Simulation Method Improvement of Confined Concrete Box Steel Arch Supports in Soft Rock Tunnels

The confined concrete box steel arch support—composed of box steel and core concrete with a geometrically symmetric double-chamber cross-section—is an innovative solution for effective deformation control in soft rock tunnels. To address the longstanding challenge of balancing accuracy and efficiency in numerical simulations of such supports, this study proposes an improved simulation method. Specifically, a compression-bending yield criterion for the steel–concrete composite structure was derived based on the plane section assumption and full-section plasticity criterion. This criterion was then embedded into a pile element via Fish programming to develop an improved pile element, enabling accurate simulation of the bending moment–axial force coupling effect. Verification results demonstrate that the derived yield criterion precisely captures the axial force–bending moment coupling behavior of the confined concrete box section. Numerical simulation data exhibit excellent consistency with the theoretical m-n curve, with the maximum deviation in yield axial force and bending moment not exceeding 10%. Compared with the solid element model, the improved pile element model shows differences of less than 6%, 13%, and 21% in key indicators of surrounding rock stress, deformation, and plastic zone, respectively—meeting engineering accuracy requirements. Notably, the mesh count of the improved model is only 5.5% of that of the solid element model, achieving over a 40-fold increase in computational efficiency. Furthermore, this study identifies section type and section size as the dominant parameters governing surrounding rock stability, providing valuable insights for the design and numerical simulation optimization of soft-rock tunnel support systems.