ELASTO-PLAS'T`IC ANALYSIS OF CAVERNS.

Abstract

The analysis of large- size underground opening in jointed rock masses is an extremely complex process. The problem is of basically three dimensional in nature. The complexities in analysis arise- due to in homogeneity and anisotropy of the rock mass, the presence of joints, faults and shear zone, insitu stresses and non-linearity of the material behavior. In this study, an attempt has been made to model Sardar Sarovar Powerhouse (SSP) cavern excavated in a complex geology in the state of Gujarat, India. The SSP cavern is 210 m long, 23 m wide and 58 m high and is located in basalt intruded by dolerite 'dykes. Both contacts of this dyke with adjoining basalts are sheared. The cavern is located at about 30-65 m below the average ground level. Both basalts and dolerites consists of three joints sets each, in general, have rough surfaces, extremely narrow aperture and medium persistence. There are three main approaches namely analytical, observational and empirical, available for analysis of tunnels. The empirical approaches are based on rock mass classification systems. Observational approach basically represents a 'built as you go philosophy, and the analytical solutions are mostly based on some simplifying assumptions about the behaviour of surrounding medium. In most of the analyses, surrounding medium is assumed to be elastic or elasto-plastic and the effects of time dependent mechanical properties of medium are generally ignored. In this study, analysis of a cavern -under Sardar Sarovar Powerhouse (SSP) has been considered using both elastic and elastoplastic properties of the medium. Among various numerical techniques, such as finite element method (FEM), the boundary element method (BEM) end the distinct element method (DEM), FEM has emerged as the trendy technique in geotechnical engineering especially when constitutive laws are non-linear. In the present study, a software package for FEM program (ANSYS 7.0) has been used on the basis of Drucker Prager criteria for 3D non-linear Analysis of stresses in anisotropic rock masses for cavern in Elastic and Elasto-Plastic Media. The software ANSYS 7.0 is capable of simulating the effect of insitu stresses, deformation and geological discontinuities like faults zones and anisotropy