Evaluation of seepage under the Ghazi Barrage: A case study

To design proper underseepage control measures for the Ghazi Barrage to be constructed on Indus River as a part of Ghazi-Barotha Hydropower Project, extensive underseepage studies were carried out. Both electric analogue and finite element computer programme were used for evaluation of exit gradients and uplift pressures. The presence of openwork gravel layers in the river bed and the gap graded nature of sandy gravel bed material prone to internal piping presented unique problems. After the completion of tender design, piping tests carried out on the gap graded materials at Imperial College, London and WAPDA’s Central Materials Testing Laboratory, Lahore, indicated that gradients at which internal erosion/piping is initiated in such materials are much lower than the theoretical values of the critical gradients. As a result, additional studies were carried out during detailed design in order to arrive at the most feasible arrangement of underseepage control measures. Different combinations of cut-offs, relief wells and drainage blanket were analysed under various hypothetical conditions likely to occur during the life time of the Project. This paper presents a discussion on the studies and reasons for adopting the recommended solution.