TUNNELING THROUGH THE YOUNG HIMALAYAS - A CASE-HISTORY OF THE MANERI-UTTARKASHI POWER TUNNEL

A 8.56-km long circular tunnel with a finished diameter of 4.75 m has been constructed for the Maneri hydel (hydro-electric) project on the River Bhagirathi. The tunnel passes through quartzitic and metabasic rock formations of the young Himalayas. The tunnel excavation was accomplished from four headings, one from the upstream end at Maneri, two from an intermediate adit at Heena, and one from the downstream end near Uttarkashi where an 80 MW power house is located. Problems of tunnel face collapse, with or without heavy ingress of water, cavity formation and large tunnel closures leading to buckling of steel ribs on account of squeezing ground conditions were encountered. Methods to combat these problems are described. The absence of advance knowledge of the frequently changing rock mass and ground water conditions and, therefore, the inability of the tunnelling engineers to modify the construction method and the support system was responsible for these problems. Predicted values of support pressure obtained from the empirical methods of Terzaghi*, Barton et al.** and the one proposed by Singh et al.*** have been compared with the observed values for checking the reliability of various methods particularly for poor rock masses.