Hydroelectric power projects are pollution free and renewable sources of energy. These projects involve the construction of dams across the flowing stream ofwater. Construction of dams in rock regions requires stability analysis of the foundation and the supporting rocks at the banks of the reservoir area. Stability of the concrete foundation over the rocky strata and stability of the banks depend upon the shear strength of the concreterock and rock-rock interfaces, respectively. Therefore, evaluation of shear strength parameters (cohesion and friction angle) at rock-rock and concrete-rock interface is the most important primary step for stability analysis and design of dams. Determination of shear strength parameters involves the in-situ tests in drifts on the dam axis site and interpretation of test results. In view of the above, site-specific shear strength parameter interpretation is carried out for Amochu hydroelectric power project site, Bhutan. This project envisages construction of 175m high concrete gravity dam across river Amochu. The existing rock type on both left and right banks of the site is observed to be weathered Phyllites. From the analysis of 20 field tests in 2 drifts, it has been found that for rock-rock interface shear strength values are approximately 50% higher at the right bank as compared to the left bank. On the other hand, for concrete-rock interface, this variation is around 10% only. Therefore, seeing the large variation of estimated shear strength in rock-rock interface, it is proposed to use individual bank stability analysis whereas, marginal variation in concrete-rock interface suggests the use of the combined analysis of the data from both the banks for foundation stability analysis.

1 Introduction

The design of foundations of dams in rocky strata includes determination of bearing capacity, settlement analysis and sliding/slope stability analysis. Bearing capacity and settlement analysis involve the ability of the rock foundation to support the imposed loads without shear failure and without excessive settlements. Sliding stability analysis involves the ability of the rock foundation or slope to resist the imposed loads without shearing or sliding. Both analyses must be coordinated and satisfied in the design. For performing theses analyses shear strength parameters viz. cohesion and friction-angle are used as input parameters. There are two major categories of analysis, a) Analysis at the concrete foundation and rock joint interface and b) Analysis at the rock-rock interface. Henceforth, determination of shear strength parameters is required for the concrete over rock (C/R) interface as well as rock over rock (R/R) interface on the project site. Former is for stability analysis of concrete dam resting over rocky strata while later is concerned with the ability of the surrounding rocks to hold the reservoir.

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