Abstract:

To investigate the mechanical behaviors of tunnel crossing coal seam strata under the action of gas and ground stress, a case study is conducted on the Tianchengba tunnel in Guizhou, China. A threedimensional experimental physical model is constructed using a similar model test to examine the mechanical behaviors of the reserved rock wall under two tunnel spans. The variation patterns of the rock wall thickness during tunnel excavation under different factors are studied using numerical simulations. The findings are the following: (1) Tunnel excavation changes the initial stress state of the surrounding rock. This results in stress concentration in the rock wall ahead of the tunnel face. The unloading of stress on the side of the tunnel face after excavation leads to the maximum difference between vertical and horizontal stresses. (2) Under identical geological conditions, the greater the vertical stress on the upper part of the reserved rock wall and the tunnel span, the thinner the reserved rock wall becomes. This increases the likelihood of failure and instability of the reserved rock wall. (3) During tunnel excavation, the vertical stress of the reserved rock wall ahead of the tunnel face and the vertical displacement at the corresponding monitoring point increase simultaneously. When the vertical stress of the reserved rock wall ahead of the tunnel face reaches a peak, the vertical displacement deformation measured at the monitoring point also reaches a peak. Then, the vertical stress of the reserved rock wall decreases, whereas the vertical displacement deformation measured at the monitoring point gradually stabilizes. A sudden change in vertical stress can lead to the failure of the reserved rock wall ahead of the tunnel face, resulting in the sudden and rapid increase in the displacement at the tunnel monitoring point. (4) Numerical calculation models are established under different gas pressures, rock mass ratings, and tunnel spans. Through orthogonal experimental design encompassing eight different scenarios, the variation patterns of the reserved rock wall thickness under different factor combinations are obtained. Finally, a multiple regression analysis method is used to fit an empirical formula for calculating the minimum reserved rock wall thickness.

Key words: coal seamcrossing highway tunnel, rock wall stability, model test, numerical simulation