1940
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
1 Geological
mapping.
Over 0.75 Sq.Km on 1:1000 scale.
2 Core drilling Double tube barrel
–
over 3000 m
Triple tube barrel
–
over 500 m
3 Trenches
Three parallel trenches of +30 m.
Six cross trenches of + 15 m
4 Shafts
Six shafts of 9.5 to 18m depth, 3.5
m dia.
5 Drifts
Four drifts of +16
–
23 m.
6 Bore hole
camera studies
In 2 drill holes to study cavitations
in silt stone/bedding shear zones.
This Data indicated that the sheared /crusted rock mass shall
get consolidated if consolidation grouting is carried out and
after completion of coffer dam, curtain grouting is also
recommended, 1 m from the u/s face of coffer dam. Both these
treatments were absolutely necessary in view of typical
geotechnical problem faced. These treatments were carried out.
3. COFFER DAM - UPSTREAM AND DOWNSTREAM
The u/s coffer dam was necessitated to stop and divert the post
monsoon river flow, to facilitate construction of main dam.
This post monsoon flow ranging from 300 cumecs to 100
cumecs was required to be diverted from a diversion tunnel
which was under construction through the left abutment.
As the construction of diversion tunnel was getting delayed
considerably, provision of six sluices in the body of u/s coffer
dam was envisaged to pass the post monsoon flow.
3.1 Concept.
The work consists of design and construction of upstream and
downstream coffer dam of Indira Sagar Project. This was the
new concept of design used for first time in any country to
construct a coffer dam using 5 tons pre-cast blocks as
shuttering on both faces u/s and d/s side of a dam, and filling
the enclosure with boulders and stonecrete them under water.
All work of coffer dam has been done underground and under
water upto RL 193.5 m.
The maximum height of u/s coffer dam was 24 meter, 12 m
under water and 12 m above water. The lowest foundation
level in river bed was ± 180.60 m. For under water portion 5
ton hollow pre-cast c.c. blocks were casted and placed on the
u/s and d/s face of dam, as a shuttering in a cell of 15 m length
with the help of divers. Before placing the blocks, river bed
was leveled by using blasting under water and with special
technique, PC blocks were placed. Boulders were filled in the
enclosure of a 15 m long cell and grout pipe with safety
reinforcement were placed. The cell of 15 m was thoroughly
caulked from outside, so that river flow does not have any
effect in the cell. A colloidal grout, made out of sand, cement
water and super plasticiser was pumped through the grout pipe
at bottom and level of grout slowly built up from down upward.
Thus colloidal grout (colcrete) replacing the water in the voids
of the boulder and converting the boulder mass into concrete.
This under water work was carried out upto 193.50 m level.
About six pre-fabricated construction sluices of 2 m x 3 m were
placed to take care of post monsoon discharges upto 300
cumecs as the diversion tunnel was under construction and was
not ready.
Stone masonry was constructed above water level over the
underwater works from RL 193.5 m and raised upto RL
204.50 m.
3.2 Design.
Height of u/s coffer dam was 24 m with the lowest foundation
level kept at 180.6 and top of coffer dam as RL 204.5m. It has
been designed on the principle of gravity dam, duly checked,
for stability in various condition like Reservoir empty and
Reservoir full etc. with following parameters :-
3.2.1 Coffer dam:
a) Above water level, Randam rubble masonry with a
compressive strength of 10.5 N/mm2 (cement mortar 1:4
approximate) with selected stone placed in u/s and d/s face.
b) Under water stonecrete masonry in 1:2 and 1:3 colcrete with
P.C.C. Blocks on u/s & d/s faces.
c) PCC blocks in M-15 grade Concrete.
d) Coping : Concrete of M20 grade (c.c. 1:1.5:3) with 20 mm
graded metal.
It may be observed that for underwater work, PCC blocks were
kept on u/s and d/s section of the coffer dam and they were
considered as a homogenous part of the full section. Dam cross
section and L-section can be seen vide sketch
–
3 and 4.
