FINITE ELEMENT ANALYSIS OF UNDERGROUND OPENING

Abstract

The ground is in a state of prestress due to non uniform solidification of its various layers at the time of its formation and the subsequent tectonic activities. In its virgin state, rockmass is restrained against any movement. While excavation for an underground space is carried out, the in-situ rock stress is released. Consequently, restraint of some of the rock mass vanishes on atleast one side and causes displacement perpendicular to the excavation boundary and towards the excavation. These result in loosening of the rock mass and opening of the rock joints. In the case of a highly jointed rock mass, some of the blocks may become unstable and may eventually fall down. This process of redistribution of stresses continues till a new equilibrium state is reached. This new state is characterized by the concentration of compressive stresses in some locations and tensile stresses on the other. The objective of analysis of an underground excavation is to determine these stress concentrations, created by the excavation and the associated displacements. Boundary Element Method, Distinct Element Method, Finite Element Method are some of the many numerical techniques available to simulate and model the rock structures. These techniques have repined the technique of physical modeling of the structure. With the advent of computer and advancement in numerical technique, the problem of solving complex geo-material properties, geometrical shapes and stress fields haVe become possible. The present work is an attempt to use the finite element method to evaluate the change in stresses and strains caused due to excavation. Finite elethent method is widely used tool in the computer aided analysis of underground structures. iii The actual problem of underground construction is three dimensional in nature, the finite element analysis of which is time consuming. Methods have been devised to reduce the solution of actual three dimensional problem to either a series of two dimensional problems or to two different two dimensional analysis in two perpendicular planes. By doing an analysis in the longitudinal plane, the variation of longitudinal strain is determined in a polynomial form which is then used in the analysis in transverse plane. Keeping in view of the above stated advantage of two dimensional finite element analysis, computer program was developed in FORTRAN77. An anisotropic stiffness matrix has been derived for two dimensional problem, which includes the effect of joint set and their orientation, on the stress distribution of the said problem. Equivalent nodal load vector, caused due to the release of pressure as a result of excavation of material has been incorporated. The deformation of the cavern wall and the stress distribution around the opening for isotropic and anisotropic behavior of the rock mass, as obtained from the numerical analysis suggest an anisotropic behavior of rock mass. The empirical correlation proposed by Barton(1998) shows close agreement with the numerical value obtained in the numerical analysis used in the present work. The deformation as in figure given be Bhasin et. al. (1995) also obtained from numerical analysis (two dimensional UDEC-BB) shows the same result as obtained in this report.