EFFECT OF PRESSURE DEPENDENT MODULUS ON STRESS-DISTRIBUTION IN TUNNELS

Abstract

In recent times, there has been increase in the number of underground hydro-power projects in India because of economic and environmental reasons. The construction of diverging, pressure tunnels is must for the hydro-power projects. Stress analysis using numerical modelling is increasingly being used to study rock mechanics problems associated with excavations in complex geologic formations. Deformability of rock-mass significantly influences its behaviour and is, therefore, an important consideration for the design of underground openings. Weak, porous and closely jointed rocks often have elastic modulus which are not constant but increase with increasing confining pressure. The use of classical constant modulus linear elasticity in these cases can lead to erroneous predictions of the deformations and of the initiation and extent of failure around underground excavations. In the present study the rock mass is considered to have power law variation of deformation modulus with confining pressure. For stress and deformation analysis of rock-structures, finite element software package has been updated taking into account the effect of pressure-dependent deformation modulus and varying in-situ stress field conditions. The effect of pressure dependent modulus on stress-distribution pattern around the periphery of circular tunnel have been analysed. It has been found that maximum stress concentration do not occur at the excavation boundaries for in-situ stress mobilization factor z 1.0 and decrease with increasing pressure-dependent deformation modulus parameter ( a ). The offered conclusion from the present research work is that the stress-analysis of rock-structures should be carried out using pressure-dependent deformation modulus in weak, porous and jointed rock-mass.