135. Hurdles and Achievements – An Example from Tunnelling in Himalayan Geology

Tunneling in Himalayan geology during the course of hydroelectric project has involved the construction of long tunnels of varying diameters and shapes through geologically complex terrain in sedimentary, metasedimentary and igneous rocks. Extensively folded, faulted and crushed rocks with residual tectonic stresses led to difficult tunneling problems and complicated design evaluation in recent decades posing challenging task to engineering geologists and construction engineers to negotiate. During the geotechnical appraisal of these tunnel routes, efforts were made to modify the alignment to suit the geological terrain and to avoid or reduce tunneling problems. Considerable experience has been gained by those engaged in these projects. This paper highlights hurdles and achievements during the course of tunneling 7.2 km of 23 km long 7.6 m to 8.2 m excavated dia. (6 m to 6.8m finished modified horse shoe shaped) head race tunnel for Tala Hydroelectric Project, which is located in southwestern part of Royal Kingdom of Bhutan in Eastern Himalayas.