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International Conference on Case Histories in (1988) - Second International Conference on Geotechnical Engineering Case Histories in Geotechnical Engineering

02 Jun 1988, 10:30 am - 3:00 pm

History of Tehri Rockfill Design

Bhagwat V. K. Lavania University of Roorkee, Roorkee,

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Recommended Citation Lavania, Bhagwat V. K., "History of Tehri Rockfill Dam Design" (1988). International Conference on Case Histories in Geotechnical Engineering. 24. https://scholarsmine.mst.edu/icchge/2icchge/2icchge-session3/24

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This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International Conference on Case Histories in Geotechnical Engineering by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. Proceedings: Second International Conference on Case Histories In Geotechnical Engineering, June 1-5, 1988, St. Louis, Mo., Paper No. 3.53

History of Tehri Rockfill Dam Desig~ Bhagwat V.K. Lavania Professor of Engineering, University of Roorkee, Roorkee, India

SYNOPSIS : A 260.5 m high rockfill dam is one of the main components of Tehri Dam Project which is under construction for the last over seventeen years. During this period detailed site investigations have been carried out and design of the dam is kept under constant review. A number of studies are conducted to provide feasible solutions to anticipated problems. The paper describes main design features of the dam mentioning the changes necessiated time to time with regard to these.

INTRODUCTION The Tehri Project is an irrigation-power project of the of tectonic deformations suffered by them, have been broadly State of , in India, envisaging construction graded as follows: of: (1) A 260.5 m high rockfill dam across the to store 2,880,000 acre-ft of water; (2) underground Phyllites of Grade I. - This rock unit is predominently power house, having installed capacity of 2000-2200 11:1W; arenaceous, massive in character, and distinctly jointed and (3) spillway for design discharge capacity of 11,700m /s. at phritiferous places that have lenticular elongated streaks The four diversion tunnels of 11.00 m diam each have already of brown colored calcarious material. been constructed. The layout of project works is given in Fig. 1. The dam is to be placed at the site such that its Phyllites of Grade U - This rock is conspiciously banded due to the rapid alterations of arenaceous and argillaceous material. In physical quality and competenance this unit is considered next to grade I phyllite.

Phyllites of Grade m - This unit is composed mainly of the argillaceous components with lesser amounts of arenaceous material. It carries quartz veins and is traversed by closely spaced foliation planes, cleavages, and joints. The unit is generally weathered. It is involved in minor folds and puckers.

Sheared Phyllite - The sheared phyllites constitute the weakest bedrock unit in the gorge and have resulted from crushing.

Shear Zones - A number of shear zones have been noticed at the dam site and its vicinity. Some of them are quite prominent. Shear zones are alined mostly along the foliation direction and trending in N 40° W-S 40"E to N 70" W-S Fig. 1 - General Layout of the Project works 70°E direction with dips of 30"-45° in the southwest direction, i.e. the downstream direction. These shear zones vary in foundation will be highly jointed and sheared phyllitic rocks thickness from fraction of centimeter to about half a meter. and its abutment contact slopes be steep. The area is also Apart from these, transerse shear zones, cross shear zones, seismically active. The construction of the project was and longitudinal shear zones (with respect to axis of dam) taken up in 1970. However, the activities did not move are also present. fast mainly because of the paucity of funds. But this provided the design engineers enough time to tackle the complex 1. Transverse Shear Zones - There are a number of problems of this gigantic structure. The problems examined transverse shear zones in the area. During initial investi­ are: (l) The location of the da:Tl axis; (2) the dam section gations, a very prominent, about (10-m to 15-m thick) shear (3) design of coffer dam; (4) the approach to slope stability zones was interpreted from drill hole data in the river analysis under static and dynamic conditions; and (5) the bed. It was therefore proposed (design of Dam, 1973) to foundations treatment. construct an across the river tunnel to confirm its presence. However, the subsequent drilling and electrical logging GEOLOGY OF AREA did not indicate the presence of such a prominent feature, and the tunnel was not constructed. But, now, after a period The rocks exposed in the area of the project site are phyllites of fourteen years, its construction has been started so as of Chandpur-Series (Lavania, 1975, Gupta et al., 1979). to leave no doubts about its presence before construction The rock bands are of variable thicknesses have dips of of the dam. 45"-60° in the downstream direction, and strike almost east-west. The river flow in this reach is north-south. On 2. Cross Shear Zones - There are also a number of cross the basis of their physical condition, the argillaceous and shear zones in the area. The most prominent of these is arenaceous materials present, and the varying magnitude exposed in the vicinity of the dam. It has a strike

Second International Conference on Case Histories in Geotechnical Engineering 615 Missouri University of Science and Technology http://ICCHGE1984-2013.mst.edu of N 85° W-S 85° E and dips at 55• in south-north direction. in Fig. 3, zone A was proposed to consist of better clay, It appears at both banks of the river, as shown in Fig. 2, with a PI greater than 7% and zone B, enveloping zone and its thickness in this area is about 4.0 m. A, was to consist of clay from other borrow areas.

Gl Et.&O-:

A Betle'da, C f'1llet B Ollatr day 0 River lenxe .alerill Fig. 3 - Initially Proposed Section

Further investigations for the dam fill materials suggested that in case the clay in the top layer of the river terraces is mixed with the underlying pervious material consisting l2ill Pllyllllt GtaM I IZJ Shar zone f•hal of silt, sand, gravels, and boulders, a very good well-graded (§ Pllylllletmen rnMr~ impervious material can be obtained for the core. The pro­ ~Phylli!eCr.otlll I2:J F.,l!,.. posed gradation envelope of such a blended core material is given in Fig. 4. Such blended materials have already Fig. 2 - Geology of Dam Site been used in the construction of the core of high like Nurek (300-m) in USSR, Mica (244 m) in Canada and DAM AXIS Oroville (234 m) in the United States. In 1960 Bleifuss and Hawke recommended the use of a well-graded mixture of A number of alternative axes were considered for the dam. fine and coarse material for the core of the dam. Initially dam axis No. I, as shown in Fig. 2, was considered the suitable axis. It provided excellent seating for the dam...... , But with it, the major cross fault was coming under the ioo core of the dam and therefore any movement along the : I/ fault due to earthquake or otherwise would have created / some damage or even rupture of the core. The problem "" /,t became more important in view of steep valley slopes. 70 v It was then proposed that the axis be shifted upstream of .. / A this abutment shear so that the dam core lies clear of ./ /I it upstream. The shifted axis is shown as No. 2 axis in Fig.2. .-" The investigations of the No. 2 axis pointed out that very I / heavy overburden, of the order of a 40-m depth, would 20 /""' most likely be met on the right abutment. Because of this, P'"v 10 ..---11 I contact of the right abutment with the dam would have _...... ::r: I I to be shifted further away into the hill, thereby shifting 0 0.0:11 I:JI':Z OJIIS 0.01 o.m o.1o .Jo 2G so 101 the spillway to the right, and thus involving increased excava­ Sizi!,Jaeilliltelers tion or a massive retaining wall about 50-m high, which would have to be constructed at the contact of the dam Fig. 4 - Proposed Gradation Envelope for Core Material and spillway. This necessitated modification in the dam axis No. 2. Axis No. 3, which is on two spures as shown The project lies in the seismically active area of the country. in Fig. 2, was considered. However, this was also not agreed with an 8-MM scale intensity have occurred upon because of the valley divergence downstream of it. on this seismic belt. Webster (1970), while describing the Finally, axis No. 4 was recommended for the axis of the crack resistant measures taken at Mica Dam, mentions dam. At this axis, the downstream convergence is available the use of blended material in the core as one of them. and the overburden on the right abutment is less compared Hjelness and Lavania (1980) have also found such material to axis No. 2. to be leakage, erosion and crack resistant. However, the practical difficulty of removing oversize boulders still persists. Curvature in the dam axis was proposed to give a little In case of Nurek Dam, it was achieved by rolling down convexity to the dam axis upstream, with a view toward the material on a hill slope. providing some sort of arch action in the dam which might help in reducing the tendency of transverse crack formation Zoning of Dam Section - To be practical, all the materials in the body of the dam. used in the dam fill must be obtained from river terraces. The core material has to be obtained from the terraces DAM SECTION where the thickness of upper clay layer is sufficient to impart the desired impermeability to the blended material­ Available Fill Materials - Phyllitic rocks are not suitable without adding fine material from other areas. The pervious for use in the dam section where high pressures are expected material for upstream and .downstream shell of the dam to occur. Thus only grade I rocks, available from chute has to be obtained from terraces where the thickness of spillway excavation, will be utilized in the upper portions upper impervious material from it has good drainability; of the dam. The survey for construction materials for the this must be done without screening off the fine material. dam indicated that the river terrace material containing The material for the filters between core and shells has silt, sand, gravel, and boulders can be utilized in pervious to be obtained either from a river bed where clean gravels zones of the dam. Therefore, initially it was decided to are available, or screened from the terrace material. Thus have the core of the dam consist of two zones. As shown the zoning of the dam section became a very simple job.

Second International Conference on Case Histories in Geotechnical Engineering 616 Missouri University of Science and Technology http://ICCHGE1984-2013.mst.edu The proposed dam section is shown in Fig. 5. logical parameters to determine the height of the coffer dam corresponding to 1 in 1000 year frequency flood and it has been found to vary between 85.0 m to 115.0 m. There have been discussions also regarding the design flood fre­ quency and the decision has been in favour of 1 in 1000 year mainly because of not taking any risk for expected unprecedented flash flood in the absence of long-term hydro­ logical data of the region. For the coffer dam section shown in Fig. 6, about 2.3 million m 3 of fill is to be placed in GfEI.~ one non-monsoon period of about six months in a very narrow

Eteval•on. in meters gorge of the river. Corresponding to 1 in 100 year flood the fill quantities reduce to half. Alternate designs for A. Biuded~r-2teriar 8 f'iltet placing the fill in two working seasons were worked out C flrw:r terrace material (upto et a!., 1980). The sections were designed to withstand, (i) the overtopping (Fig. Nos. 7a and 7b) and , (ii) through Fig. 5 - Proposed Section flow and overflow simultaneously (Fig. Nos. 8a and 8b). Shape of Core - Both the vertical and inclined cores have I been provided in high dams. The core of the Mica Dam E L.704·5~·J-12 m. is also moderately inclined with an upstream slope of 0.4-:1 and a downstream slope (sloping towards upstream) of 0.1:1; ! the core of the Oroville Dam has an upstream slope of 0.9:1 and a downstream slope (sloping towards upstream) of 0.5:1; and the core of Nurek Dam is a central core, with both upstream and downstream slopes of 0.25:1. The model studies carried out at the University of California in 1958 and at University of Roorkee in 1962 have indicated that an inclined core is somewhat more earthquake resistant. A vertical clay core was initially proposed for the section shwon in Fig. 3. It was thereafter proposed to have a moder­ Fig. 7a - Overflow Section with u/s Blanket ately inclined core for the dam, as shown in Fig. 5.

Thickness of Core - Site investigations revealed the presence of sound rock in the river bed at EL 579 m. The core was therefore extended upto this level. Also, it was considered to have a seapage gradient of 0.5 and therefore the thickness of the core has been increase, as shwon in Fig. 6, at the foundation contact level to 0.5H.

EL 704.50m -j)-IOm Impervious blanket/ core ~- .... , CCir&:R OAM ;;.-"' R ' I 2. T ransltlon - sand and gravel ~ @I I ~ I I , m 3A. Shell- sand, \)ravel and boulder up to 600mm. 1 I Wll (lnldld mhcf\1"' al'diJv, 5111, smd, Lrze--/ {. Shell-greater 75mm. Vow! G!'ld bOJJder upro J50mrn ~ 38. than n11. Solid l11y~r 0 35 70 ll' Sll"d one! oraw:f tllmltllll/lllttr toy•n lOrn. T CUA.TAJN (!.ttl) 4. River bed material. m ldhctwt of s•t, saM!, grOWl and bolkill' l.lpiO 5CIC"'mlrom riYtr r.nac:~t ·· IV' RO(k~ 1111 Fig. 7b - Overflow Section with Central Core Fig. 6 - Main Dam and Cofferdam Section Design of Filters - For high dams, specially in seismically EL.677·5J~-<-10m. active areas, well designed thick filter layers are preferred ~.~ 600mm. size rock. as compared to transition zones that roughly satisfy the ~~· ··. filter criterion. However, it was not possible to design filter ·t: lCD . layer with respect to the proposed blended material for the core using Terzaghi or U.S.B.R. Criterion. Sherard •E1..600·00···~ ·• _ ...... ~ •• (1979) provided solution to this problem of broadly graded ZONE coarse soils that are internally unstable and accordingly fine to medium sand layer has beeh provided on upstream and downstream faces of the core as shown in Fig. 6. The Fig. Sa - Through Flow Section rest of filter layers on upstream and chimney drain on down­ stream have been designed with respect to this sand layer as the base layer.

DESIGN OF COFFER DAM

It was estimated that a 104-.5 m high coffer dam would be needed to safe-guard against 1 in 1000 year frequency flood with a preceeding flood of 1 in 25 year and 2.7 m freeboard. Fig. 6 shows the placement of the coffer dam as part of the main dam. Its upstream and downstream slopes are 2:1 and 1.85:1 respectively. There have been Fig. 8b - Through Flow Section with Raised Section and a number of exercises with various combinations of hydro- u/s Blanket.

617 Second International Conference on Case Histories in Geotechnical Engineering Missouri University of Science and Technology http://ICCHGE1984-2013.mst.edu There have been also suggestions for construction of concrete overflow section, below the core of the main dam, to serve as coffer dam or to reduce the fill quantities. There is 150 yet another problem with the placing of coffer dam or ~ 451-+1-1--f---1--++~""1 the first stage dam as shown in Fig. 6. The axis of coffer dam, about 300 m upstream of the axis of main dam, would z ~ 40r-~-+--+-~~~-S~~ have all the problems anticipated for axis No. 2 of the ... dam as mentioned before. 0 ~ 3Sr--rr-r.z~--~~~--1 Study to construct the first stage dam by directional blasting ... technique was also conducted in 1978. The inference drawn 0 30r-~-t--t---4------4 w from the study was not in favour of adopting the technique ..J mainly because of presence of very .weak (sheared phyllites) I!) 25 1-...L..L-l..---J'--..___ --:' rock bands, effect of heavy blasting on other structures ~ 4·75 6·3 10 20 6.3 450 of the project and, proximity of the site to the town. GRAVEL SIZEC~U,f.) Fig. 11 - Extrapolation of Values Against Maximum STABD.ITY ANALYSIS OF DAM SLOPES Particle Size (I.R.I. 1977)

Nonseismic Condition - The outer slopes of the dam have be done for higher sizes of materials along any of the three been fixed by carrying out the stability analyses by the indicated paths. The lower most curve (No. 3) indicates Fellenius method of slices (of circular slip surface) and angle of internal friction as 42.5°. The effect of confinement by wedge method for the conditions of, (i) end· of construction, was later studied by conducting tests on 38 em. dia size (ii) steady seepage, (iii) sudden drawdown upto dead storage samples in the triaxial apparatus for shell and core materials level and, (iv) sudden drawdown upto partial pool levels. (Lavania 1982). The angle of internal friction for the shell material (Zone III) adopted for the design is 38°. For core material friction angle is 27° and Cohesion 10 kilonewtons For determining the properties of fill materials, extensive per square metre. For transition zone or filter layers friction sampling and testing has been done. In the borrow area angle adopted is 32°. for shell material (Dobata), 53 number of pits having depth of 9-10 metres were excavated (Gupta, et a!., 1979). The Seismic Condition - The conventional pseudostatic analysis gradation envelope of the material as in the borrow area was also carried out along with nonseismic condition analysis is shown in Fig. 9. Initially the shear parameters of this and the outer slopes were fixed on the base of minimum material were determined by 30 em x 30 em size direct safety factor of 1.5 and unity for nonseismic and seismic shear box. The maximum size of rock particle which could condtion with seismic coefficient of 0.15. Thereafter, a be used in the test apparatus was 25 mm only. The gradation number of studies were taken up for seismic stability of curve for the specimen material was made in such a way the dam. Design earthquake parameters were determined that the curve for the coarser 70% of the material is parallel for the site (1983). Fig. 12 shows the normalised plot of to that of borrow material as shown in Fig. 10. In order the design time history for the site. The recommended to find out the effect of particle size, plot was obtained values (max.) of design base earthquake (DBE) and maximum for maximum particle size tested versus angle of friction credible earthquake (MCE) are 0.125 g and 0.25 g respectively. vlaue, as shown in Fig. 11. Extrapolation of this curve can

!'-HYDROMETER .. ::;0 3 SIEVE ANALYSIS ANLYSIS mm mm 0 0 g ...,..._0 o coo-. • 0 c .... "' 0 ,.. "'" 100 "' .... "' .., - .... ~~~~ 80 IIIVL a:•o / - 1&1 ~v ~40 I ~~0 __ ...... -- A' 1&1 (.) ~ 1&1 0 --rrl Ill a: SILT I SANO GRAVEL II. I 0.002 I 0.01 I 0·06_1. 0·2 0·6 2·0 10 100 1000 0.006 0.02 0.1 1.0 0-00 ... (;:<; PARTICLE SIZE IN 1\4.1\4. Fig. 9 - Gradation Envelope of Dobata Material Fig. 12 - Artificial Earthquake for Tehri Site. NQI'malized to g. 100r------.------,~~~----~~ &.3mm Attempt has been made to estimate the non-recoverable 80 12·'!1 deformations (1983) due to MCE, on the basis of evaluation : 6 19·0 of yield acceleration for rigid body movement of the sliding ..: 25·0 mass. As shown in the Fig. 13, only about 15.0 m and 10m ii: 40 depths of the upstream and downstream slopes are likely ... to be affected. Therefore, it has been proposed to place 20 m deep grade I rock (instead of gravel and boulder fill), z 20·rJ~~~~~~~~::~~r==-______j which has angle of friction of about 45°. With this provision, ~ 0~ as shown in Fig. 6, there is no liklihood of any slope defor­ ~ LJs~t~L!T __ ~~==~~C::::::LJU~II=-~~E!:j mation. d-75 4-75 GRAIN SIZE M. tot. Tests were conducted at site on fill and foundation materials for determining the numerical values of dynamic shear Fig. 10 - Gradation curves fQI' Parallel Gradation Modelling modulus and damping coefficient. The plots of shear modulus Technique versus strain level for the shell and core materials are

618 Second International Conference on Case Histories in Geotechnical Engineering Missouri University of Science and Technology http://ICCHGE1984-2013.mst.edu given in Figs. 14 and 15 (1983). The damping ratio of 10% FOUNDATION PROBLEMS has been adopted for these. The foundation problems, which include abutment contact problems, are of the following two types: (1) Routine problems of earth dams; (2) special problems of the Tehri Dam arising out of the geological topographical conditions (Design Memo. 1973, Lavania 1975 and Gupta et a!., 1979).

Routine problems that are characteristic to any earth dam generally relate to: (1) Seepage through foundation rock; (2) development and accumulation of water pressure in abutments; (3) removal of weathered or undesirable zones in foundation; (4) easing off the overhangs and local steep slopes in the area and; (5) treatment of local pockets of weak material, seams, and joints. Fig. 13 - Affected Wedges of Dam PROPOSED REMEDIAL MEASURES

Routine Foundation Problems

• W.l'l! FllOIII.t.GA.TfO"' UST )!, FRU V!8~4t!ON TEST Foundation Grouting - For making the foundation sufficiently 0 'o:>cfo vre~.t.Tro" nsr impervious so as to prevent excessive seepage, curtain grou­ ting and blanket grouting in the core and transition filter contact was earlier proposed. In view of the highly jointed and sheared phyllitic rocks in the foundation, a three-line grout curtain was proposed. The depth of the primary holes at 12.0 m spacing would be 0.3 H, with a minimum of 25.0m at the crest. The secondary and tertiary holes would be about two-thirds and one-third the primary hole depth. This depth can be modified depending on the grout intakes.

Fig. 14- Dyanmic Shear Modulus Versus Shear Strain Area grouting for the core contact area and downstream {Dobata Material) fine filter has been planned at 3.0 m spacing 10.0 m deep upstream of the curtain and 5.0 m deep downstream. The foundation grouting plan is given in Fig. 16. I • II'AYE PRO.OAt>AliON TUT ll( 'AU Y18UTION TUT 1400 o fOII!:f.C VIBUTIOH TUT ~ ~ ~lOll i ~ ~ ~" k

I.,® I •Priruryh:l1t •secondary bOle "Te:rliaryhGie Sc:·!fl·2~ . ,.. ,,., '.., tRiver Q,l'llnSUln$. i!IIICfefJ ...... ~=-' ~~Uolt iTA.\1111

Fig. 15- Dynamic Shear Mod.' v/s Shear Strain, Core M~terial Fig. 16 - Foundation Grouting Plan The shell material as available in borrow areas has quite There has been lot of thinking on this aspect of foundation high percentage of silt and sand (Fig.9). Though it has been treatment during last about ten years and a number of decided to restrict the silt size particles to 6% by selective alternative proposals have been studied. Though, the grou­ borrowing but still the development of excess pore water tability tests conducted at the site have not shwon any pressure during earthquake could not be ruled out without cause of concern; but the unprecedented height of the dam tests on such a meteria!. Similarly, there could be develop­ made some engineers to think for post construction groutting ment of excess pore water pressures in the core. Therefore arrangements and accordingly the proposals of having foun­ tests were conducted (1983) on shake table by placing the dation grouting tunnels as provided in Nurek Dam and Chico­ material in saturated condition with desired density and asen Dam were prepared. Recently, proposals have been subjecting it to the sinusoidal cycles equivalent to the MCE prepared that have grouting tunnel below the core either design time history. It was observed from these tests that fully or partly (cut and cover) in the foundation rock and there would be no excess pore water pressure development connected to shafts or abutment drainage tunnels on both (and thereby reduction in shear strength) for more than 70% R.D. the banks.

A number of finite element analyses have been carried Drainage Tunnels - For preventing development and accum­ out for the dam. Two dimensional linear and non-linear ulation of pore-water pressures in the abutments, drainage analyses do not show any sign of distress to the proposed tunnels are proposed at EL 700 m on both banks. A rock dam section. There is, of course, slight concentration of cover of 30.0 m (100 ft) minimum has been proposed over stresses upstream of the core (in the shell) where it changes the tunnel below the dam fill. slope (from 0.5:1 to 1:1). But this does not seem to be of any consequence. The three dimensional analyse, which Foundation Stripping - The ASCE Committee (1972) found, are in progress, are likely to present a more rational picture for the dams studied, that the overburden-having dei?th as the valley is very narrow and confinement effect would of ± 10.0 ft (3.0 m) and less was removed from the ent1re be substentia!. area. In case of the Mica Dam, the overburden (called common material) was removed only below the core foun­ dation; at Oroville Dam, the removal was done over the entire area; and at Bennet Dam it was in the area below

Second International Conference on Case Histories in Geotechnical Engineering Missouri University of Science and Technology 619 http://ICCHGE1984-2013.mst.edu core and filters. The depth of overburden at the Tehri Dam structures. The main problem is to obtain a secure junction area is not uniform. It has been proposed that in the area between the core of the dam and the retaining wall. This under upstream and downstream shells where overburden problem is likely to be greater importance when the behaviour is met, stripping of a maximum 6.0 m (20.0 ft) be done. of this junction is visualized during an earthquake. At present, But the area under the dam core would be excavated upto the wall of counterfort type is proposed. sound rock level to provide an effective cut-off. The core foundation level and shell foundation level will be joined CONCLUSIONS by excavation at a gentle slope not steeper than 1H:lV. The paper describes the design of main features of a 260.5m Abutment Easing - As shown in Fig. 17, the gorge section high Tehri rockfill dam with brief explanations as to what has slopes of right and left abutments as 1.1:1 and 0.95:1, changes have been made time to time. This is to augment respectively. Therefore, it is proposed that the local steep the rockfill dam technology and invite suggestions. slopes and overhangs if any, be eased at a slope of 70• (0.36H:lV), as was done at the Mica Dam. However, the REFERENCES depth of silt, gravel, and boulder-fill in the river bed is expected to be of the order of 10-15 m. The gorge in this "Applicability of Directional Blasting Technique at Tehri depth is likely to have almost vertical walls. Therefore, Dam Site" (1978), Report, U.P. State Irrigation Design considerable excavation may have to be done. Keeping this Organisation, Roorkee. in view, another proposal, as mentioned in the following section on "Special Treatment", is under consideration. Bleifuss, D.J. and Hawke J.P. (1960),"Rockfill Dam: Design and Construction Problems", Transaction, ASCE, Vol.l25, Tapoldim£1.839.SIII Paper No. 3072, Part II, pp. 275-300. Clough R.W. and Pirtz D. (1958), "Earthquake Resistance of Sloping Core Dams". Transactions, ASCE, Vol. 123, Paper No. 2939, pp. 792-816. "Design of Dam" (1973), Design Memo. No. 3, Tehri Dam Project Designs, Roorkee. Sirippedsurliceprofilt olueavatf!lnfardamCOit "Design Earthquake Parameters for Tehri Dam Site" (1983), Earthquake Engg. Studies, Report No. 83-04-, Earthquake Engg. Deptt., University of Roorkee. "Dynamic Soil Parameters and other Studies for Tehri Dam" (1984-) No.84--10, Earthquake Engg. Deptt. Uni. of Roorkee. "Foundation Treatment for Dam" (1973), Design Memo. Proposedcanatteplur No. 5, Tehri Dam Project Designs, Roorkee. Fig. 17- Proposal of Concrete Plug "Foundation and Abutment Treatment for High Embankment Dams on Rock" (1972), Committee on Embankment and Treatment of Joints and Seams - It is proposed that the Slopes of Soil Mech. and Found. Dn., Proc. ASCE, Vol.98, foundation below core and filters be cleaned of loose and SM5, paper 9269, pp. 1125-1128. foreign materials, joints, faults, shear zones etc.; that it be cleaned upto a depth at least three times their width; Gupta, S.K., Lavania, B.V.K., Gupta, R.L. and Pathak, R.C. and that it be back-filled with concrete or pneumatically (1979), "Design on Dam Embankment and Foundation", applied mortar. Larger depressions would be backfilled with Proc. Workshop on Tehri Dam Project, University of concrete. Roorkee, Vol. I, pp. 76-98. Study has also been done to assess foundation settlements Gupta, S.K., Lavania, B.V.K. and Singh, N. (1980),"Coffer after the dam construction on the basis of plate load test Dam at Tehri Dam Project", Proc. GEOTECH-80, Indian data (Gupta et a!., 1979). Institute of Technology, Bombay, Vol. I, pp. 132-137. Hjeldness E.I. and Lavania B.V.K. (1980), "Cracking, Leakage Special Foundation Problems and Erosion of Earth Dam Materials", (1980), Proc. JGTD, ASCE, Vol. 106, GT2, pp. 117-135. Concrete in River Section - The abutments of the Tehri Dam are steeper than those of the Mica, Oroville, and Bennet Lavania B.V.K. (1975), "Tehri Rockfill Dam", Proc. JGTD, Dams, and the water head is also greater than for these ASCE, Vol. 101, GT9, pp. 963-976. dams. However, the abutment slopes of the Nurek Dam Lavania B.V.K. (1982), "Testing of Materials for use in High (300 m) are of almost the same order, and the water head Earth and Rockfill Dams", Proc. 4-th Int. Congress, Int. is also greater than that for the Tehri Dam. For the Tehri Assoc. of Engg. Geology, New , Vol. VI, pp. 24-3-251. Dam, in case the excavation of abutments is done at the limiting slope of 70° from the core foundation level in the Odintov A.K. and Gladkov B.F. (1971), "Construction of river bed, it would involve a huge excavation of the rocks the Concrete Plug at the Base of Nurek Dam", Proc. and for a considerable height the abutment contact would Hydrotechnical Const. Jul., No. 1, pp. 31-35. be at a slope (70•) steeper than that available naturally Sherard J.L. (1979), "Sink-holes in Dams of Coarse Broadly from above the river water level. In the Nurek Dam, for Graded Soils", Trans. Thirteenth Int. Cong. on Large a similar situation, it has been considered economically Dams, Vol. II, Q.No. 4-9, New Delhi, pp. 25-36. and technically sound to have the portion at the river bed filled with concrete (Odintov and Gladkov, 1971). This con­ Webester J.L., (1970), "Mica Dam Designed with Special crete block in the river bed, under the core contact area, Attention to Control of Cracking", Trans. Tenth Inst. is called a "concrete plug". It is !57 m long, 30 m - 60 m Cong. on Large Dams, Vol. I, pp. 4-87-507. wide, and accommodates three grouting galleries. Similar proposal (Fig. 17), with and without grouting galleries have been proposed for the Tehri Dam.

Counterforts at Junction - As the spillway is very close to the dam, a retaining wall of the order of about 25 m hight is to be constructed at the junction of the two

620 Second International Conference on Case Histories in Geotechnical Engineering Missouri University of Science and Technology http://ICCHGE1984-2013.mst.edu