Probabilistic modeling of excavation-induced damage depth around rock-excavated tunnels

The cracked zones caused by tunnel excavations are among the major reasons for structural instability in mechanized tunnel projects. Thus, various methods have been proposed to evaluate such cracks in terms of size and depth. However, deterministic estimations have been associated with a margin of error due to the inherent uncertainties of the respective problems as well as the approximation nature of the existing methods. Therefore, in this study, a probabilistic approach was adopted to assess the depth of a cracked zone induced around a tunnel. For this purpose, the problem was formulated as a reliability model, and the Monte Carlo method was utilized to calculate the exceedance probability for the crack depth around the tunnel. The exceedance probability was then calculated for any different depths of crack and presented as probability contours around the excavation zone. It was demonstrated that the exceedance probability is reduced sharply with an increase in the crack depth such that it fell below 5% for the cracks deeper than 2.28 times the tunnel radius. Effects of the maximum tangential stress at excavation boundary and crack initiation threshold on the exceedance probability were further calculated using the reliability sensitivity analysis. Eventually, the influence of in-plain stress ratio on the failure probability was investigated, and the results were reported for three rock types, including limestone, granite, and mudstone.