Tehri Dam (India)

Tehri Dam is 260.5 m high rockfill dam with clay core, recently constructed the U.P. Himalayas on the

Bhagirathi River, a tributary of the Ganga. The reservoir has a gross storage of 3.53 cubic km and live storage of

2.5 cubic km. The dam is a multi-purpose project, which involves an underground hydropower plant (HPP) with

installed capacity of 2000 MW. It provides irrigation facilities to an additional area of 2.70 lakh ha besides

strengthening irrigation in the existing Ganga Canal Command.

Geology

The dam site is located a narrow gorge with steep slopes near the bottom of the valley but flattening out towards

the top. The rocks exposed in the vicinity of the site are in general banded phyllites, the bands consisting of

mudstone and sandy material. On the basis of the quantity of argillity and sandy materials present in the rock and

the varying magnitudes of tectonic deformation suffered by them, the phyllites can be broadly grouped into three

grades in descending order of quality: grade l constituting about 45%, grade II about 25% and grade III about

30% of the total rock exposures in the gorge.

The contact between the phyllites and Shimla slates on one side with the younger quartzite and dolomite of

Garhwal group on the other, is marked by the Srinagar thrust. The Srinagar thrust is a regional feature having a

strike continuation of over 100 km approaching within 6 km of the dam site. There are a few other minor faults, 8

to 10 km from the dam site. Sandstone with low permeability was found in the core contact area after removal of

overburden and weathered rock. The power plant cavity is also located in mostly sound rock. The seismicity of

the region is high but comparable with existing Bhakra and Tarbela dams in the Himalayas and many other high

rockfill dams with clay cores such as Nurek, Chicoasen, Ataturk and El Infiernillo in other countries. The design

of the dam is conservative to withstand earthquakes of maximum anticipated intensity.

General layout

The general layout of the dam and appurtenant works is shown in Fig. 1. The axis of the dam is straight. There

are four diversion tunnels of horseshoe section with 11 m equivalent diameter, two on each bank. The diversion

tunnels are later intended to be used as shaft spillway tunnels. The main spillway js a chute spillway on the right

bank of the dam with a discharging capacity of 13150 cubic m/s. The HPP is located on the left bank.

Fig. 1. General lay-out of Tehri dam project

Fig. 2. Typical section of Tehri dam.

Dam zones: 1A and 1B well-graded impervious blended core material with maximum particle size up to 200 mm; 2A -

well-graded terrace gravelly material, maximum size 600 mm, fines (< 4.75 mm) less than 35% silt content (0.075 mm) not

more than 5% provided further that 80% of material should not contain silt exceeding 30%; 2B - well-graded terrace gravelly

material, maximum size 600 mm, silt content (< 0.075 mm) not more than 5%; 2C - well-graded terrace gravelly material,

maximum size 600 mm, fines (< 4.75 mm) between 10 and 22%; 3A -fine filter for main dam, silt content less than 5%; 3B -

fine filter for coffer dam; 4 - coarse filter sand gravel mixture, maximum size < 60 mm; 5 - well-graded hard blasted rock

with maximum size up to 1200 mm.

The dam section is shown in Fig. 2. The core is nearly central. The outer upstream slope is 2.5:1 while the

downstream slope is 1.7:1 broken by a number of berms to give an average slope of 2:1. The core consists of

well-graded impervious blended material with maximum particle size up to 200 mm. The shells consist of well-

graded terrace gravelly material with a maximum size of 600 mm with silt content (less than 0.075 mm) not

exceeding 5%. Suitably graded filters are provided on either side of the core.

Foundation Treatment

Overburden and weathered rock have been removed in the core contact area. A maximum valley slope of 0.75 :1

(H:V) has been adopted.

Curtain grouting has been done in three rows. The guideline for primary holes is 0.3 H, that for secondary holes

2/3rds that of primary and for tertiary holes 1/3rd of primary. The primary holes are spaced 12 m apart and the

minimum depth for these holes at crest is 22 m. Area grouting in the core contact zone has been done through

holes at 3 m spacing, the depth of grouting being 10 m upstream of the curtain and 5 m downstream.

Compaction

Self-propelled, smooth single-drum vibratory rollers with a minimum static weight of 15 t are being used. The

drum is not less than 1.5 m in diameter & 2.0 m in length. The vibration frequency is 1500 to 2000 per minute.

Zone I, core material: Zone 1 fill material has a moisture content varying between 1% above and 1% below

optimum with an average equal to optimum. However, for the material placed adjacent to foundations, abutments

or concrete surfaces, the moisture content was maintained at 2% to 3% above OMC.

Fill was placed in layers of 40 cm loose thickness. The lower layer was scarified before placing material for the

next layer. 8 passes of the roller to achieve an average dry density of 1.90t/cubic m, with a minimum of 1.85

t/cubic m for minus 4.75 mm material were specified. Adjacent to the rock foundations, near abutment contacts,

where the usual roller cannot reach, special compactors was used to achieve average dry density of 1.85 t/cubic m

with a minimum of 1.80 t/cubic m.

Zones 2A and 2C, rockfill material: Rockfill in these zones will be compacted in 80 cm (loose thickness) layers

by a minimum of 8 passes to attain an average dry density of 2.35 t/cubic m with a minimum of 2.3 t/cubic m.

Water will be added to the rockfill at the rate of 125 litres/cubic m before and during dumping and an equal

amount after spreading and levelling.

Zones 3 and 4, filter material: The material was placed in 40 cm thick layers and watered before compaction.

The specified dry density is 2 t/cubic m. Zone 5: This is a zone of 1.5 m thickness on both outer slopes of the

dam. This was compacted to a dry density of 2 t/cubic m. The dumped riprap on the upstream face is not to be

compacted.