Earth Pressure Calculation Method for Shallow-Buried Loess Tunnels Considering Support and Size Effects

Tunnel excavation provides an efficient solution for utilizing underground space and enhancing traffic efficiency. Large-section loess tunnels address the growing demand for large-span tunnel construction in Northwest China. Previous studies have thoroughly examined the calculation methods for surrounding rock earth pressure in typical cross-sectional loess tunnels. However, when applied to large cross-sectional loess tunnels, these methods yield significant discrepancies between calculated and field-measured values, making accurate estimation of the surrounding rock earth pressure impossible. Therefore, this paper develops an earth pressure calculation model for shallow tunnels based on the failure mechanism of shallow-buried loess tunnels, proposes a method for calculating the surrounding rock earth pressure in large-section shallow-buried loess tunnels, considering the effects of support and size, and further clarifies the applicable range of burial depth for this method. Considering the effects of the burial depth, tunnel size, lateral pressure coefficient, and internal friction angle of shallow tunnels, the equations for calculating the fracture angle (β) of the surrounding rock are derived. By comparing the measured earth pressure values with the theoretical calculations, the error is found to range from −4% to 27%, which is significantly smaller than that of other existing methods, thereby verifying the rationality and applicability of the proposed approach. Finally, the effects of tunnel burial depth, internal friction angle, rise–span ratio, and cohesion on earth pressure and the impacts of burial depth, internal friction angle, rise-span ratio, and lateral pressure coefficient on the fracture angle in shallow-buried tunnels are analyzed. The results show that, after considering these factors comprehensively, the calculated earth pressure and fracture angle closely align with the actual field conditions, further validating the applicability of the proposed calculation method.