by flooding the space between theupstream coffer dam and the main dam. This enabled water tobe raised almost 10Om against the major section of the dam.Good agreement was found between observed and predicteddeformations. It lent reassurance that the structure wouldbehave, as it did, in a satisfactory manner during the very rapid,irreversible impounding which took only 14 days. The mainconcrete section of the dam is flanked by embankment dams,forming a total length of 1172 m. Individual lengths are given inTable 18.9.

Table 18.9

M

Hollow gravity of double buttress monoliths 612Diamond-headed buttress 1450Mass gravity section 532Rockfill embankment 1984Earthfill embankment 3194

Figure 18.26 ltaipu dam: double buttress monolith of hollowgravity section in river channel, (a) Section; (b) plan, where (1) isthe grout curtain, (2) contact grouting, (3) consolidation andcontact grouting, (4) drainage holes, (5) drainage tunnel, (6) shearkeys, (7) dense basalt, (8) breccia, (9) vescicular amygdaloidalbasalt, (10) discontinuities, (11) contraction joints, and (12) is thepowerhouse

Figure 18.27 ltaipu dam: diamond-headed buttress of main dam.(a) Section; (b) plan, where (1) is the contraction joint, (2) groutcurtain, (3) consolidation grouting, (4) drainage holes, (5) drainagetunnel, (6) shear keys, (7) dense basalt, (8) breccia, (9) vescicularamygdaloidal basalt, and (10) represents discontinuities EL 125 andEL 112

EL va

riable

EL variable

Example 18.8

The diamond-headed buttress - Wimbleball dam, southwest Eng-land (63m) built 1975-79 (Figure 18.28).

The foundation consists of highly folded and contortedsandstones and siltstones with intercalated beds of slatey shalesand mudstones. To limit underseepage, grouting was extendedto 40m below formation level and relief wells were drilledbetween the buttresses. Fly ash was used in the concrete toreduce the heat of hydration. Contraction joints between dia-mond heads were sealed by a 300 mm rubber waterbar upstreamof a 200 mm Paracore plug.

Figure 18.28 Wimbleball dam: a diamond-headed buttress, (a)General arrangement; (b) section through dam and valve tower

The diamond heads, forming the upstream face of the dam,sloped at 1 vertical:0.383 horizontal.

The heights achieved by the various types of buttress dam areshown in Figure 18.29.

18.4.5 Arch dams

An arch dam can be likened to an arch bridge lying on its sidewith the abutments acting as springings for the arch. The archtransfers pressure from the reservoir water to thrust on theabutments, which must be strong enough to carry the thrustwithout permitting unacceptable deformations. The joints andbedding planes in many bedrocks provide potential weaknesses.Valley formation releases horizontal stresses and can be accom-panied by strains and movements such as cambering and valleybulging which can loosen the rock mass. Great care is needed toensure that the rock forming the abutments can accept both themagnitude and direction of the thrust that will come from thearch dam. As the reservoir filled for the first time, the inability ofthe left abutment rock structure to support the thrust from the61 m high Malpasset dam brought about its collapse on 2December 1959 causing serious damage to the downstreamtown of Frejus, and killing 421 people.

18.4.5.1 Arch gravity dam

A mass gravity dam requires much less weight to keep it stable ifit is built curved in plan (just as a piece of cardboard will standon its edge if it is curved). Stresses thrown on to the abutmentsare less than with a thin arch dam but, unlike a true gravity dam,no separate section would be able to support the reservoir waterpressure without the benefit of the arch action.

Spillwaychannel

Spillway

Tail bay

Pumphouse

Generator house

Valve tower

Drawoffpipework

Tailbay

Valve tower

YearFigure 18.29 Maximum heights of buttress dams since 1900

Example 18.9

The Pieve di Cadore dam, Italy (112m) constructed 1946-49(Figures 18.30 and 18.31). This dam is founded in dolomiticlimestones of the upper Trias. The modulus of elasticity of therock lay in the range 2000 to 3000 MN/m2: this was improved bygrouting to 5000 to 6000 MN/m2. The lowest part of the gorgewas filled in with a mass-concrete plug to a height of 57 m. Thearch gravity dam above this level was constructed in 33 mono-liths, each about 12 m wide.

The upstream face, varying in thickness from 1.5 m at the topto 4 m at the base, was made with concrete containing 250 kg ofcement per cubic metre. The remaining major part contained200kg/m3.

At the interface between the two concretes, there was adrainage system of 300 mm diameter pipes over the whole heightof the dam.

To allow cooling and shrinkage, alternate monoliths were

constructed ahead of the others. On completion, and aftersufficient cooling, the 34 vertical construction joints weregrouted and coated with bituminous material to protect anytensile cracking that might develop.

18.4.5.2 Double curvature arch

To avoid tensile stresses in an arch resisting reservoir waterpressure, some curvature is desirable in the vertical section.With a perfect shape, putting the concrete into pure compres-sion, the section could be relatively thin without exceeding thesafe working compressive strength of the concrete.

A model using an elastic sheet in pure tension, bulging underthe water pressure, gives the shape in mirror image for purecompression. In practice, some abutment deformation, temper-ature stresses and uplift pressures from water in the foundationinterface and joints in the concrete affect the pure compressivestress ideal and require greater thickness to be used.

Flat slab (Ambursen)

Contiguousbuttresses

Multiple arch

Max

imum

heig

ht (m

)

D. Jo

hnso

n

ltaip

u

S. C

hiara

Dixe

nce

Prele

Rodr

iguez

Figure 18.30 Pieve di Cadore dam: plan

Access road to construction plantand inspection shafts \)YV

Spillway waste channel

Access gallery to intakecontrol devices

Central controlroom

Monte Zovo cofferdam

Bottom outlei

Intake

Crest of dam(685.00)

Intermediateoutlet

SpillwayAccess to outletcontrol devices

Construction plant

Figure 18.31 Pieve di Cadore dam: cross-sections

(634.00)(627.00)

(635.84)

(660.00)

Concrete with 250 kg of.cement per cubic metre Concrete with 200 kg of

cement per cubic metre

Crest of dam(685.00)

Section B-B

Concrete with 200 kg ofcement per cubic metre

Concrete with 250 kg ofcement per cubic meter

Slide-gauge shaftsPendulum shaft

Concrete with 250 kg ofcement per cubic metre

300 mm diameter drain

Section A-A

Crest of dam(685.00)

Concrete with 200 kg ofcement per cubic metre

Crest of dam

Inspection shaft

Example 18.10

Neves dam, Italy (94.7m) constructed 1960-63 (Figures 18.32and 18.33). Neves dam is founded in a gneissic granite forma-tion. Design, based on a mathematical model using finiteelement techniques, was based on a modulus of elasticity for the

rock of 14000MN/m2 and a value of 28000MN/m2 for theconcrete in the lower third of the dam, 30 000 MN/m2 for theupper part. The radius of the arch increased with height asshown in Figure 18.34.

The dam was built in 12 sections, each about 15m long withconcrete containing 240 kg/m3 of pozzolanic cement.

Figure 18.32 Neves dam: layout

Downstream cofferdam

Watchhouse

Spillway

Lateral spillway L = 29.00 mOverflow sill at EL 1856

Bottom outlet gate controlroom

Gate controlroom

Normal WL(1856.00)

Bottom outlet entrance(formerly diversion tunnel)

Bottom outlet tunnelIntake structure

Power tunnel Drain outlet

Upstream cofferdam

Normal WL(1856.00)

Crest of dam(1857.66)

Normal WL Crest of damRaising

Concrete with 240 kg/m3 of pozzolaniccement, slightly reinforced at the faces j

Inspection bridges

Figure 18.33 Neves dam. (a) Main section with data; (b) mainsectionPerimetral joint

Drain outlet

Normal WL

18.5 Design concepts

18.5.1 Embankment dams - homogeneous section

Two important points must be borne in mind:

(1) It is essential to prevent the phreatic surface from reachingthe downstream slope. This is most effectively achieved byplacing a filter drain to separate the upstream and down-stream shoulders (see Figure 18.15, page 18/13).

(2) Foundation deformations and settlements over abutmentirregularities such as those indicated in Figure 18.35 maycreate tensile strains that can cause visible cracks and maypermit leakage to the central drain.

18.5.2 Embankment dams with central clay cores

The traditional British section had upstream slopes of 1:3 anddownstream slopes of 1:2.5 (Figure 18.36).

18.5.2.1 Shoulders

The function of shoulders is to support the clay core. They mustresist sliding along the foundation and any plane through the fillagainst horizontal thrust from the core.

Slope stability should be checked along a slip surface thatmight pass through core and foundation, or within the fill (seeChapter 9).

High construction pore pressures in the shoulders should beavoided by:

(1) Use of permeable fill.(2) Provision of drainage layers.(3) Placement of fill at a water content which allows a maxi-

mum dry density to be developed by the compactingmachines to be used.

A transition is required between permeable shoulder fill andsemi-impervious core material. This is often achieved by select-ing borrow material so that the coarsest is placed in the outerportions of the shoulders, with the finest fraction adjacent to thecore. It is good practice to place a graded filter between core andshoulder fill.

Figure 18.34 Neves dam: various radii of arches

Radius atextrados (m)

Radius atintrados (m)

Exaggerated crestsettlement Open cracks

Plane of lowembankmentstress but noopen crack

Alluvialfoundation

Rockfoundation

Figure 18.35 Zones of tensile strain

Figure 18.36 Traditional puddled clay core dam

18.5.2.2 Filters

The aim is to use a well-graded material, with pores smallenough to prevent entry by particles from the core. At the sametime, the filter material must not be so fine that it would be lostin voids in the shoulder fill. To satisfy these requirements, it maybe necessary to use several different grades of material in acomposite filter with finest next to the core graduating tocoarsest in contact with the shoulder fill.

Traditional filter design is based on the concept that the poresin a granular material are only about one-fifth the diameter ofthe particles. Thus, the grain size of a filter can be about 5 timesthat of core material. In practice, filter material is well gradedand the controlling size is often taken as the maximum of thesmallest 15% of the mixture. When a sieving analysis has beenmade, producing a grading curve of the type shown in Figure18.37, the controlling size is Z)15.

A filter rule is:

(1) Z)15 maximum of the filter must be equal to or less than 5times Z)85 minimum of the core (or smaller-size filter zone).

(2) Z)15 minimum of the filter must be equal to or greater than 5times Z)15 maximum of the material to be protected toprovide adequate permeability.

Specifications usually contain other restraints such as maximumand minimum sizes, absence of gap grading, etc.

The smallest particle in the clay core may be of clay size, i.e.< 0.002 mm and it is unlikely to be practical to provide atheoretically correct filter to trap this size of particle. Fortu-nately, most clayey fills that are used for cores are well-gradedmaterials containing coarse sand or even pebbles. The clayparticles tend to floe (unless they are dispersed by the chemicalcomposition of the reservoir water) so that it is only necessary tofilter the floe size.

Even though, initially, some clay floes may pass into the poresof the filter, the slightly larger sizes of the clay core cannot passinto the pores and, after an initial slight loss of material, a finerfilter of core material builds up against the provided filter. It isnecessary to ensure that the grading of the core material willallow this to happen and that the various zones of the filter willprevent loss of filter material into the shoulder fill.

Vaughan and Soares18 have developed a method for designingfilters by using their permeability to indicate pore size. Thepermeability of a filter is proportional to the square of its poresize and if retained particle size is used to represent this, then:

k = Adx (18.1)

where k represents filter permeability, d, particle size, A is a

constant depending on other geometric factors, and x is a powerof about 2.

Tests with a number of clays used for the cores of British damsgave A = 6 . Ix 10~6 and x= 1.42.

The floe size d can be found from hydrometer sedimentationtests, using the local, natural water with a soil concentration of25 g/1. It was found that floe size could be determined with equalaccuracy by observing the settling velocity of the clearing frontof the suspension.

Filters for the small Ardingly dam in Sussex were designed bythis method. Sedimentation tests with river water gave a floe sizeof 6 to 15 um, with an average of 10 um. The above values of Aand x gave a required permeability for the filterA; =1.6 x 10"4m/s. A natural, medium-sized sand was foundwith Z)50 = 0.4 mm and Z)15 = 0.23 mm. Its permeability was0.9 x 10~4m/s and it has been used successfully.

Figure 18.37 Grading curves showing filter rule, (a) Large grainsscreen small grains at filter opening; (b) grain .size criteria for soilsused as filters

Grain diameter (mm)

Limits of filter D15

Good filter

Soil beingfiltered

Large grain

FilteropeningLarge grain

Top W LSelected fine material Puddle clay core

Embankment fill(as excavated)

Puddle-clay-filled cutoff trenchNatural ground

Smallgrain

Fines

(%)

Further work by Vaughan and Scares indicated values ashigh as ^ = 6.7.x IQ-6 and x=1.52. The small differencebetween these sets of values may be regarded as the currentlatitude in this new design method.

Drainage layers under and in the shoulder fill can be designedas filters to prevent fill finding its way into them. This aspect isnot important on the downstream side where water flows arelikely to be small. In the upstream side, however, it may bedesirable to design the drains as filters, particularly when thereservoir will be subjected to repeated large fluctuations, as inthe case of a pumped storage hydro-electric scheme.

18.5.2.3 Clay core

The permeability of particulate materials is dependent on stresshistory, as well as, for example, grain shape, size and grading.

A heavily overconsolidated clay may show a low per-meability, e.g. 1 x 10~9cm/s when an intact sample is tested inthe laboratory. Field tests from piezometers usually show muchhigher values due to the joints, fractures, bedding planes, siltinclusions, etc. which form the fabric of the clay.

When excavated and compacted into a rolled clay core, intactfragments may retain the low permeability of the intact sample,but the mass permeability of the core may be controlled more bythe joint surfaces between fragments. Compaction to optimumdensity will ensure that these joints are not only tightly closed,but carry across them a prestress induced by the compactionmachinery - a stress which will be increased as the height of fillrises.

Hydraulic fracture. Water from the reservoir tends to perco-late along a myriad joints. If its pressure exceeds the total stressacting across the joints, they may be forced apart, producing afracture that will penetrate into the core until it meets a zone ofhigher stresses.

The amount by which the joints are forced open depends onconfining conditions and the compressibility of the compactedclay. The fissure may not become very wide and as watermigrates into the pores of the clay forming its walls, reducingthe effective stresses to zero, the clay will swell and may close thefissure, if the reservoir water pressure ceases to rise.

A fissure which extends until it finds an outlet may permitsufficient flow to cause erosion which will increase fissure width,thereby further increasing flow and leading to piping. Rate oferosion may depend on how well the clay will disperse into thereservoir water. The degree of dispersion of a clay may bemeasured by the VoIk19 double hydrometer test or the Sherard etal.20 pinhole test. A fairly clear indication may be obtained,however, from the ball test which used to be carried out on claysamples to check their suitability for use in a puddled clay core.

Ball test. Samples of the clay at the proposed placement watercontent were rolled by hand into balls about 50 mm diameter.These were carefully placed in a bucket of river water (to be thereservoir water) and left for 24 h. The degree of dispersion couldbe judged from the amount and fineness of the material that hadfallen away from each ball. In the extreme, a ball coulddisintegrate into a loose heap and produce a muddy suspensioncovering the bottom of the bucket. A ball of nondispersive claywould remain intact in clear water.

Wet seams. Materials of low plasticity, such as silts, erodereadily. If a fissure develops slowly, without appreciable flow,i.e. before it has reached an outlet, then, under the release ofeffective stress, the silt can expand into the water to loosely fillthe fissure. As it progresses through a wide core, under theaction of rising reservoir pressure, it may fill up with loose siltuntil, when it reaches an outlet, the permeability of the loose siltmay be sufficiently low to limit flow to nonerodable amounts.The resulting seam of loose, saturated core material may be

found if dry excavations are made into the core. Boreholesdrilled with flush water will not disclose such seams, althoughwhen they are reached, loss of washwater can be expected.

Sherard21 has described wet seams found in the cores ofManacouagan 3, Yard's Creek and Teton dams. He believedthey originated in hydraulic fractures and proposed the abovemechanism to account for their development. A more detaileddiscussion about the wet seams found in the Teton core duringthe post-failure investigation were given by Sherard.22 The wetseams in the main body of the Teton core were not the cause offailure.

Total pressures in a core. The risk of hydraulic fracture can bereduced by increasing total stresses in the core. The maximumtotal stress on a horizontal plane could not be expected toexceed the overburden pressure. More usually it is much less.

It requires relatively little differential settlement between coreand shoulders to develop the full shear strength of the clay. Anarrow core can resemble material in a silo and its attachmentto the silo walls supports some of its weight. This silo, or archingaction, reduces the vertical total stress in the core, below itspotential value of av = yh = a0 the overburden pressure, where av

represents the vertical total stress, y the bulk density of corematerial, and h the height from considered point to crest.

If the core is assumed to have vertical sides, as indicated inFigure 18.38, then a guide to the vertical total stress at any levelin the core can be obtained from:

o,=h(y-c-£)

where a represents the half width of the core and cu theundrained shear strength of the core.

In an axial direction, where the core is restrained by the valleysides, the total pressure <ra, which may control hydraulic frac-ture, is given by:

(J3 = A0 (<TV-W) + W

where K0 represents the coefficient of earth pressure at rest, andu the pore pressure in the core.

In order to maximize <rv, a dense material should be used for thecore and adjustments made by reducing cu and increasing a toobtain desired values.

In general, reducing cu increases K0 so that maximum valuesof cra can be obtained with suitably low values of cu.

An increased water content will reduce cu and increase B (seeChapter 9), thereby increasing construction pore pressure u.This increase in the value of u also helps to increase <ra.

It is desirable to have the clayey core material in such acondition that, at the end of construction, piezometric level inthe core is above crest level. This may fall by dissipation asreservoir level rises and an ideal solution would be for thepiezometric level to have fallen to top-water level in time to meetthe reservoir water at that level.

18.5.2.4 Below-ground cutoff and core contact area

Traditional British treatment at a site on alluvium was toexcavate a below-ground trench along the core centreline, takendown to a suitably impervious strata.

The trench was filled with concrete to form an imperviouscutoff and this was taken up a short distance into the clay core.The top of the wall was sometimes shaped like a spearhead sothat it would push up into the clay as the core settled. To avoidflow of water along the interface between concrete and clay it

Figure 18.38 Silo or arching action in a clay core, (a) Totalvertical stress at any level in a vertical core; (b) core restrained byvalley sides

was necessary to have a total pressure across the interface inexcess of reservoir pressure.

The deep trench required a strong support system whichmade the whole operation expensive. A more modern alterna-tive is to support the trench with a slurry during excavation andplace the concrete by tremie-pipe, displacing the slurry from thebottom upwards.

Since the purpose is to stop water flow under the dam, there isno need of a strong concrete wall. To allow for consolidation ofthe ground under dam weight, a compressible concrete issometimes used to fill the trench.

Another method is to form the wall with contiguous boredpiles constructed of concrete. This can be made by boring holesfor alternate piles and then, after they have been cast, borebetween them, cutting a small crescent from each.

The most common below-ground treatment is the injection ofgrout from small-diameter boreholes with the aim of forming acontinuous wall of ground, made watertight by the grout. Thetype and extent of treatment depends on ground conditions.Sometimes a grout curtain is formed from one line of holesalong the core centreline, but it is more usual to have three lines.The grouted width is usually increased near ground surface to atleast core base width by blanket grouting from shallow holes, asindicated in Figure 18.39.

Grout can be injected as the hole is drilled, but it is morecommon to drill to full depth and grout in stages from thebottom. It is now usual practice to use perforated tubes coveredwith rubber tubes (tube a manchette) which are grouted into thebored hole with a weak grout. A double packer is then passedinto the steel tube so that grout can be injected horizontally atany level. The rubber tube acts as a flap valve and allows theperforated steel tube to be washed out, so that further groutingcan be carried out if necessary.

A test for determining whether grouting is necessary and tocheck the effectiveness of grouting was devised by Lugeon.23

This requires injection of water in a borehole at a pressure of10 bar. Flow is measured and expressed as litres per metrelength of hole. A flow of 1 litre per metre per minute (referred toas a Lugeon) is regarded as the limit before grouting is needed.

This test, intended for bedrock, is dangerous because the top-of-hole pressure of 10 bar will exceed overburden pressure to adepth of about 50 m. Many sites show much higher Lugeonvalues over the upper 20 to 30m, and this is often taken toindicate the degree of weathering, but it can also be due toexcessively high water-pressure. Borehole diameter was notspecified by Lugeon so it is difficult to convert Lugeon values toin situ permeability. In bedrock, water loss is along fissures, sothat a determination of in situ permeability which would becomparable with a sand or gravel is not possible.

Figure 18.39 Grout holes under a clay core

Centre line

Today, the danger of causing hydraulic fracture in the groundthrough use of excessive pressures is generally recognized, soalthough tests carried out are still referred to as Lugeon tests,water pressures are related to overburden pressure. This alsoapplies to grout pressures. It is very easy to discharge largevolumes of grout into the ground, but it may be opening fissuresrather than sealing them.

A great deal of money can be wasted on extensive precon-struction grouting programmes on a dam site. Tests to measurein situ permeability should be designed on soil mechanicsprinciples (see Chapters 9 and 11) and be carried out during thesite investigation. If the results indicate that dam underflow willnot cause unacceptable water loss or dangerous build-up ofpressure under the downstream shoulder, consideration shouldbe given to omitting a grout curtain.

In the interests of security, an inspection/drainage galleryshould be built in a trench at ground surface to form part of thebase contact area for the core.

On completion of the dam, the underflow condition can beassessed from instrument readings as the reservoir fills, whenany necessary extra drainage or grouting can be carried outfrom the gallery. Much higher grouting pressures can then beused because the foundation is held down by the weight of thedam.

It is unfortunate that financial arrangements can seldom bemade that will allow construction of a below-ground groutcurtain after dam completion. If the cost of the groutingprogramme is not included in the estimated cost of the dam, it isextremely difficult to obtain money at the end of construction. Itis also impossible to avoid the notion that the design has been afailure if grouting is found to be necessary as the reservoir fillsfor the first time.

18.5.2,5 Rockfill

Earthmoving machinery can handle rockfill containing pieces of1.5 to 2m. Heavy vibrating rollers can compact layers of 2mthickness and these sizes have commonly been used in rockfilldams.

The rockfill should be well graded with a tendency towards anexcess of the smaller sizes to ensure that the large pieces are fullybedded and do not touch. In general, most of the material froma quarry can be accepted. A limit to the amount and size of finesis when the in situ permeability of the rockfill is reduced to1 x 10~5m/s. A soft rock such as a sandstone may produce anample quantity of the finer material, whereas a very hard rocksuch as basalt may be short of fines. Because of this, thesituation often arises in which a rockfill of soft rock suffers lessdeformation than one composed of hard rock.

Placement. Segregation may be reduced by tipping the rockfill4 or 5m back from the advancing edge of the layer, thenbulldozing it over to the level of the new layer. The larger piecesfall to the bottom and are covered by fines which produce asmooth surface that is kind to the placing machinery and makesgood contact with a smooth vibrating roller, transmitting thecompaction energy into the fill. Provided there are sufficient ofthe small fines, the voids between large pieces become com-pletely filled and pressed into the lower surface so that surfacesbetween layers cannot be detected except when markers (such ascoloured sand) are used. Water content should be sufficient forworkability, typically 5 to 10%. An excess of water is notharmful because the rockfill should be sufficiently permeable forit to drain freely.

Compaction control. It is not practical to control rockfillcompaction by in situ density measurements. Specification isusually of method (size of roller, number of passes, etc.) butroller performance can be measured with a compactionmeterwhich will show when a desired compaction has been achieved.An apparatus designed by Geodynamik in conjunction with

Dynapac Research has been described by Forssblad24 andThurner and Sandstrom.25

Stability of rockfill. The failure envelope for rockfill is curved.A typical example is shown in Figure 18.40.27 The value of shearstrength can be expressed by:

T = A(J?

where r represents the shear strength of the rockfill in kilonew-tons per square metre, a' represents the normal effective stress inkilonewtons per square metre and A and b are constants.

Values of A and b found from tests on several rockfills are givenin Table 18.10.

Testing equipment used to obtain the parameters A and brequires to be fairly large, but clearly it is impractical to test thefull-size rockfill. It has been found that samples obtained bysieving off the larger sizes give results more representative of thewhole rockfill than samples with parallel, scaled-down gradingcurves. Test samples of 300 mm diameter and 700 mm heightshould be regarded as a minimum. Density and water contentshould be as close as possible to those expected in the field.

Design of slopes can be simplified by use of stability numbersgiven by Charles and Scares. 26 They have shown that the factorof safety:

F- rA_^

where F represents the stability number, H the height of theslope, and y the bulk density of the rockfill.

Normal stress (a') (kN/m2)Figure 18.40 Curved failure envelope: rockfill, where (1) issandstone, (2) slate, (3) is also slate, and (4) is basalt. (After:Charles and Watts (1980) The influence of confining pressure onthe shear strength of compacted rockfill'. G6otechnique, 30, 4)

The stability number for a given slope (1 vertical: jc horizontal)is given in Figure 18.41.

As a guide to the confining pressures to be used in the tests forthe determination of A and 6, Charles and Scares have provideddesign curves to give values for 0^n, the maximum normal stresson the critical failure surface. The curves are reproduced inFigure 18.42.

Shea

r stre

ss (T

) (kN

/m2)

18.5.3 Roller-compacted concrete

A well-graded sandy gravel composed of rounded particlesforms an excellent shoulder fill for embankment dams. It readilycompacts to a maximum density and is usually sufficientlypermeable to be classed as rockfill. Addition of some cementi-tious materials can both increase its strength so that it can beplaced at steeper slopes, and reduce its permeability so that itbecomes suitable for the construction of gravity section dams.By restricting heat liberation during hydration, temperaturerises can be kept low enough so that long, continuous lengthswill not develop shrinkage cracks. This enables it to be placedlike earthfill. For a given height of dam, less volume of fill isrequired than with earth or rockfill, so more rapid constructioncan be achieved.

Mixes. Cement-stabilized earth and lean mix have been usedto construct road sub-bases for half a century. Rollcrete used byLowe10 for the core of the Shihmen cofferdam in Taiwan wasmade from a concrete-type aggregate to a grading as shown inFigure 18.43. It had a maximum size of 76mm and containedonly 53 kg Portland cement and 53 kg fly ash per cubic metre(traditional concrete would contain about 300kg Portlandcement per cubic metre). Lowe used the modified AmericanAssociation of State Highway Officials (AASHO) test (seeChapter 9) to determine optimum placement water content. Hefound this also gave maximum compressive strength, as shownin Figure 18.44.

inches US standard sieve size

Slope 1 (vertical) inx (horizontal)Figure 18.41 Stability numbers: rockfill slopes

Slope 1 (vertical) inx (horizontal)Figure 18.42 Maximum normal stress on critical failure surface

Figure 18.43 Rollcrete for Shihmen core: grading curve

Table 18.10 Values for the parameters A and b for several compacted rockfills

Rock type

DioriteSilicified conglomeratePizandarau sand and gravelNelzahualcoyotl conglomerateMalpaso conglomerateCarboniferous sandstonePalaeozoic slatePalaeozoic slate (weathered)Basalt

A

2.02.62.22.13.86.85.33.04.4

b

0.8700.8460.8760.8810.8080.6700.7500.7700.810

Reference

Marsal28-29

Marsal28'29

Marsal28

Gamboa and Benassini30

Marsal29

Charles and Watts27

Charles and Watts27

Charles and Watts27

Charles and Watts27

Fines

by w

eight

(%)

Grain size (mm)

Coars

e

Mediu

m

Fine

Coars

e

Mediu

m

Fine

Silt

Clay

GRAVEL SAND SlLT and CLAY SOILS

Water content (% dry weight)(b)

Figure 18.44 Rollcrete and water content, (a) Density; (b)compression strength

The mix used for Willow Creek dam (51.5m), the firstAmerican dam built with roller compacted concrete and com-pleted in 1982, contained 36.3 kg Portland cement and 14.5 kgfly ash per cubic metre. Segregation at the base of each layerproduced porous joints which had to be grouted at a later stage.

The British approach outlined by Dunstan16 has been to fillcompletely the voids in the aggregate to both minimize per-meability and improve workability at low water contents. Theair-.voids ratio in a compacted fine aggregate passing a 5mmsieve is typically 0.35:0.38.

A paste of this volume is therefore required to fill the voidscompletely. A study of the behaviour of joints between success-ive layers showed that even higher proportions of paste wereadvantageous.

The Milton Brook trial bank used a crushed limestoneaggregate and an excess of paste with very low water content.

Table 18.11 Mixes for the hearting of several roller-compacted dams

In Japan, the roller compacted dam method of constructionhas tended to follow the American roller compacted concreteapproach, namely of a dry, lean-mix concrete. Fly ash is used toreduce heat of hydration but in general much less has been usedthan at the Milton Brook trial. Mixes that have been used forthe hearting of several roller-compacted dams are given in Table18.11. To allow for some shrinkage, transverse contractionjoints, made by vibrating a cutting plate into freshly placedlayers, have been spaced typically at 15m.

Consistency tests. Concrete for roller compaction is too dryfor the traditional slump test. Instead, vibrating equipment isused. The Japanese consistency meter is a steel cylinder of480 mm diameter and 400 mm height which is filled with the mixand mounted on a vibrating table. It has a transparent plasticplate/piston, loaded to 200 kN, on the surface of the mix. Thetable vibrates with an amplitude of 1 mm at a frequency of4000c/min. The vibrating compaction value is the time, inseconds, for the cementitious paste to have risen so as to makecomplete contact with the underside of the transparent plasticplate.

There is a close relation between number of passes required tocompact the mix with a vibrating roller and the vibrationcompaction value.

In designing a mix, the test can be used to adjust sand content,water content, etc. to give optimum values. The effect of varyingsand content is shown in Figure 18.45.

Dam section. In a dam section, two approaches may be used:

(1) Placing a waterproof element i.e. steel sheet, bituminousmembrane, or dense concrete with contraction joints con-taining waterstops, on the upstream face. With this arrange-ment, the main body of rollcrete does not have to have lowpermeability and can be provided with open contractionjoints.

(2) In the homogeneous section, upstream and downstreamfaces may be formed by horizontally slipformed kerbs,precast units being held on by reinforcement laid into therollcrete, etc. The rollcrete mix must be given low per-meability by use of excess paste content - this will alsoenable tight joints to develop between successive layers. Aswith homogeneous embankment dams, drainage should beprovided to prevent water reaching the downstream face.

There may be an advantage in sloping both faces: this maysimplify provision of finishes to the faces and will assist stabilityby giving a vertical component to the thrust from the reservoirwater.

Galleries. Drainage/inspection galleries can be formed:

(1) With traditional concrete, using formwork.

Name

ShimajigawaOhkawaShin-NakanoPiricaTamagawaManoAsahiogawaSakaigawa

Aggregate size(max.)(mm)

8080

15080

150808080

Coarseaggregate(> 5mm)

14761500146815881544155215001182

Fineaggregate(< 5mm)

(kg/m3)

749686685668657726706752

Cement

9196848491969684

Fly ash

3924363639242436

Water

105102909095

102102105

Date

1977-801978-791979-821982-881983-871985-881986-891988-92

Compaction energy:equivalent MOD. AASHO

Water content (% dry weight)

Mix 84 Portland cement (kg/m3)53 Fly ash (kg/m3)

WaterCement + fly ash

Dry

dens

ity (M

g/m

3)Co

mpre

ssive

stre

ngth

MN/

m2

% Sand of total aggregate

Figure 18.45 Sand : aggregate ratio and vibration compactionvalues

18.6 Legislation

Dams are subject to legislation in most countries. In theinterests of public safety, there are usually conditions imposedon the qualifications of engineers permitted to design andsupervise the construction of dams. There is also often a systemof inspection of dams during their operation.

In the UK, dams retaining a reservoir of more than 24 000 m3

are controlled by the Reservoirs Act 1975. This supersedes theReservoirs (Safety Provisions) Act 1930, which required en-gineers to be approved and registered by the Secretary of Statebefore they were permitted to design and supervise dam con-struction. Dams had to be inspected at certain times and at leastevery 10 years, when a report had to be made available tointerested parties.

The new Act continues this general principle, but providespowers to implement recommendations for repairs or modifi-cations contained in the Inspector's report. Appointments to apanel are for 5 years only and all reservoirs must be registeredand continuously supervised by a qualified civil engineer, inaddition to being inspected periodically by an independentengineer.

18.7 Further reading

The International Commission on Large Dams (ICOLD), holdscongresses every 3 yr in various parts of the world. The transac-tions of these congresses contain a wealth of information andform milestones along the path of developing technology. The

breadth of subject discussed at each congress is kept withinreasonable bounds by being addressed to only four questionschosen internationally prior to each congress. Abstracts ofICOLD publications covering the contents of the transactionsare available in two volumes. A list of all ICOLD publications,which includes special volumes such as Lessons from damincidents and World register of dams as well as numerousbulletins covering specific subjects, is available from the centraloffice of ICOLD in Paris. In the UK, information may beobtained from the Institution of Civil Engineers, where meetingsare held by the British National Committee of ICOLD.

A few of the numerous publications on dams are listed in thebibliography. Innumerable papers relating to dams can befound in the Proceedings of civil engineering institutionsthroughout the world.

References

1 Kerisel, J. (1985) 'The history of geotechnical engineering up until1700'. Proceedings, llth international conference on soil mechanicsand foundation engineering. San Francisco. Golden JubileeVolume, pp. 3-93.

2 Rao, K. L. (1951) 'Earth dams ancient and modern in Madrasstate'. Transactions, 4th International Congress on large dams, NewDelhi, vol. 1, pp. 285-301.

3 Buckley, R. B. (1898) 'Discussion on reservoirs in India'. Proc.Instn. Civ. Engrs, 132, 213-217.

4 Pinto, N. L. de S., Materon, B. and Marques, P. L. (1982) 'Designand performance of Foz do Areia concrete membrane as relatedto basalt properties'. Transactions, 14th international congress onlarge dams, Rio de Janeiro, vol. 4, pp. 873-906.

5 Steffen, H. (1982) Bituminous cores for earth and rockfill dams.International Commission on Large Dams, Bulletin No. 42,ICOLD, Paris.

6 Kjaernsli, B. and Sahde, A. (1973) A new waterproofing techniquefor Norwegian dams. Norwegian Technical Institute, PublicationNo. 98, pp. 1-4.

7 Lohr, A. and Feiner, A. (1973) 'Asphaltic concrete cores:experiences and developments'. Transactions, llth internationalcongress on large dams, Madrid, vol. 3 pp. 827-42.

8 Penman, A. D. M. and Charles, J. A. (1985) 'A comparisonbetween observed and predicted deformations of an embankmentdam with central asphaltic core'. Transactions, 15th internationalcongress on large dams, Lausanne, vol. 1, pp. 1373-89.

9 Sherard, J. L. (1973) 'Embankment dam cracking', in:Embankment dam engineering. Casagrande volume, Wiley,Chichester, pp. 271-353.

10 Lowe, J. (1962) Discussion on The use of rollcrete in earth dams'(unpublished discussion). 1st American Society of Civil EngineersWater Resources Engineering Conference, Omaha (reproduced inpart in Proceedings, international conference on rolled concrete fordams (1981), pp.Wl-W5, London). Construction IndustryResearch and Information Association, London.

11 Gentile, G. (1964) 'Study, preparation and placement oflow-cement concrete, with special regard to its use in solid gravitydams'. Transactions, 8th international congress on large dams,Edinburgh, vol. 3, pp. 259-77.

12 Wallingford, V. M. (1970) 'Proposed new techniques forconstruction of concrete gravity dams'. Transactions, 10thinternational congress on large dams, Montreal, vol. 4, pp. 439-52.

13 Moffat, A. I. B. (1973) 'A study of dry lean concrete applied tothe construction of gravity dams'. Transactions, llth internationalcongress on large dams, Madrid, vol. 3, pp. 1279-99.

14 Dunstan, M. R. H. and Mitchell, P. B. (1976) 'Results of athermocouple study in mass concrete in the Upper Tamar dam'.Proc. Instn Civ. Engrs, Part 1 60, 27-52.

15 Dunstan, M. R. H. (1976) The Upper Tamar dam' (discussion).Proc. Instn Civ. Engrs, Part 1 60, 670-71.

16 Dunstan, M. R. H. (1981) Rolled concrete for dams: a resume oflaboratory and site studies of high fly-ash content concrete.Construction Industry Research and Information AssociationReport No. 90, CIRIA, London.

(2) As precast units, placed by crane.(3) As sandbags and loose fill.

The first two systems interfere with placement of rollcrete andcause time delays. In the third system, the shapes of the galleriesare defined by laying sandbags in the rollcrete and filling thespace between with loose fill (sand and gravel or other suitablefill). In this way, the placing machinery can work over thegalleries without interruption. When the rollcrete work is com-plete, the uncemented fill is dug out to form the galleries.

Vibr

ation

com

pacti

on v

alue (

s)

17 Schnitter, N. J. (1984) 'The evolution of buttress dams' in: IntnlWat. Pow. and Dam Constn., 36, 6, 38^42 and 36, 7, 20-22.

18 Vaughan, P. R. and Soares, H. F. (1982) 'Design of filters for claycores of dams'. Proceedings, Am. Soc. Civ. Engrs Geotech. EngrgDiv., 108, 17-31.

19 VoIk, G. M. (1937) 'Method of determination of the degree ofdispersion of the clay fraction of soils'. Proc. Soil Sc. Soc.America.

20 Sherard, J. L., Dunnigan, L. P., Decker, R. S. and Steele, E. F.(1976) 'Pinhole test for identifying dispersive soils'. /. Geotech.Engng Div. Am. Soc. Civ. Engrs, 102, GTl, 69-85.

21 Sherard, J. L. (1985) 'Hydraulic fracturing in embankment dams',in: R. L. Volpe and W. E. Kelly (eds), Seepage and leakage fromdams and impoundments, American Society of Civil Engineers,pp. 115-41.

22 Sherard, J. L. (1987) 'Lessons to be learned from the Teton darnfailure', in: Engineering geology - special issue on dam failures.Workshop on dam failures, Purdue. Elsevier, London.

23 Lugeon, M. (1933) 'Barrages et geologic'. Bulletin TechniqueSuisse Romande, Lausanne. Publication No. 58, 225^40.

24 Forssblad, L. (1980) 'Compaction meter on vibrating rollers forimproved compaction control'. Proceedings, internationalconference on compaction, Paris, pp. 541-46.

25 Thurner, H. and Sandstrom, A. (1980) A new device for instantcompaction control. Proceedings, international conference oncompaction, Paris, pp. 611-614.

26 Charles, J. A. and Soares, M. M. (1984) 'Stability of compactedrockfill slopes'. Geotechnique, 34, 1, 61-70.

27 Breth, H. (1964) 'Measurements on a rockfill dam withbituminous concrete diaphragm'. Proceedings, 8th internationalconference on large dams, Edinburgh, vol. 2, pp. 305-315.

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