Evaluation of rock spalling prevention measures considering tunnel face extrusion in 3D elasto-plastic finite element analysis: An occupational safety perspective

During tunnel construction, workers are at risk of injury or fatality due to rock spalling from the tunnel face, particularly during blast charging and the installation of steel arch supports which require workers to enter the tunnel face. In this study, several countermeasures are evaluated for their effectiveness in preventing spalling accidents, including auxiliary benches, the umbrella arch method (long-length forepiling), long-length face bolts, primary shotcrete (firstcrete), face shotcrete (facecrete), and short-length forepoling. Their effects are analysed using three-dimensional elasto-plastic finite element analysis (FEA), with the tunnel face extrusion displacement and its reduction rate used as key indicators. Furthermore, multiple regression analysis and machine learning are applied to the FEA results. The target variables are the tunnel face extrusion displacement and its reduction rate, while the explanatory variables include key rock mass properties, such as Young’s modulus, Poisson’s ratio, cohesion, internal friction angle, tensile strength, and unit weight. Young’s modulus and the unit weight are identified as the most influential factors. Additionally, a simplified formula is derived for predicting the tunnel face extrusion displacement, and a multiple regression model (or machine learning model) is developed to estimate its reduction rate. The simplified formula is crucial to rapidly estimate extrusion displacement and assess rock spalling probability, without relying on computationally intensive numerical simulations.