RESPONSE OF TUNNELS IN SQUEEZING GROUND CONDITION

Abstract

During the last five decades, utilization of underground space has proliferated with the advent of construction technology in difficult-to-construct terrains. Many civil engineering activities involving the construction of large size tunnels and caverns are undertaken as a part of construction of many hydro-electric projects, large underground petroleum and defense storage facilities, and the road and rail projects in hilly regions. In India, the lion’s share of these underground projects have been executed in the Himalayan region. Geology of this region is very complex and fragile. The rock masses are in a highly jointed, fractured, and weathered condition. The mechanical behaviour of jointed rock masses is very complex and becomes further more complex due to occurrence of major geological discontinuities like folds, faults, fault zones and shear zones. The situation gets further aggravated due to presence of severe hydro-geological conditions at many places along the length of tunnels. Moreover, the uneven mountainous topography and the ongoing tectonic activities in the region lead to high in-situ stresses, which form the primary loading component of these underground structures. A particular combination of large size of cavity, high depth of overburden, low rock mass quality, and high in-situ stresses gives rise to high squeezing (large convergence) of rock masses upon excavation and other stress-induced instability problems. Mechanical response of these rock masses therefore governs not only their analysis and design methodology, but also the construction technology. It also governs the overall cost of the project.