A new classification of failure mechanisms at tunnels in stratified rock masses through physical and numerical modeling

Abstract

Stability of tunnels in stratified rock masses under low stress conditions is governed by the rock mass structure (blocks) and excavation geometry and dimension. A physical model was built and the effects of bedding dip, discontinuity spacing, and tunnel dimensions on tunnel failure mechanisms were examined. Based on the image analysis, three zones were recognized around the excavation consisting: stationary, collapsed and buckling zones. The buckling zone was more affected by layer dip angle. The collapsed zone occurred in three modes of block falls and sliding, and toppling. Also, numerical simulation using distinct element method was performed and verified. After model calibration, numerical simulations were extended to a wide range of block size and discontinuity gradients. Finally, considering physical test results, image analysis, and numerical simulation results, failure mechanisms in stratified rocks were classified using “bedding dip” and so called “dimensional ratio”. Results lead us to a deep insight of ground behavior around the shallow underground excavations in stratified blocky rock masses under low stress condition. Additionally the deformation mechanisms of blocks such as buckling, shearing, sliding are discussed in detail.