INFLUENCE OF ANISOTROPY AND SHEAR ZONES ON STABILITY OF CAVERNS

Abstract

A number of hydroelectric projects are in design and construction stage in India for utilising untapped hydroenergy. The construction of large caverns for powerhouses, particularly in complex geological conditions, is a new challenge. The behaviour of the caverns in massive competent rock, although complicated, has already been studied using theories of elasticity, plasticity and rheology. The behaviour is further complicated by the complex geological conditions such as, presence of joints and shear zones. Under similar complex geological condition powerhouse cavern of Sardar Sarovar multipurpose project is under construction below the dam on river Narmada situated at Kevadia village in Narmada District of Gujrat state. In the present study, an attempt has been made to study the Sardar Sarovar powerhouse (SSP) cavern using 3 dimensional non-linear elastic stress analysis. The non-linear elastic analysis is justified as the cavern is located in the non-squeezing rock mass and the shear zone is non-plastic. Further, the thin shotcrete lining has been analysed by substructuring method. Limitations of the physical - geomechanical models of hydroelectric projects motivated the research workers in the early 70s to develop empirical models based on field observations and case histories. Later, with the realization of limitations of the empirical models and advent of personal computers, research efforts were directed towards several numerical techniques, e.g.. the finite element method (FEM), the boundary element method (BEM) and the distinct element method (DEM). Out of these, FEM has emerged as the most popular technique among civil engineers due to its generality and ability to model rock mass discontinuities and the associated anisotropy. Review of the literature suggests that the analysis and design of caverns is generally based upon 2-D linear elastic finite element analysis. The general approach, is to assume reduced modulus of deformation and perform analysis considering the rock mass as isotropic if the joint sets are three plus or random or both and there is no major shear zone. However, it is a gross simplification if the presence of one joint set with or without a major shear zone is ignored and the rock anisotropy is not considered in the analysis, as brought out in the present study. Also an accurate and realistic prediction of the structural behaviour of cavern requires three dimensional finite element analysis to consider the effect of complex geological features such as shear zones and major discontinuities and intrusions. In the present study, a general purpose software package, for 3-D non-linear elastic analysis of stresses in anisotropic rock masses (ASRAM) has been developed using finite element method with the capabilities to simulate in-situ stresses and geological discontinuities like shear zones, fault zones and anisotropy associated with jointed rock masses. A pre processor to support the main program for assistance in mesh / input-data generation and its graphical checking and a post-processor for the presentation of results have also been developed. The software has been used for the analysis of SSP cavern to study the influence of anisotropy and shear zones on its stability. In order to simulate the major discontinuities in SSP cavern, like shear zones, a generalised formulation of the three dimensional joint / interface element has been proposed to model interface of jointed rock mass and shear zone taking into consideration the dilatancy, roughness and undulating surface of a discontinuity. The constitutive equations have been derived to account for the complex anisotropy of the rock mass according to its micro-structural nature. The compliance matrix of the rock mass is assumed as equal to the sum of compliance matrices of the isotropic rock material and all the joint sets. The SSP cavern is 210 m long, 23 m wide and 58 m high. It is located in basalts intruded by dolerite dykes. A 2 m thick shear zone exists at the basalt-dolerite interface which intersects the cavern. The material boundaries have been chosen as the finite element boundaries in order to simulate complex geology of the area. The rock mass parameters of SSP project site