Evaluation of rock bursting phenomena in a tunnel in the Himalayas

Tunnels passing beneath deep rock cover (overburden) are subject to instabilities caused by induced rock stresses. In a relatively unjointed massive rock mass the instability is associated with rock spalling/rock bursting, while if the rock mass is weak, schistose, sheared and deformed, squeezing is more likely. This paper reports a study which reviews stress-induced instability along the Parbati II headrace tunnel, evaluates the rock mechanical properties directly linked to the stress-induced instabilities and back-calculates the magnitude of the in situ stress state using finite element numerical modeling. An attempt is made to evaluate the magnitude of the tectonic horizontal stress component and to estimate the rock burst depth-impact. It is emphasized that more cases of tunnel damage should be studied to verify the applicability of the proposed equations and to establish the approximate range of the horizontal tectonic stress component along the Himalayan chain.