SEISMIC STABILITY ANALYSIS OF TUNNEL IN JOINTED ROCK MASS USING DISTINCT ELEMENT METHOD

Abstract

Tunnels have long been assumed to have the ability to sustain earthquakes with little damage because of their redundant support from the surrounding medium. However, failure of several underground structures during recent earthquakes proposes a deeper consideration in detail design of these structures. This dissertation work is on seismic stability analysis of tunnels using distinct element method (DEM). Distinct element method code UDEC is used for the analysis. Few tunnels are considered for analysis from Tehri dam project in northern India. The approach gallery tunnel 3 (AGL 3) and the portal region of the intake tunnels are considered for seismic stability analysis. The internal forces and stress concentrating on the tunnel support systems are found out. In the present study an attempt has been made in the seismic analysis of tunnel through geological discontinuities like joints, shear seam, and tectonic fracture zones along with active faults. Preliminary parametric study is carried out to determine the size of the finite model size. Comparison study has been made between finite element method (ANSYS) and distinct element method (UDEC) for continuum static analysis. Assumptions are made where there is lack of data. Modeling parameters of a clay gouge are selected based on• a sensitivity study before dynamic analysis is performed. The critical orientation of the shear zone is determined for the approach gallery tunnels. The effect of initial in situ stress state, joint friction angle and joint shear stiffness are studied. Stresses and forces acting on the lining are plotted and interpreted with respect to the stability of the tunnels. Comparison is also made from results obtained in the gallery tunnel analysis with the shear zone and with out the shear zone. The intake tunnels are also analyzed considering free field boundary condition. It is tried to simulate the excavation stages during construction of the series of tunnels.