Safety and stability are important issues in underground excavations. Making tunnels water tight plays an important role in this respect. In general, most of the rocks are significant barrier against groundwater movement and have very low porosity, with the exception of certain young sedimentary and volcanic rocks. However, existence of discontinuities in the rock mass increases the permeability. The Himalaya rock mass are in general deeply weathered, faulted and fractured. This leads to an increase in the rock mass permeability in the region.

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