FINITE ELEMENT ANALYSIS OF TUNNELS IN ROCK MASS

Abstract

Construction of tunnel, by and large is an art. The engineers have constructed tunnels for many centuries. Many kilometers of tunnels are still being constructed throughout the world. A large percentage of these are constructed in rock. When excavation is made through rock mass, surrounding rock loses its support. As a result, the surrounding rock deforms towards the excavation. Due to change in deformation stresses are developed in the surrounding rock, which influence the initial equilibrium of the rock mass. New equilibrium is established with changed stresses and increased deformations and at certain position the stresses developed in the rock mass due to excavation exceed the strength of the surrounding rock mass. In such situation the rock mass falls towards the tunnel. Supports are provided to prevent the failure of tunnel as well as other under-ground structure. The supports are designed on the basis of the induced stresses. The objective of analysis of an underground excavation is to determine the induced stresses due to excavation and the associated displacements. Boundary element method, Distinct Element Method, Finite Element Method, Finite Difference Method are some of the numerical techniques available to simulate and model the rock structures. With the advent of computer and advancement in numerical technique, the problem of solving complex geo-material properties, geometrical shapes 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 element package ANSYS 5.4 has been used for analysis. Analysis has been done for a section of Delhi metro tunnels and a set of tunnels near hill site at Udampur. Effect of parallel tunnels on each other has been studied. In case of circular tunnels the effect of adjacent tunnel is insignificant if the center to center distance between the tunnels is more than six times radius of the tunnel. Also, parametric study with change in depth of the tunnel and Poisson's ratio of rock mass has been performed.