Subansiri Lower Project: Design and Construction Challenges of the Concrete Cutoff Wall

The Subansiri Lower project is one of the largest hydropower projects in India, aiming to generate 2000MW via a run-of-the-river scheme. The concrete gravity dam rests on a rock foundation, on which a continuous concrete cutoff was installed into the sandstone rock mass, for both seepage control and gradient limitation purposes. The partially reinforced concrete cutoff was executed with the diaphragm wall technique, using a special-purpose compact Hydrofraise drilling rig, working from several access galleries in the dam body and in both abutments. It was necessary to break down the site into several working areas at different locations and at different heights, which complexified operations and required the frequent transfer of equipment through specialized hoisting. Furthermore, the project planning was dependent on the dry and monsoon seasons, the latter making for very difficult conditions. This paper presents the main aspects and specificities of the project both in terms of design and construction of the diaphragm wall cutoff.