CUT-OFFS IN TEMPORARY COFFER DAMS AS SEEPAGE BARRIERS FOR CONCRETE DAM CONSTRUCTION –A CASE STUDY OF TEESTA

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The Teesta-V hydroelectric project in Sikkim required excavation for a concrete gravity dam founded on rock about 40–42 m below the riverbed. The riverbed consisted of permeable alluvium with numerous large boulders, creating a serious risk of seepage and piping beneath the temporary upstream and downstream cofferdams. Controlling seepage was essential to protect the cofferdams and maintain a workable dam pit, particularly during the short construction season.<br /><br />NHPC selected jet-grouted cutoff walls, supplemented by permeation grouting, rather than diaphragm or concrete walls or a conventional grout curtain. Jet grouting was considered faster and less vulnerable to large boulders. Double-fluid jet grouting formed interlocking soil-cement columns extending through the alluvium to about 1 m into bedrock. Trial works were used to establish construction parameters and test permeability. Permeation grouting was added to address potential gaps between columns, while conventional grouting treated the underlying rock.<br /><br />The cutoff walls were constructed beneath both cofferdams within a tightly planned six-month non-monsoon period. The upstream wall was approximately 161 m long and the downstream wall 159 m long. Quality was assessed through borehole water tests and pumping tests supported by piezometers and finite-element seepage analysis. Measured cutoff permeability was approximately 0.5–0.7 × 10⁻⁶ m/s, meeting the project’s performance targets.<br /><br />The walls greatly reduced seepage through the riverbed, with remaining inflows mainly from the valley abutments and manageable by pumping. This enabled uninterrupted dam excavation and concrete placement and improved cofferdam stability. The project demonstrated that deep jet-grouted cutoffs can provide an effective, timely seepage barrier in bouldery alluvium under challenging river and geological conditions.

Keywords

Teesta-V hydroelectric project

Sikkim

jet grouting

cutoff walls

seepage control

bouldery alluvium

cofferdams

hydroelectric dam excavation

permeation grouting

concrete gravity dam