Variation of In-Situ Stress: Case Studies from Bhutan

The origin of in-situ rock stress is due to the movement of the Earth's tectonic plates, residual stresses generated by metamorphism the presence of overburden. There can be additional causes of stress, such as water pressure heat. Alterations of in-situ stress are induced by a variety of factors, especially the pre-existing discontinuities in the rock mass, inhomogeneity, anisotropy inelasticity. In published literatures on Himalayan region, the direction of in-situ stresses usually trends towards North-North-East (NNE) to North-East (NE) direction. However, there are cases, where average maximum horizontal stress has been found to be in North-West (N49⁰W) to East (N86⁰E) direction, with its magnitude varying from 4.02 – 21.65 MPa at six different location/elevation of two projects viz; Punatsangchhu-II and Tala HEP, Bhutan, respectively. In the present paper, the variation of rock stress is discussed with respect to factors like inhomogeneity, rock anisotropy, discontinuity, tectonic stress activity, residual stress phenomena gravitational stress activity (overburden stresses) influenced by topography and free surfaces.