Brittle Failure Simulation of Pahang Selangor Raw Water Transfer Tunnel Under Rock Overstressing Available to Purchase

In the recent years, the underground constructions have been developed such as transportation, water conservancy, general road traffic, hydroelectric power and energy, etc. The excavation of these underground facilities changes the in-situ stress state, leading to new sets of stresses and deformation around the opening. Therefore, varies instability problems may arise, such as ground squeezing, rock burst or rock swelling because of the induced stresses (Gong et al. 2012; Ortlepp 2001). Understanding and simulating the rock failure process is the major issue of a deep excavation to achieve an appropriate rock support system that provides a stable construction with low cost. In underground openings, when the maximum stress at the tunnel boundary is larger than the rock strength a brittle failure occurs in form of spalling. Spalling occurs as a violent compressive stress causing crack growth behind the excavated surface and buckling of thin rock slabs. Nowadays, tunneling under high overburden has increased due to the construction technology development and the need for deep underground excavations. Increasing the tunnel depth causes many instability problems because the stress magnitude increases with depth. The extent and depth of failure in deep excavations are controlled by the in-situ stress magnitudes, whereas the discontinuities are the controlling factor in the failure process at shallow depth openings (Hoek et al. 1995). Slabbing or spalling is the most popular failure mode in massive to moderately jointed rock masses under high stresses (Kaiser et al. 1996; Martin 1997). It is a gradual process of stress-induced slabs formation and initiates in the region of maximum tangential stresses on the underground excavation boundary and results in a V-shaped notch. Breakouts or V-shaped notches are generated around the excavation openings of a circular tunnel if the maximum principal stress σ₁ is larger than the minimum principal stress σ₃. The fractures caused by the generated stresses are parallel to the maximum principal stress σ₁ direction during the initial formation of the V-shaped notch. The stress-induced fractures are revealed by the tension mechanism due to the variation in material properties. An increase in the stresses leads to increase the fractures at the fracture side and the adjacent elements. The final notch failure occurs when these fractures connect to the excavation boundary (Zhao et al. 2010). Recently, a rock mechanical analysis is used widely to simulate the failure depth and investigate its process. This analysis decreases the underground construction hazard and the construction cost (Edelbro 2010). Rock failure simulation requires the in-site stress magnitude and direction and the rock mass spalling strength. Many methods have been applied to simulate the rock mass failure, most of which are based on the findings of the Underground Research Laboratory in Canada (Martin 1997; Hajiabdolmajid et al. 2002; Diederichs 2007). The present study adopts different material models to predict the rock brittle failure utilizing a numerical stress modelling. The failure observed in the critical section of the Pahang Selangor Raw Water Transfer Tunnel under high overburden is predicted numerically. The constitutive models include the elastic analysis model, elastic-perfectly-plastic model and elastic-brittle-plastic model with (m_r = 0 and s_r = 0.11) and cohesion-softening friction-hardening (CSFH) model. A parametric analysis of the CSFH strength parameters is also performed to show the effect of these parameters on the failure depth and determine which approach can capture the actual failure depth.