1939
Design and construction of a coffer dam on Narmada River for Indira Sagar project
in central India: a case study of innovative foundation
Conception innovante et construction d’un batardeau provisoire pour le barrage sur la rivière
Narmada dans le cadre du projet Sagara en Inde centrale
Bidasaria M.
Ferro Concrete Const. (India)
ABSTRACT: Indira Sagar Project in M.P., a multipurpose project, a 92 m high dam on Narmada river was required to be constructed
but during off monsoon period flow of 300 cumecs of river Narmada was to be diverted so that dam can be constructed. For this, it
was necessary to construct a 24 m high coffer dam to divert the off monsoon flow through a diversion tunnel from left abutment. This
coffer dam was to be founded on a complex geological strata. To construct this 24 m high coffer dam, a new concept of innovative
design used for first time in any country to construct a coffer dam, using 5 tons pre-cast blocks as shuttering on both faces of a coffer
dam and filling the enclosure with boulders and stonecrete them under water. The work of 24 m high coffer dam has been done
underground and under water for 12 m height using stonecrete and balance 12 m by conventional stone masonry.
RÉSUMÉ : Pour la construction du projet de barrage principal de 92m de hauteur sur la rivière Narmada des ouvrages provisoires de
détournement de la rivière étaient nécessaires dans un contexte géologique difficile
. L’article présente la conception innovante
de
batardeaux utilisant des blocs creux préfabr
iqués en béton avec bétonnage sous eau après mise en place d’enrochements.
1 INTRODUCTION
Indira Sagar Project is situated near Punasa about 60 Km. from
Khandwa Town in Madhya Pradesh. To construct this dam, it
was essential to divert Narmada river during off monsoon
period from the original flow route, so that dam can be
constructed without any hindrances. For this, conceptually it
was necessary to have two important Component i.e. one u/s
and d/s coffer dam to stop and divert Narmada river flowing
from the original flow route, where proposed main dam was to
be constructed and another a diversion tunnel through which
Narmada post monsoon flow of 300 cumecs can be diverted
during working period. That is how the construction of u/s and
d/s coffer dam was necessitated. A sketch showing the various
component like, u/s and d/s coffer dam diversion tunnel,
proposed main dam etc. is shown in sketch
–
1.
2. FOUNDATION AND GEOTECHNICAL PROBLEMS
AND GEOLOGY
The Geological survey of India had carried out elaborate
geological studies at the India Sagar Project dam site. It has
been indicated that the dam site is located in the upper
vindhyan inter bedded sequence of tough quartz arenites
(quartzite), sand stones with minor silt stones. The bed in
general has an ENE-WSW strike and dip by 15-25 towards
NNW with exceptional steep dips of 40-45 due to local
warping. Bedding shears of 10-25 cm thickness confined to the
silt stone beds are common features. The dam area in the river
bed is occupied by a number of ENE-WSW trending vertical
fault/shear zone, indicating horst/grabben structure showing
relative vertical displacement of blocks.
It is WSW
continuation of the Sone-Narmada fault. Mapping of the area
has identified about five shear zones ranging steep dipping to
vertical. These zones are braided with clayey gougey shear
seams of 0.10-0.75 m thickness enveloping competent fractured
lenses of quartzites and sand stone of 0.50 m to 2.5 m width
sketch-2 shows geological L-section along main dam axis.
The fault zone includes the shearing (Sz-5), by virtue of its
disposition and continuity, extends beyond the coffer dam
located about 80 m upstream of the exposed section and opens
in to the water pool, created by the upstream coffer dam inspite
of being directly connected to this water pool, created by the
upstream coffer dam. Inspite of being directly connected to
this water pool with a head difference of about 18-20 m the
exposed section is completely dry and points to the near
impervious nature of the fault zone material. Permeability test
carried out in the new test holes in the fault zone confirm this
observation (< 1 lugeon).
Except in the highly crushed zone/intrusion dyke, the fault
zone material looks well compacted and was expected to have a
high in situ density in the range of 2.2-2.3. The shear strength
parameters were high and grain porosity may not exceed 20 %.
The material did not show any significant deterioration
notwithstanding the fact that there has been water to a depth of
1-2 m standing on it for a considerable time.
Even though the strata for foundation of coffer dam looked
positive, for further dam design and stability of coffer dam
foundation, detailed geotechnical investigation were necessary
to eliminate geological surprises. Due to lukewarm report
about highly Crushed zone/Intrusion dyke the foundations of
coffer dam needed additional treatment to make it water tight.
Further site investigation were carried out as given in the table
below.
It may be observed that during pre-construction stages, to
determine the detailed design parameters, following method of
site investigations were performed :-
