Influence of Insitu Stress on the Alignment of Head Race Tunnel due to the Intersection of Anticlines at one of the Hydroelectric Projects

It has been the regular practice that the geological structures such as faults, folds and dykes are used in geology as indicators of paleo stresses. But the stresses that created these geological structures might have suffered by different geological activities over time due to additional tectonic events, erosion, glaciation etc. (Chaudhary et al, 1974; Dasgupta et al, 1982) Hence the current rock fabric may or may not be correlated with the current insitu stress field. The geological structures may be helpful in predicting the present state of stress but always not sufficient enough for design of any engineering applications.

The alignment of 14.5 km long Head Race Tunnel (HRT) for 434 MW Rampur Hydroelectric Project was expected to intersect the axes of two anticlines. These anticlines are part of the fold systems that were expected to align the maximum compression direction along the alignment of the HRT thus providing favorable condition for the Head Race Tunnelin accordance with stress state.

Keeping in view of above, stress measurements have been conducted by hydraulic fracture method inside three vertical boreholes drilled near the axes of the anticlines. The results revealed that the present state of stress around these anticlines was governed by topography (Valley) and not by geological structures (folds). This topography induced maximum compression direction perpendicular to the alignment of the HRT and thus unfavourable to the HRT alignment. To address the instability conditions while driving the HRT through these two anticlines, heavy support system in the form of steel lining was required in place of simple concrete lining.