Discussion and application of a risk assessment method for spalling damage in a deep hard-rock tunnel

Abstract

Abstract Quantitative risk assessment of spalling damage needs to address three key tasks: risk probability estimation, loss estimation and risk level determination. Here, based on uncertainty analysis of the damage initiation and spalling limit method, a general risk assessment method for spalling damage in hard-rock tunnels is proposed. First, with a reliability-based design method, a probability reliability estimation model for spalling damage depth was established to determine the probability of roof and wall spalling damage based on the stochastic response surface of the Hermite polynomial chaos expansion. Second, the direct economic loss due to damage was calculated by using the spalling damage depth of a unit length tunnel section and the average loss cost; thus, the risk estimation of spalling damage was carried out. Last, a theoretical analytical formula for the spalling damage expected cost ratio was derived to determine probability level thresholds, and the risk grading criteria were calculated. The method was applied to the case of the Canadian deep geologic repository project for low- and intermediate-level nuclear waste. The risk levels of roof spalling damage, wall spalling damage and both types of spalling damage were obtained, and the influences of the commonly used empirical formula for depth estimation, the coefficient of variation and the correlation coefficients of random variables on the estimated probability of spalling damage depth were analysed. The method was shown to be feasible for accurate risk assessment of spalling damage, and the results can be used to guide dynamic risk prevention and control and to support design optimization for hard-rock tunnel construction.