This paper describes the formation of a large cavity in an underground complex at a hydropower plant in the Himalayas. The geometry of the cavity is controlled by the rock joints and foliation that intersected a major steeply dipping shear zone. For stabilization of the cavity, three-dimensional numerical modelling was performed using the distinct element analysis approach. The numerical model consists of the powerhouse complex including the machine hall, transformer hall and the downstream surge chamber. The results from numerical simulations indicate that the cavity has redistributed the stresses in the rock mass and that small deformations along an existing shear zone can cause significant instabilities in the surrounding rock mass resulting in an increase in the size of the cavity. Supporting of the rock mass and backfilling of the cavity with materials having a Young's modulus of larger than 300 MPa has been proposed based on the numerical results.

1 Introduction

The Himalayas has a relatively large potential of hydropower and many hydropower projects are currently under construction. Many of these projects involve large underground constructions including headrace tunnels, large powerhouses, transformer halls, desilting caverns and surge chambers. It is a well-known fact that the Himalayas, which is more than 3000 km long and 300 km wide, is generally characterized by steep slopes, lofty hills, and complex geological and tectonic settings. Therefore, detailed engineering geological assessments are warranted at the project sites for understanding the behavior of rocks in underground openings (see e.g. Bhasin and Pabst, 2015, Goel et al. 2012).

The problems faced in underground constructions in the Himalayas include squeezing ground if the rock contains a considerable amount of clay minerals, and loosening of the rock mass in the case of layered and jointed rock masses. Loosening results in the separation of the rock mass from the main body and produces a dead load. A combination of loosening and squeezing may also occur over time. In general, the geo-mechanical properties of the jointed rock masses are important for the design and performance prediction of structures built in and on rock masses.

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