Geological, Engineering Geological and Geotechnical Studies of the Upper Trishuli – 2 Hydroelectric Project Rasuwa, Central Nepal

Abstract

The Upper Trishuli – 2 Hydroelectric Project is located on the northern part of Rasuwa District, Central Nepal. Geographically, the study area extends from longitude 85° 17' 44" E to 85° 20' 37" E and latitude 28° 07' 36" N to 28° 10' 06" N. Its estimated capacity is 100 MW with net head of about 98 m. The study is concerned with geological, engineering geological and geotechnical studies of the Upper Trishuli – 2 Hydroelectric Project. The rocks of the area belong to the Ranimatta Formation, Midland Group of Lesser Himalayan succession. They comprise quartzite, phyllite and schist. Most of the area comprise higher proportion of quartzite than that of phyllite The foliation plane of the area is along (020° to 040°) with dip amount of (45° to 30°). All the hydraulic structures lie within the bedrock. The structures like desander basin and powerhouse are underground. The headrace tunnel alignment passes through two passive faults namely, Syo Khola Fault and Gre Khola Fault. Adjusted RMR and Q ranges from 46 to 64 and 1.68 to 8.66 respectively. Generally, rocks belong to poor to fair rock class. Geotechnical studies include preliminary stress analysis and rock support design along the underground structures. Average in-situ deformation modulus ranges from 6.7865 to 22.719. Vertical and horizontal stress as well as horizontal to vertical ratio ranges from 0.74-3.9394, 1.5029-5.7050 and 0.623-3.262 respectively. Damage index varies from 0.0367 to 0.1158. Support design for construction of the headrace tunnel based on different system suggests the combination of local to systematic bolting and reinforced shotcrete as per requirement. Geophysical survey shows the thickness of alluvial and colluvial overburden ranges from 15 m to 60m. Construction material can be extracted from 14 sites while 4 sites are suitable for muck disposal.