Geotechnical Investigation and Its Challenges in Landslide Dammed Valley of Nepal Himalaya: A Case History of Upper Tamakoshi Hydropower Project

Abstract

The natural damming of rivers by landslides is a prominent geomorphological phenomenon in the high, rugged mountains of the Nepal Himalaya. One such example is the wide valley of Lamabagar, which was formed by the landslide damming of the Tamakoshi River where the damsite of Upper Tamakoshi Hydropower Project (456MW) is constructed. Geotechnical investigations in such complex terrain are critical and typically involve multiple stages, including site investigation, soil and rock testing, modeling, empirical and mathematical analyses, field measurements, monitoring, and ultimately, the engineering design of structures. These steps are essential for ensuring stability and suitability of the site prior to construction. During the construction phase of the Upper Tamakoshi Hydropower Project, the 2015 Gorkha Earthquake caused substantial damages, triggering the need for further geotechnical evaluation of the dam site area. The primary objectives at that time were to delineate the bedrock depth on the left and right abutments as well as at the dam center and to determine the geotechnical parameters of both the rock and overburden materials. This study specifically focuses on the characterization of materials extracted from a borehole located at the left abutment. The analysis includes interpretation of assessment of surface observation, borehole data interpretation and in-situ tests. Findings indicate that a large boulder is resting over the alluvial deposits of the Tamakoshi River at the left abutment. This observation is significant for understanding the mechanical behavior and stability of the dam foundation in this type of area. The characterization of these materials provides essential input for structural design, seismic assessment, and long-term performance evaluation of the dam infrastructure.