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Introduction
The National Hydro Power Corporation is building the Teesta Low Dam-IV for a Hydro Power Plant (160MW) near siliguri in West Bengal, India. A cofferdam was planned to divert the river water and allow 20m deep excavation in the river bed. The river bed alluvium is made up of a highly permeable mix of sand, gravels and boulders followed by bedrock. A grout curtain was required to reduce the permeability to 10-6 m/sec and to allow the construction of dam foundation in a relatively dry condition. The depth of bedrock varied between 5m and 20m from top of the cofferdam.
Permeation Grouting
Permeation grouting was chosen to form the grout curtain in the river bed alluvium along the cofferdam. Permeation grouting is the process of injecting a grout suspension (e.g. cement slurry, silica gel, etc.) into a granular stratum. The grout suspension permeates through the inter-granular voids and produces an integrated mass of soil and grout suspension to lower the permeability. The cofferdam with permeation grout curtain supported a max. excavation depth of 20m and a max. water head difference of 19m.
Quality Control & Testing
As part of the quality control during construction, computerised devices were used to monitor the various operating parameters (e.g. depth, flow rate, grout volume, grout pressure, etc.). In addition, the automated monitoring system delivers the quality control records in real-time to assist the termination of particular stage of grouting.
Post construction in-situ permeability tests i.e. pump-in test (above water table) and pump-out test (below water table) were conducted in the check holes drilled within the grout curtain at regular intervals along the axis of the cofferdam.
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