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18 Dams

A D M Penman DSc, CEng, FICEGeotechnical Engineering Consultant

Contents

18.1 Definition 18/318.1.1 Types of dam 18/3

18.2 Brief history 18/3

18.3 Embankment dams 18/418.3.1 Introduction 18/418.3.2 Rockfill dam with upstream reinforced

concrete membrane 18/418.3.3 Rockfill dam with upstream asphaltic

membrane 18/518.3.4 Rockfill dam with central asphaltic core 18/518.3.5 Rockfill dam with central clay core 18/1118.3.6 Earthfill dam – homogeneous section 18/1118.3.7 Earthfill dam with central clay core 18/13

18.4 Concrete dams 18/1318.4.1 Introduction 18/1318.4.2 Gravity dams 18/1318.4.3 Rollcrete dams 18/1418.4.4 Buttress dams 18/1718.4.5 Arch dams 18/23

18.5 Design concepts 18/2918.5.1 Embankment dams – homogeneous

section 18/2918.5.2 Embankment dams with central clay

cores 18/2918.5.3 Roller-compacted concrete 18/34

18.6 Legislation 18/36

18.7 Further reading 18/36

References 18/36

Bibliography 18/37

18.1 Definition

In the UK, the name 4dam' is given to a civil engineeringstructure built across a valley to form an artificial lake as areservoir of water. There are numerous variants. Some reser-voirs are formed on relatively flat land by building long dams toencircle the required areas. Others are built to store materialsother than water. In South Africa and some other countries theword 'dam' is used for the reservoir which is retained by a 'wall'or 'dam wall'.

18.1.1 Types of dam

Dams are separated into two main types by the choice ofmaterial used for their construction: (1) embankment; and (2)concrete dams.

(1) Embankment dams are made from nonorganic particulatematerial excavated from the Earth's surface local to the damsite and used more or less as excavated. They are subdividedinto earthfill and rockfill dams, although many embankmentdams contain both types of fill. Further subdivisions can bemade, according to the material used, to make the water-proof element, e.g. central clay core, sloping clay core orupstream membrane of asphalt or reinforced concrete.

(2) Concrete dams are made from a carefully selected andprocessed harder fraction of this material, bound togetherand strengthened by an hydraulic cement. They are sub-divided according to their mechanism for remaining stable.(a) gravity dams: these are the simplest because they rely on

gravitational force to oppose the overturning momentcaused by the pressure of the reservoir water on theirupstream faces.

(b) hollow gravity dams: these require less concrete andtherefore cost less to construct. Foundation require-ments are more critical.

(c) buttress dams: these also require less concrete thangravity dams. The buttresses support the upstream faceof a buttress dam. The upstream edges of the buttressesare commonly widened so that they join, forming thecontiguous buttress dam. As an alternative, theupstream face may consist of small arches betweenbuttresses, forming a multi-arch dam.

(d) arch dams: these may be constructed as a whole in onelarge arch, spanning the valley sides and relying on themto carry the very large thrusts caused by reservoir waterpressure. This type is the most sophisticated of theconcrete dams and may be subdivided into single-curvature and double-curvature, according to whetherthe vertical section is straight, or is curved to furtherreduce bending moments in the concrete.

18.2 Brief history

Dams have made a major contribution to the development ofour civilization. Their earliest role was to provide storage forirrigation water; now, they also provide hydro-electric powerand water for industry and large cities.

Early dams were all of the embankment type, of necessitybuilt from the earth and stones found at the site. Helms (quotedby Kerisel)' states that the oldest dam in the world is at Jawa inJordan, dating from about 4000 B.c.and was built of earth witha masonry facing. Perhaps the second-oldest is the Sadd el-Kafara on the Wadi el-Garawi near Helwan in Egypt, builtabout 2900 B.C. It was 11 m high with upstream and down-stream rubble masonry walls, each 24m wide at their bases,separated by a central earthfill section 36 m wide.

Rao2 reports that there was a tradition of dam building inIndia where it was once considered as one of the seven meritor-ious acts which a man ought to perform during his lifetime.During the period of British tenure, many embankment damswere constructed by traditional methods and were accepted as ameans of famine relief, giving employment to thousands.According to Buckley,3 the completed schemes were not onlyprofitable, but brought happiness and contentment to thepeople by ensuring reliable crop production.

In the UK the industrial revolution required water fortransport, industrial processes and a growing population. In theeighteenth century, dams were built to store water for canals;during the nineteenth, the majority were for water supply, andearly in the twentieth century, dams were built specifically toprovide power for aluminium smelting in Scotland and Wales.

Before 1800, almost all the world's dams were of the embank-ment type. During the nineteenth century, concrete technologyand methods of structural analysis were developed. The manysites then available with sound rock foundations at shallowdepth created increasing interest in concrete dams.

The increasing size of the human population of the world,together with ever-rising demands for irrigation water andpower, caused rapid increase in the number of dams built.Figure 18.1 shows, from 1800 to the present, the increase inworld population, together with the number and height ofembankment dams. Although the numbers of dams increased inresponse to the rise in population, the number of embankmentdams fell during the second half of the nineteenth and earlytwentieth centuries due to the number of concrete dams beingbuilt in that period. The proportion of the total that wereembankment dams being built during any 5-yr period fell to aminimum of about 30% by the end of the first quarter of thiscentury.

Since that period, the proportion has increased until currentlymore than 80% of dams being built are embankment dams. Thehighest dam in the world (Nurek, 300 m) is of the embankmenttype and is soon to be exceeded by another (Rogan) which willbe 325 m high when complete. Both are in the Soviet Union.

This reversal of trend can be attributed to:

YearFigure 18.1 Numbers of people and embankment dams,1800-1985. Curves also show heights of embankment dams andtheir proportion of total world dams

Number of largeembankment dams

Height of embankment'dams

World population

Numb

er of

large

emba

nkme

nt da

ms (X

103)

Dam

heigh

t (m)

Emba

nkme

nt/tot

al da

ms (%

)Wo

rld p

opula

tion

(X 1

09)

(1) Improved understanding of the behaviour of embankmentdams due to advances in the art and science of soil mecha-nics since publication of Terzaghi's Erdbaumechanik in1925.

(2) Increased capacity of earthmoving machinery after theintroduction of the internal combustion engine and caterpil-lar tracks.

(3) Reduction in the number of sites with bedrock at shallowdepth suitable for concrete dams.

(4) Increasing cost of labour.

At present, even at sites with strong rock near the surfacesuitable for concrete dams, e.g. the Lesotho Highlands schemes,embankment dams are often found to be cheaper. They cantolerate relatively poor foundation conditions and, when com-bined with their low price, this makes them a most attractiveoption.

18.3 Embankment dams

18.3.1 Introduction

Embankment dams can be subdivided according to type andposition of the waterproof element. In this section, a descriptionof the salient features of each type will precede an actualexample.

18.3.2 Rockfill dam with upstream reinforcedconcrete membrane

This type is currently rising in prominence and so will bedescribed first. (This does not mean that it is more importantthan other types of embankment dam.)

The Foz do Areia (160 m) on the River Iguac.u in Brazil is theworld's highest dam of this type and is a typical example

(Figures 18.2 and 18.3). It was required for hydro-electric powerand river control. The region of the dam site is made up ofmedium to thick basaltic flows of 25 to 55m depth. As iscommon with volcanic rocks, the interface between successiveflows forms a zone more pervious than the massive basalt. Thesite investigation showed that about 70% of total volume waspredominantly dense basalt with about 30% consisting mostlyof basaltic breccia. Table 18.1 gives some geomechanicalproperties of the rocks.

More than 14 million m3 of rock excavation was required fortunnels, power station and spillway chute. This excavated rockwas used as fill for the dam. It was placed in layers up to 1.6 mthick, sluiced with water at a rate of 25% volume of placed rockand compacted by four passes of a 10-t vibrating roller. Detailsof layer thickness and zones of rockfill are given in Figure 18.2,showing a cross-section of the dam.

Fragmentation of the hard rock by blasting produced a fillthat was rather too uniform in size. To reduce compressibilityunder the upstream membrane, a transition zone of rockfillcrushed to a specified grading with a maximum particle size of150 mm was placed in 400 mm layers, compacted by the main10-t vibrating rollers. The upstream face of this zone wassmoothed and coated with a bitumen emulsion covered withsprayed sand to prevent erosion and facilitate compaction. Thiswas achieved by six passes with the smooth 10-t roller pulled bycable up and down the slope, vibration only being used whilemoving upwards.

Great care was exercised in constructing the plinth to ensure asatisfactory water-tight connection between upstream mem-brane and valley sides. The plinth section is shown in Figure18.4.

The reinforced concrete membrane was made 800 mm thick atthe base, tapering to 300 mm at the top of the dam. The mainpart of it was cast as slabs 16 m wide, placed in slipforms. Beforeslipforming could start, the bottom piece of each strip, at itsjunction with the plinth, had to be formed in fixed shutters toproduce a square end for the slipforms.

1C Basalt rockfill. 1.6 m layers compacted four passes 10 t vibrating roller water 0.25 fill volume1B Basalt rockfiii. 0.8 m layers compacted four passes 101 vibrating roller water 0.25 fill volume1E Basalt rockfill. selected pieces placed1A Basalt rockfill. dumped1D Basalt and breccia 0.8 m layers compacted four passes 101 vibrating roller water 0.25 fill volume

11B Well-graded crushed basalt maximum size 150 mm 400 mm layers111D Impervious earthfill 300 mm layers

Figure 18.2 Foz do Areia: cross-section

PlinthGrout curtain

Top of firm rock Toe dyke

Clay protection

E L 685.00 (1st stage)

Concrete face

E L 744.00 (WL)E L 745.25 Axis of dam

E L 748.00 (crest).EL 738.00

Access road

Control slope

EL varies

EL 625.00E L 610.00

Sound rock

E L 633.00

E L 656.00

E L 615.00

EL varies

Construction of the dam began early in 1977 and the dam hadreached full height in 1979. Impounding began in March 1980.Details of the design and performance of this dam have beengiven by Pinto, Materon and Marques.4

A list of six other concrete-faced, compacted-rockfill damsand four proposed dams is given in Table 18.2.

18.3.3 Rockfill dam with upstream asphalticmembrane

The superior flexibility of asphalt makes it appear attractive asmaterial for an upstream membrane. Its use avoids the construc-tion of contraction joints and the reinforcement of a concretemembrane, but considerable care has still to be exercised at theplinth, where excessive relative movements can tear an asphalticmembrane and permit leakage. Asphaltic membranes have beenused on dams up to 83 m high. Slopes vary from about 1:1.5 to1:2 (Figures 18.5 and 18.6).

Winscar dam (54m) was constructed on the River Don inSouth Yorkshire between 1972 and 1975. The site is in theMillstone Grit series which includes alternating bands of sand-stone and shale. The shale was moderately strong in situ, butdeteriorated rapidly on exposure and had to be immediatelycovered with a fine granular fill where it was encountered atformation level. The sandstone, particularly those seams knownas the Huddersfield White rock, was most stable and was usedfor the rockfill.

The pronounced fissure structure of this rock tended tocontrol the shape and size of the pieces produced in the quarryby blasting. Fragmentation produced plenty of the finer sizesand the resulting rockfill was relatively well graded. It wascompacted in 1.7m layers by four passes of a 13.5t vibratingsmooth roller. Water was added to the rockfill when necessaryto bring the water content up to 6%.

A selected finer fraction of the fill was placed under the 1 : 1.7upstream slope and rolled with the 13.5-t roller, hauled by cablefrom the crest, but without use of vibration. The surface wassprayed with a tackcoat of bitumen before being covered with alevelling layer of porous asphalt placed by a paving machine. Itwas followed by two layers of dense asphalt, placed withstaggered joints and compacted by small, smooth vibratingrollers. It was covered with a sealcoat of bitumen and treatedwith a white finish above low-water level as a protection fromsunshine. Figure 18.6 shows conditions while the membrane wasbeing placed.

18.3.4 Rockfill dam with central asphaltic core

In the absence of any suitably impervious fill that could be usedfor a core, asphalt has been used to construct a central water-proof element.

The practice may be said to have started with the constructionof the 45 m high Vale de Gaio dam in Portugal in 1948. A tightlypacked rubble stone wall was formed to a slope of 1 (verti-

Figure18.3 Foz do Areia

Table 18.1 Foz do Areia: geomechanical properties of bedrock

Specific gravityPorosity (%)Compressive strength (MN/m2)

Modulus of elasticity (MN/m2)

Soundness testSodium sulphate (% of loss)

Los Angeles abrasion test(Type E grading, 1000 rev.) (%)

DrySaturatedDrySaturated

Coarse aggregateFine aggregate

Dense basalt

2.801.30

233190

6669064725

25

11

Basaltic breccia

2.3011.803725

2550023550

5035

20

Name

Foz do AreiaNew ExchequerSalvajinaAlto AnchicayaKhao LaemShiroroMiel 1SegredoXingoHa

Country

BrazilUSAColombiaColombiaThailandNigeriaColombiaBrazilBrazilBrazil

Height(m)

160150148140130125185145140125

Year completed

198019661984197419841984ProposedProposedProposedProposed

Figure 18.5 Winscar dam: cross-section

Table 18.2 Concrete-faced compacted-rockfill dams

Groutcurtain

Relief wells Selected fillScale

Selectedfill

Rockfill

Sown and planted

NWL 343.8 mDense asphalticconcrete: two layers

Beaching1 in 1.7 345.5 m

Figure 18.4 Plinth detail

E L 646.35

Mass concretePlinth.

AditMassive basalt

F low contact

Weathered breccia

Detail 1: Treatment Gallery

Stress relieffractures

Plinth

Detail 1

Hard rock surface

Flow contact

Pervious zones

Figure 18.6 Construction of asphaltic membrane: Winscar dam

cally):0.8 (horizontally) against the downstream fill andsmoothed with a lean concrete that acted as a drainage layer.Asphalt with a maximum particle size of 9 mm was then placedunder timber shuttering to a thickness of 200 mm, tapering to100 mm near the top of the dam. As the upstream shoulder wasraised, so the shuttering was moved up to form the sloping core.

According to Steffen,5 this method of construction was notpopular. In 1954, the 58m high Henne dam in Germany wasbuilt with a 1 m wide sloping core formed between shutters. Asoft bituminous concrete overfilled with bitumen and filler wasstiffened by adding clean, dry stones that were vibrated into themix. The shutters were moved up at each lift. This technique wasused to form cores for several dams in Germany, Austria andFrance until the end of the 1960s.

A similar, but perhaps simpler, approach was used in Norwayin 1969 for building a vertical asphaltic core without the use ofspecial plant. The idea was to construct the core of aggregateand then fill the voids with bitumen. To economize on bitumenthe voids should be small, but to obtain deep penetration theyshould be large and, preferably the aggregate should be warm.Laboratory tests showed that a rounded aggregate graded from19 to 76 mm, not colder than 3° C, was penetrated to a depth of20 to 30cm by bitumen poured at 160° C.

A core was built by this method in the 12m high approachdam at the south end of the spillway structure of the 67 m highGrasj0 dam near Trondheim. Timber shutters were used toproduce a core width of 0.5 m and the aggregate was compactedwith adjoining filter material. A 180- to 200-penetration bitu-men at 170° C was poured into the voids in the core aggregate.The core, as shown in Figure 18.7a and b, was extended to the

very top of the dam to ensure that the head of bitumen washigher than the head of water in the reservoir to prevent it frombeing displaced by the water pressure (hydraulic fracture) andso that the bitumen could be topped-up if required. Electricalbitumen pressure gauges were installed in the core duringconstruction. Measured pressures were less than the hydrostaticbitumen pressure, as shown in Figure 18.7c. As the reservoirrose to top water-level, the bitumen pressure increased to valuesshown in Figure 18.7d and remained above water pressure. In adescription of this work, Kjaernsli and Sande6 say that theperformance of the core during the first 2 yr of operation wassatisfactory with negligible leakage through the dam.

Machines were developed in Germany in the early 1960s toplace narrow cores of dense bitumen-concrete. Since 1962, atleast 23 dams have been built with machine-placed bitumen-concrete central cores. Those higher than 50m are listed inheight order in Table 18.3.

The first, completed in 1962, was the Dhiinn valley rockfilldam (35m high), described by Breth.27 It was founded on rivergravel over a bedrock of greywacke sandstone and argillaceousslate: the shoulder fill was crushed slate placed in 0.6 m layersand compacted by a 71 sliding vibrator. Concern was expressedthat the asphaltic core should be compatible with the slaterockfill. A system of measurement was therefore devised (Figure18.8) consisting of a vertical inspection shaft lined with concreterings each separated by a few centimetres to allow for settle-ment, built with the dam and situated 6 m downstream of thecore at about the major section. Six horizontal cross-pipes,spaced 5.5m vertically, facilitated observation of the down-stream face of the core.

Figure 18.7 (a) Dam section; (b) Asphaltic core and filter;(c) and (d) bitumen pressures in the core wall

Pressure t/m2

Table 18.3 Dams (> 5Om) with machine-placed bitumen-concrete central cores

Dam

High Island - eastHigh Island - westFinstertalKleine KinzigDhihm main damMeggetWiehl main dam

Height(m)

105.096.092.067.562.558.054.0

Core thickness(cm)(max.) (min.)

120-80120-8070-5065-506090-6060-50

Completiondate

1977197719811981198119831971

Country

Hong KongHong KongAustriaWest GermanyWest GermanyUKWest Germany

a, Asphaltic core; b, filters; c, transition zone; d, rockfill placed in1.5 m layers and sluiced; e, riprap; f, larger-size riprap for waveprotection

Max. WL484.0 m

486.5 m

Waterpressure

Figure 18.8 Arrangement for core measurements in the Dhiinndam

The very bottom of the core was made 1 m wide where it wasin contact with a concrete groutcap, but the main core was700 mm wide over the lower part, 600 mm in the middle sectionand 500 mm at the top. The results of measurements publishedby Lohr and Feiner7 are given in Table 18.4. As the reservoirwas filling for the first time, the upper part of the core movedupstream presumably because of the effect of wetting theupstream rockfill, but subsequently the horizontal movementshave been in a downstream direction. Some part of this down-stream movement could be caused by spread of the core undervertical loading. Core settlement at each measuring point tendedto exceed settlements of the fill slightly, indicating that the coresettlements were caused by self-weight rather than downdrag bythe fill.

Example 18.1

In the UK, two dams have been built with this type of core; (1)the Sulby rockfill dam (60 m) on the Isle of Man, completed in1982; and (2) the gravel-fill Megget dam (56m) in Scotland,completed in 1983.

The Sulby dam. (Figure 18.9) was originally intended to be builtin two stages. The first was to be a 35 m high structure with acentral core wall of machine-placed bituminous concrete. Thesecond stage, which was expected to be built at some future dateto raise the height to 60 m, was to consist of rockfill placeddownstream of the first dam and fitted with an upstreammembrane of bituminous concrete (b) connected to the top ofthe central core wall (a).

While the 35m high dam was under construction, it wasrealized that it made economic sense to build both stages at thesame time, and so the contractor continued with the 60 m highdam (68 m above lowest foundation) using an upstream mem-brane for the upper part.

Figure 18.9 Sulby dam: cross-section.a, Central core of bituminous concrete; b, membrane of bituminousconcrete

The vertical core was placed by a Teerbau machine, whichused steel sideplate shutters to form the core to the requiredwidth of 750mm with vertical sides. Hot asphaltic mix wasdropped into the hopper and fed down to the shutters as themachine advanced along the dam, the machine's tracks bearingon the transition material, adding a lift of 200 to 250 mm to thecore. A second part of the machine, linked to the first, placedtransition material on either side over a width of about 1.5 m.After it had passed, the machine left a level compacted surfaceof transition material with the asphaltic core in the middle. Damfill was brought up on either side to support the transitionmaterial.

Example 18.2

The Megget dam. The vertical core of this dam was formed witha Strabag machine which also travelled with its tracks bearingon a 1.5 m width of transition material on either side of the core.The machine (Figure 18.10) had a long steel nose which pro-jected over the core and contained preheating equipment. Hotasphaltic mix passed from a hopper through an adjustable

Position

Height of measuringposition above base (m)

Settlement during constructionto 1962 (mm)

Movements during operationto 1970 -vertical (mm)

Horizontal (mm) maximum upstreamon impounding

Total downstream to 1970

Base 1

3.4

35.0

50.0

0.033

2

8.9

60.0

78.0

0.039

3

14.4

130.00

200.0

0.081

4

19.9

170.0

290.0

8.071

5

25.4

100.0

253.0

14.063

6

30.9

20.0

202.0

20.052

Crest

35

Cement injections

Cutoff wall Collecting channel

Slope paving

RockfillAsphaltic concrete

Originalsurface

coreRolled loam

Cofferdam.

RockfillObservation andmeasuring shaft

Filter 25/80 mm

Observation pipes

Topsoil

Table 18.4 Measured movemements of the asphaltic core of Dhunn main dam. (After Lohr and Feinner (1973) 'Asphaltic concrete cores:experiences and developments'. Transactions, 11th international congress on large dams, Madrid)

Figure 18.10 Strabag machine placing core at Megget dam

aperture to give the desired width of core and was immediatelysupported by transition material that had been dumped over thenose. As the machine advanced, guide plates spread the finegranular transition material which was compacted, togetherwith the core leaving, as with the Teerbau machine, a firm levelsurface to the 200 to 250 mm lift of core plus transition.

This method of construction gave an inverted 'fir tree'-shapededge to the core. Although the aperture gave the design width tothe bottom of the lift, compaction caused the upper part tospread slightly and press into the transition fill. Thus, the designwidth of 900 mm at the base, reducing in steps to 600 mm in theupper part of the dam, was never decreased: it was slightlyexceeded at the top of each lift.

The surface of each lift was kept clean by a strip of tarpaulinlaid over it. This was very effective and even allowed coreplacement to continue immediately after snowfall. The machinewas guided by a string pegged out at a specified distanceupstream of the core. The Megget core was curved in plan andthe pegs were positioned with the aid of a theodolite-mountedelectronic distance-measuring device stationed over a referencepoint above crest level on the left abutment.

Shoulder fill was a well-graded gravel that was placed in400mm layers and compacted with four passes of a 5.5tvibrating roller. This produced a very high density and a verystiff material Numerous instruments installed in the dam duringconstruction included horizontal plate gauges taken through thedownstream fill to touch the core and thereby measure anymovements that occurred. Figure 18.11 is a section of the damshowing the positions of these gauges, and the movementsobserved during first filling of the reservoir are given in Figure18.12. A comparison between observed and predicted deforma-tions of this dam have been given by Penman and Charles,8 whoconclude that the asphaltic concrete core acted as a thindiaphragm. It had little effect on construction deformations andduring reservoir impounding simply transmitted the increasedlateral thrust caused by the impounded water on to the fill of thedownstream shoulder. During dam construction, there werevirtually no downstream movements of the downstream face ofthe core, indicating that, unlike a clay core, it did not exert alarge lateral thrust on the gravel fill because of its own weight.

18.3.5 Rockfill dam with central clay core

At sites where there is a source of suitable clay as well as rock,this type of dam may prove more economic than a rockfill damwith an upstream membrane, particularly if delivery of cementor asphalt to the site would be expensive. This design avoids thedetailed handwork and special machinery required for slipformwork or placing and compacting asphalt. It also obviates thedangers of damaging, concentrated movements near the plinthof upstream membranes, and the very high hydraulic gradientunder the plinth that could cause internal erosion in somebedrock formations.

The width of the rolled clay core is usually between 0.5/f+ Cto 0.33H+ C where H represents height and C is the minimumwidth at dam crest. This provides a theoretical hydraulicgradient of 2 or 3 along the contact between the core and thefoundation or abutment on which it rests.

In order to ensure good contact and an absence of cracks orfissures which would allow a passage for water under the clay,the cleaned formation is often coated with a layer of concreteover the area of contact with the clay core. The foundation isoften sealed by grout injected through boreholes to form a groutcurtain under the contact area to act as a below-ground cutoff.

Example 18.3

The Llyn Brianne dam (91 m) is an example of this type of dam

Figure 18.12 Downstream movements of the core onimpounding, a, Observed movements; b, asphaltic concrete core; c,foundation; d, gravel fill; e, reservoir water level

(Figures 18.13 and 18.14). The dam site, on the River Towy incentral Wales, is in a slatey, argillaceous rock which fragmentedinto plate-shaped pieces when won by blasting in the quarry. Atrial embankment showed that placing and compaction pre-vented any preferred orientation, broke up some of the piecesand produced a dense fill.

The stripped bedrock was blanket-grouted to 10 to 15mdepth over the core contact area and coated with a pneumati-cally applied mortar skin 50mm thick. In addition, a groutcurtain was formed to a depth of 45 to 75m under thecentreline.

The rockfill to form the shoulders was spread in 0.5 m layersand compacted on every second layer, i.e. on a 1 m layer, by fourpasses of either an 8.6 or 13.51 smooth vibrating roller.

Riprap on the upstream slope consisted of oversize pieces ofthe rockfill 1 to 1.5 m size, bulldozed out from the general fill.

18.3.6 Earthfill dam - homogeneous section

Dams built almost entirely from one type of fill, without

Figure 18.11 Major section of Megget dam, showing thepositions of horizontal plate gauges a, b and c where (1) is theasphaltic concrete core, (2) the transition zones, (3) gravel fill, and(4) the control gallery

Gauge D

Gauge C4

Gauge B5

Gauge A6

Figure 18.14 Llyn Brianne dam

provision for neither a less pervious core nor more stableshoulders, became popular in the Americas as the size andpower of earthmoving machinery developed in the 1920s and1930s.

A danger quickly recognized was that, if under full reservoir,the phreatic line could cut the downstream slope and local slipsdevelop, leading to backsapping and eventual destruction of thedam.

The US Bureau of Reclamation installed standpipe piezom-eters in the mid 1930s to check on the positions of phreaticsurfaces and in this way revealed the presence of constructionpore pressures. Placement at water contents below Proctoroptimum substantially reduced construction pore pressure butproduced a relatively stiff fill. Differential settlements couldcause cracking: examples of cracked and failed dams have beengiven by Sherard.9

Figure 18.13 Llyn Brianne: cross-section,(a) Clay core; (b) transition; (c) filter; (d) rockfill; (e) riprap;(f) rockfill drain in river channel; (g) excavated level bedrock;(h) cofferdam; (i) original ground level

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