Treatment of Fossil Valley in Dam Area: A Case Study

Abstract

Himalaya occupies a unique position internationally providing ample opportunities for hydro power development through its fast flowing rivers. The total potential of three major river system of Himalaya i.e. Indus, Ganga & Brahmaputra is estimated to be 65,623 MW (at 60 % Load factor). However, only about 25 % of this potential has been tapped so far. In recent times, when ideal locations for dam in Himalayan region have almost exhausted, many dams of variable dimensions are being constructed in adverse geological settings, taking into consideration various ground improvement techniques. These dam structures are often faced with the problems of sheared and fractured rock at the foundation grade, deep overburden, slope instability of the abutments, presence of solution cavities etc. Occasionally, in glaciated valleys, fossil valleys are formed due to change in river course by sudden blockade of river in geological past. These geomorphic features are easily picked up on the ground but to establish their extent at depth requires detailed subsurface investigations. Suitable remedial measures are formulated if these features are located in close vicinity of any engineering structures. The present paper deals with the fossil valley treatment carried out on the right abutment of Parbati Dam, Stage-II under construction by NHPC Ltd in Himachal Pradesh where extensive grouting using TAM technology was carried out in phased manner utilizing various grout mix, ultrafine cement, admixtures etc. Grouting pattern was evolved so that the whole fluvioglacial material lying in the vicinity of dam area could be grouted. Post grouting check holes and permeability tests showed marked reduction in permeability values before & after the grouting, ascertaining successful treatment of the fossil valley.