The lesser Himalayan zone also known as midland at the south of the higher Himalayas is characterized with valleys with very thick alluvial deposit. The 87MW Seti-3 HEP located in the western Nepal has 14.50m high 67m long concrete gravity diversion structure. The proposed location of the dam for the project lies in flat river bed of thick alluvium deposit. Geotechnical investigations were carried out to explore the subsoil strata and for the dam foundation. The investigation included core drilling and in-situ testing at different locations of the river basin. Bed rock is encountered at the depth of minimum of 28m from river bed and varies across the section. The alluvial deposit composed of 43% of boulders, 6% of cobbles and remaining fines (sand and silt). A floating foundation with thick concrete dam is proposed with secant piles as a main cutoff structure. A numerical model is built on Rock science Module Slide 6.0 to perform the seepage analysis of the foundation varying the depth of the secant pile wall. This paper summarizes the outcome of the field investigation, and presents assessment of surface and subsurface condition of dam site. The results of seepage analysis with varying depth of secant piles and permeability condition are presented and discussed. Based on the seepage analysis results, it was proposed to embed the piles 1.5 meters into the bedrock to ensure optimal stability and minimal seepage.
dam, foundation, permeability, seepage
