Brittle failure assessment for TBM tunneling under high overburden: A case study from Pahang-Selangor raw water transfer tunnel project, Malaysia

Abstract

Modelling a rock mass failure around an underground excavation is essential to evaluate the form, depth, and extent of the failure zone and achieve a sustainable tunnel support system, thereby improving underground excavation safety and minimizing the impact of high construction costs. This paper highlights the application of the 2D elastoplastic FEM analysis RS2 program to analyze the tunnel’s critical section under overstressing conditions at the longest water transfer tunnel construction in Malaysia. Site observations have shown that high in-situ stresses have resulted in a failure zone on the tunnel sides, which has weakened the steel fiber shotcrete liner and led to a rock spalling. The depth of the failure observed in several areas with overburden more than 1000 m is around 0.3 m. A numerical analysis is performed to check the magnitude, shape, and depth of the actual failure. The computational modeling methods include the elastic, the elastic-perfect-plastic, the elastic-brittle-plastic models, and the cohesion-softening friction-hardening (CSFH) models. Parametric analysis of the strength parameters of the CSFH model is performed to explain the strength parameters’ effect on the magnitude, shape, and depth of the failure. The conclusion states that no determinisitic analysis can successfully predict real failure. However, the modeling results using back analysis of the CSFH model with Cpeak and Cresidual at 21.14 MPa and 6.34 MPa and φpeak and φresidual at 16° and 52.34° verified the observed failure’s depth and form as observed by the actual phenomena.