STRESS ANALYSIS OF A POWERHOUSE CAVERN IN JOINTED ROCK MASS

Abstract

Ever since the independence hydroelectric projects are being considered as the most conservative source of power generation. A number of such projects have been constructed and are still under design and construction stages so as to utilize the enormous hydro energy, available in nature. The construction of large powerhouse caverns has always been a challenge for design engineers, as a huge amount of capital investment is involved in such projects. Powerhouse caverns are usually constructed underground to increase the head available at the turbine level, without increasing the height of the dam, as the cost of height increment of the dam is much more than that of increasing the depth of the powerhouse cavern. Hence, the excavated underground cavities demand for an overall analysis of the surrounding rock mass, and a complete understanding of the deformational behaviour of loosened rock mass prior to the excavation of cavities. The behaviour of underground caverns in massive competent rocks has been studied by using theories of elasticity, plasticity and rheology. The surrounding rock mass is analyzed by modeling its behaviour either physically, empirically or numerically, depending upon the site characteristics and also the importance of the project. Limitations of Physical modeling motivated the research workers to develop empirical models in early 705, based on field observations and case histories. With the realization of the negligence of site specific conditions in empirical methods and advancement in the computing techniques, several numerical modeling approaches have been evolved e.g., the Finite Element Method (FEM), the Boundary Element Method (BEM), and the Distinct Element Method (DEM). Out of these, FEM emerges to be the most comprehensive and most popular technique, as far as stress analysis is concerned, due to its ability to model typical anisotropic and non-linear behaviour of the rock mass. Rock masses are characterized by the existence of distributed joints. The mechanical properties of jointed rock masses are strongly dependent on the properties and geometry of joints. Jointed rock masses are usually weaker and more deformable and are highly anisotropic when compared with intact rocks. In finite element modeling of the ii rock mass, the one of the approaches is to assume the anisotropic rock mass as an equivalent isotropic continuum, explicitly reflecting the properties of the jointed anisotropic rock mass. In the present study, a Hydro-Electric Powerhouse composed of three adjoining caverns has been analyzed using Finite Element Modeling of the jointed rock mass surrounding the cavities. A 2-dimensional elastic analysis considering the plane strain condition has been carried out with the help of a general purpose software package, i.e., Analysis of Stresses in Anisotropic Rock Masses (ASARM, Samadhiya, 1998). The program written in FORTRAN is a powerful numerical tool, with the capabilities to simulate in-situ stresses and major geological discontinuities, like shear zones, fault zones and anisotropy associated with jointed rock masses. The various cases of analyses considered in the study, i.e. Linear Isotropic, Linear Anisotropic and Non-Linear Anisotropic predicts different results, for the full face single stage excavation of the multiple caverns. Out of these cases, the non-linear anisotropic analysis has been found to give realistic results, and the results from this analysis have been recommended as the final solution. To simulate the excavation process, which is generally carried out in various stages, an attempt has been made to predict the deformational behaviour of the surrounding rock mass, by analyzing the multiple cavities in fifteen stages of excavation. The results of the analyses being displacements and stresses at the nodal and typical Gaussian points respectively, are analyzed and correlated with that of closed form solutions and the results from empirical solutions. The displacements at the crown and side walls of the caverns resulting from non-linear anisotropic analysis have been found to be in good agreement with those from empirical methods. An empirical approach has been used to suggest the support measures for the major powerhouse caverns, i.e. Machine Hall and the Transformer Hall.