Effect of tunnel overburden stress on the rock brittle failure depth

Tunneling under high overburden stresses results in many tunnel instability problems due to the rock overstressing. Understanding and simulating the rock failure process is the major issue of a deep excavation to achieve an appropriate rock support system that provides possible cost-effective and stable construction. The excavation of the Pahang Selangor Raw Water Transfer Tunnel is considered in this paper. Three critical cases of the project are analyzed. A possible rock brittle failure was predictable at the tunnel sidewalls under a depth of more than 500m. The rock overstressing is analyzed based on the in situ stress conditions, intact rock strength, and actual failure depth observed at the site. Failure zones are simulated using the cohesion softening-friction hardening model and compared with the site observed failures. A review of underground openings excavated in different rock mass conditions showed that the ratio of the maximum boundary stress to the uniaxial compressive strength (sigma(max)/sigma(ci)) is suggested as the key parameter to determine the tunnel instability problems. In this study, an attempt is made to investigate the influence of the maximum tangential boundary stress to the uniaxial compressive strength ratio(sigma(max)/sigma(ci)) on the rock brittle failure depth, stress distribution, and displacement of the rock mass around the tunnel. A parametric study is implemented using different tunnel depths including the actual tunnel depths. The results show that with increasing tunnel depth or (sigma(max)/sigma(ci)) ratio, the risk of spalling, rock burst, and other tunnel instabilities are increasing.