Safety measures in Tehri dam, India

SAFETY MEASURES IN TEHRI DAM

• Tehri Dam is a 260.5m high Earth & Rockfill dam across river Bhagirathi, which originates from Gaumukh glacier of Himalayas.

• To safeguard dam against ill effects of earthquake and other factors, a number of safety measures have been adopted during Design & Construction stages.

TEHRI HYDRO DEVELOPMENT CORPORATION TEHRI HYDRO DEVELOPMENT CORPORATION LIMITEDLIMITED

TEHRI TEHRI PSPPSP

(1000 MW)(1000 MW)

KISHAU DAMKISHAU DAM(600 MW)(600 MW)

UTTARANCHAL (INDIA)

SAFETY MEASURES - DESIGN STAGE

Location of Dam :Dam is located in a narrow, S- shaped valley; left abutment of valley providing toe support to the dam.

Geological Explorations through exploratory drift in river bed area :A 3m to 20m thick major shear was suspected under the river bed. An exploratory drift parallel to dam axis was excavated 30m below river bed.


BHIL

LANG

NAA

RIVE

R

BHAGIRATHI R

IVER

BHAGIRATH

I RIV

ER

POWER INTAKE OF HPP

POWER INTAKE OF PSP

HRT OF PSP

SHAFT SPILLWAYS

TEHRI DAM

TRANSFORMER HALL

M/C HALL OF HPP

TRT OF HPP

CHUTE SPILLWAY

SHAFT SPILLWAYS

INTERMEDIATE

LEVEL OUT LET

PSP (1000MW)

HRT OF HPP DIVERSION TUNNEL T1&T2

TRT OF PSP

DIVERSION TUNNEL T3 &

T4


Liquefaction Studies for Dam Fill Materials :To exclude possibility of development of excess pore pressure during occurrence of strong earthquake, liquefaction studies on shell and core material indicated no significant loss of strength.

• Optimised Geometry of Dam Core :• Under dynamic loading, plastic zones are formed on

u/s face of dam with u/s inclined core, whereas alternative with vertical core was safe.

• In static condition, stress transfer from core to shell was highly pronounced in case of vertical core.


25.5 M

SHELL

FSL 830.0 M

CORE

RIP-RAP

FINE FILTER

SHELL

COARSE FILTER

RIP-RAP

COFFER DAM

2B

TOP INSPECTION GALLERY(EL.835.4 M ±)

2D

2A

2A

2D

31

3EL.638.00M

D/S

U/S

EL.839.5 M

4

Main Dam Axis

EL.635.0±

2C

EL.615.0±

INSPECTION GALLERY(EL. 725.0 ±)

2.51

12.03

1

34

2D

2A

5 5

30M

60M

D/S Row of Grout Curtain (60M) U/S Row of Grout Curtain (30M)

Grouting Gallery

Consolidation Grouting (10M)


Large Free-board :To take care of any settlements of crest due to large earthquake, very liberal freeboard of 9.5 m (above normal reservoir level) has been provided. A 1.2 m high parapet wall has also been provided.

Settlement allowance :Crest of dam has been raised by 2.5m i.e. about 1 % of dam height to allow for post construction vertical deformation resulting from compression and / or settlement of embankment.



• Flatter dam slopes :U/S and D/S slopes of dam are 2.5 H :1V and 2.0 H:1V, are flatter as compared to many other high dams in highly seismic region.

• Wide crest with Flaring near abutments :To safe guard against ill effects of magnification of earthquake acceleration near dam crest.

• High density of material compaction :Dry density of 2.36 T/m³ in shell and 1.90 T/m³ in core zone has been achieved reducing the chances of differential settlement.



Two inspection galleries through impervious core, one at middle height and other near crest:- Provide access for physical inspection of dam

behaviour.- Accommodate Strong Motion Accelerographs. - Observe 3D movement of dam core.

Instrumentation Scheme : For monitoring performance during construction, operation period:- 350 instruments in the body of dam and foundation.-145 Surface settlement observation points.


TEHRI HYDRO DEVELOPMENT CORPORATION LIMITEDTEHRI HYDRO DEVELOPMENT CORPORATION LIMITED

LEGEND

SAFETY MEASURES - CONSTRUCTION

Surface Treatment :• Foundation in core trench is prepared with max. slope

as 0.75 H : 1 V and max. permissible change in slope as 200 in longitudinal direction .

• Slope in the transverse direction is kept not more than 2H:1V dipping in u/s direction.

• To protect the exposed sound rock from weathering, 50 mm thick layer of guniting has been provided.



Subsurface Treatment :• Consolidation grouting has been done to fill the

voids, fractures and cracks around the core foundation contact and improve its impermeability.

Depth - 10m, Grout pressures 2 – 4 kg/ cm2. • Curtain grouting along center line of core trench

consists of two-rows 60/30m deep. Acceptable permeability of rockmass ~ 1

Lugeon, Grout pressure 10 –12 kg/ cm2.


TEHRI HYDRO DEVELOPMENT CORPORATION LIMITEDTEHRI HYDRO DEVELOPMENT CORPORATION LIMITED


• Horizontal Filter Drain Placement of processed shell material having high

permeability of the order of A x 10 –1 cm /sec in D/S portion of dam.

• Production of core material by blending clay with pebbles:In order to reduce compressibility & improve shear properties, core material was produced by blending clay with pebbles (60:40 ratio by volume).



Depressions in core near abutment: – To prevent transverse surface cracking in core

due to canyon effect, a crack preventive procedure based on Centrifugal Model Studies was developed.

– Core material in the potential cracking zones was placed with certain time lag as compared to central portion by providing depressions adjacent to canyon.

- Formation of depressions was timed during wet season.


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Safety measures in Tehri dam, India