Challenges in design and construction of Punatsangchhu-I dam with deep foundation and Adverse Geology

Construction of concrete dams with deep foundations poses several challenges during planning, design and construction stages. Planning of access to the dam foundation level through excavation is a major challenge. During deep cutting, abutments slopes may get destabilize and may require extensive stabilization measures. The problem aggravates if the adverse geological features are encountered in abutments or at dam foundation level. Effective measures for seepage control through foundation of coffer dams and main dam as well as through abutments are required. Well defined analysis using analytical tools are sometimes not possible due to heterogeneous and complex rock mass conditions. In Punatsangchhu-I Hydroelectric Project, which is in advanced stage of construction in Bhutan, deepest dam foundation level is 75m below river bed. Adverse geology in terms of presence of shear zones and fractured rock mass in right abutment of dam resulted in slope stability problems. Stabilization measures in terms of grouting, micropiling and cable anchoring have been adopted to make the slope stable. Dam foundation improvement through grouting and installing 2m diameter concrete piles has been carried out. For seepage control, plastic concrete diaphragm wall below upstream coffer dam and secant piles below main dam blocks resting on relatively weaker rock mass have been provided. In general, the challenges and difficulties of dam construction in deep and adverse foundations are many, but the same can be minimized through detailed investigations, meticulous planning and design and by using latest technologies during construction.