Positive Cut-Off Wall in Indian Dam - A New Technique for Difficult Geology-Paper

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The paper presents a method used for the first time in India to construct a 1-metre-thick plastic-concrete cut-off wall at the Kishanganga Hydroelectric Project in Jammu and Kashmir. The wall, built beneath the dam to limit seepage, was up to 35 metres deep and keyed into bedrock. Construction was exceptionally challenging because the overburden contained large, hard volcanic boulders, while the underlying rock reached a uniaxial compressive strength of 265 MPa.<br /><br />After evaluating options including hydromill trenching and large-diameter drilling, the contractor and its engineering consultant selected a two-stage approach: carefully controlled “surgical” blasting to fracture the boulders and form the bedrock socket, followed by excavation using heavy mechanical grabs and chisels. Borehole direction and depth were closely monitored, and explosive charges were tailored to observed geology to limit damage beyond the trench.<br /><br />Excavation proceeded in 2.5-metre-wide alternating primary and secondary panels. Bentonite slurry supported the trench walls; ultrasonic profiling checked trench geometry; and tremie pipes placed low-strength plastic concrete. Overlapping secondary panels helped ensure continuity and watertight joints. Measures to address slurry losses, piping, difficult access, equipment constraints and interruptions from nearby blasting were incorporated into the construction plan.<br /><br />The completed wall was approximately 134 metres long. Construction began in 2012 and was completed in January 2016. Water-pressure tests indicated that the blasting method did not damage the underlying bedrock. The authors conclude that controlled preliminary blasting combined with conventional grabbing and chiseling can enable cut-off-wall construction in highly bouldery, hard-rock geology.

Keywords

plastic-concrete cut-off wall

Kishanganga Hydroelectric Project

controlled blasting

bedrock socket

boulder excavation

bentonite slurry

tremie concrete

seepage control

hard rock geology

Jammu and Kashmir