Mechanical response of large-span variable-cross-section tunnels: insights from numerical simulation and field monitoring

Abstract

Surrounding rock deformation and loosening zone thickness are critical parameters for tunnel support design and stability evaluation. This study examined the variable-cross-section Qingshan Tunnel on the Ren-Zun Expressway. Systematic field monitoring measured surrounding rock pressure, displacement, and loosening zone under consistent geological conditions. A 3DEC discrete element model was developed to analyze the effects of excavation step length and tunnel span on deformation. The results showed that both pressure and displacement increased rapidly during the initial phase and stabilized after 6 days and 5 days, respectively. The spandrels experienced maximum pressure. Recorded values reached 0.32 MPa for the three-lane section and 0.69 MPa for the four-lane section. Maximum displacement occurred at the crown. The locations of maximum pressure and displacement were inconsistent. Deformation curves exhibited distinct patterns. The deformation rate showed a sawtooth pattern. The upper measuring line exhibited an inverted L-shape with stepwise features. The lower line followed an arc-shaped trend. The cumulative deformation and loosening zone thickness in the four-lane section were 1.25–1.78 times those in the three-lane section. Both parameters followed a spatial distribution order: crown > spandrel > wall foot. Numerical simulations identified higher displacement sensitivities in the four-lane section. Vertical displacements at the crown and inverted arch showed the highest sensitivity. These findings provide valuable guidance for the design and construction of similar tunnels.