Geological, engineering geological and geophysical study of the proposed Dotigad hydropower project area of Dadeldhura district, far western Nepal

Abstract

Geological, engineering geological and subsurface investigation was carried out in the Bagarkot area, with the aim of site selection for the hydropower development (5.78 MW Run-of-River (RoR) type project) from the Dotigad River in the Lesser Himalayan succession. Lithology of the area is divided into: the Schist and the Gneiss formations. Schist is distributed as finger like pattern within the gneiss mass. Petrographic analysis of representative rock samples reveals fresh rocks free from adverse micro-cracks. The proposed weir axis has well exposed bedrock on both banks, the desander and powerhouse area lies on the alluvium deposit of Dotigad, the area between the intake and powerhouse has mostly residual soil cover and the penstock alignment lies on the colluvium. The rock of the area has three plus random joint sets. The RMR and Q-value ranges from 68-57 and 24.4-9.266, respectively categorized as fair to good. In subsurface study, 2D ERT survey was conducted using dipole-dipole array to access the depth of bedrock and other sub-surface geological condition. The result was verified by the drilling method in the key project areas. Falling head field permeability test was conducted and the coefficient of permeability was found to lie between the ranges 1.25×10-5 – 7.06×10-7 m/s indicating permeable to semipermeable soil. Analysis of geo-technical parameters of rock mass was done using the data from the field and their empirical relations. Average in-situ deformation modulus ranges between 33.51-19.09 Gpa. The in-situ horizontal and vertical stress ranges from 14.89-8.70 Mpa and 8.22-2.08 Mpa respectively, where their ratio (k) ranges from 4.9-1.79. The Damage Index of the rock mass along HRT ranges from 0.19-0.11, which indicates that stress induced damage does not occur. On the basis of overall study, the area is found to be feasible and suitable site in the context of folded and thrusted mountain belts of Himalaya.