Various tunneling problems in the Himalayan rocks at depth are discussed. A study on various tunnel failures in squeezing rocks, have led to a number of conclusions which are briefly discussed in the text, besides two case histories of tunneling under squeezing conditions.
With the rapid pace of development for harnessing water resources for irrigation and power generation in India, the importance of underground diversion structures and underground powerhouse has increased enormously during the recent years. The construction of these structures is a challenge to engineering skill, when these are located in the weak and fragile rocks of Himalayan region. Himalayas are young mountains and tectonically active. The rocks of the Himalayas, as mostly experienced through tunneling, are incompetent, quasi-elastic with a number of folds, faults and thrusts of various magnitudes and are heavily charged with subterranian water. The chances of existance of the subterranian water head are much more than that of mountain ranges. The tunneling at great depth through such a terrains with mixed lithology, varying tectonic behaviour and trapped water reservoir at considerable head becomes a very costly affair and hazardous operation due to the encounter of problems of squeezing, swelling, running ground, sudden ingress of water combined with a poor state of rock, excessive temperature and gases in rocks. The case histories of some of the problems encountered during the drivage of tunnels at great depth, through Himalayan rocks are discussed briefly, along with the findings of the studies carried out during the constructions. The studies have led to a number of valuable conclusions of practical significance and have provided data for theoretical analysis. The experiences gained in combating such problems have provided the technical skill to the designers and engineers of future tunnels in the Himalayan region.
The evolution of Himalaya is as a result of repeated deformation of the sedimentary successions accumulated in the Tethys Sea. Topographically, the highest and in age, the youngest, the Himalaya is characterised by its distinctive structural architecture and unique sedimentary and tectonic history. The following four major litho-tectonic-physiographic zones have been recognised, (Valdiya, 1980) which are separated from each other by major faults and thrusts (fig. 1).
Outer Himalayan Siwalik belt
The majority of the hydro electric projects are located in the Lesser Himalaya due to deep dissected valleys, favourable for high dams, big reservoirs and availability of the water heads for power generation (fig 2). The lesser Himalaya, with elevation ranging from 1500 to 3000 m, has strongly folded and multipy thrusted sedimentary and metamorphic rocks incorporating huge bodies of plutonic and volcanic material (of Paleazoic and Precambrain age) in the dissected valleys. The Himalayan region is within major earthquake region owing to the continuous collision of the plates (Plate Tectonics), and hence is subjected to so high pressure that it is an intricate problem to understand the implication of rock stresses whenever any underground excavation is done.