Stability analysis of tunnel excavation in horizontally interbedded sandstone and conglomerate: a case study

Purpose This paper aims to report the mechanical properties and stability characteristics of horizontal sandstone–conglomerate interbedded rock masses, so as to reduce the high risk of collapse and instability during tunnel excavation. Design/methodology/approach This study combines actual tunnel engineering cases with a multivariate nonlinear regression method to prepare rock mass analog materials, conducts tunnel excavation model tests and investigates the differentiated effects of different excavation methods on rock mass deformation and stress. In addition, a three-dimensional numerical model was constructed to invert the contact surface mechanical parameters of the tunnel interbedded rock mass, establishing a theoretical model for the instability of horizontal sandstone–conglomerate interbedded rock mass during excavation. Findings The research results indicate the horizontal displacement of the arch waist is greater than the vertical displacement of the arch crown in the three-step reserved three-core soil method. While the vertical displacement at the arch crown is greater than the horizontal displacement at the arch waist in the three-step reserved two-core soil method. The extent of stress release in the surrounding rock is ranked as follows: arch base > arch waist > arch crown and arch base > arch crown > arch waist. Numerical inversion results indicate that interlayer tangential stiffness plays a key controlling role in surrounding rock deformation. Originality/value Based on the Timoshenko beam theory, the surrounding rock of horizontal sandstone–conglomerate interbedded tunnels was simplified into a two-type double-ended fixed laminated beam model, elucidating the instability mechanism of horizontal sandstone–conglomerate interbedded tunnel excavation–stiffness imbalance and deformation mode competition mechanism. The research results can provide a theoretical basis and engineering guidance for stability control of tunnel excavation under similar geological conditions.