A new evaluating approach for pressure arch stability of the surrounding rock in a shallow-buried rock tunnel

In order to identify the pressure arch of the surrounding rock in shallow-buried rock tunnels and quantitatively evaluate its global stability, a boundary identification index α and a global safety factor S of the pressure arch were proposed. When α is the change rate of the ratio of radial and tangential stresses, and the value of α at the extreme point of radial stress is taken as the critical value α′, then the contour line of α = α′ is defined as the outside boundary of the pressure arch. S is the weighted average of the strength-stress ratio (SSR) within the pressure arch, which is used to quantitatively represent the global stability of the pressure arch. The applicability of α was verified through numerical simulation and model tests, and the influence laws on global stability of the pressure arch in homogeneous stratum and composite stratum were revealed by considering four factors, including surrounding rock grade, lateral pressure coefficient, tunnel burial depth, and tunnel span. The pressure arch range identified by α is more intuitive and basically coincides with the conclusions obtained by the stress path method. Results achieved from the model test relatively accurately verified the pressure arch characteristic depicted by α. In the composite stratum, the variation law of pressure arch shape affected by different factors is generally consistent with that in the homogeneous stratum, while the pressure arch expands along the boundaries of the rock layers due to the differences in rock mass parameters.