Abstract：
Tunnelling through weak tectonised schist bands is a challenge in Nepal Himalaya due to low strength rock and high horizontal stress condition. In this paper, a tunnelling challenge is considered from 650MW Tamakoshi-3 Hydroelectric Project where a large span, 17km long tunnel and large underground power house are proposed through strong augen gneiss with embedded weak tectonised schist bands. These bands, aligned parallel to the foliation plane (20o -30o/000o-010o),are-characterised by weak (<15MPa), crenulated, sheared, chlorite-sericite-talcose schist with clay gouge with thickness varying from a few centimetres to a maximum of 60m. About 15% of the tunnel length has to be excavated through the schist bands. Rock squeezing and tunnel collapse are expected challenges when tunnelling through such bands due to low strength, sheared nature, seepage and high horizontal stress. Therefore, underground excavation for the Project raised concern about large size tunnel constructability and longer construction time.
2013, Vol. 50 