Experiments and numerical simulations on pressure-arch effect for a tunnel in loose deposits

A tunnel in loose deposits, located in the National Highway No. 318, is referenced to investigate the stress disturbance characteristics and mechanism of pressure arch through a combination of physical tests and numerical simulations. The radial and circumferential stresses, formation and stability mechanism of the pressure arch are analyzed. The results show that the loose zone and extent of rock mass are larger and extend to the surface at tunnel arch, whereas a smaller zone and a larger extent are observed at tunnel sidewall. The rock mass within 120° at the arch shows an obvious radial loosing and circumferential arching effect. The rock mass within the range of 0~0.55 times the excavation span is identified to be the pressure-arch zone at the sidewall where the radial and circumferential stresses obviously increase, resulting in a high-stress concentration zone to bear the load of pressure arch and its surrounding rock. The arching coefficient has a significant spatial variation, meanwhile, it increases linearly with tunnel excavation and has the largest value at the vault, followed by that at the sidewall, in which the excavation space effect shows a marginal influence. The arching coefficient within the range of 30°~60° increases greatly when excavating within the range of 6 m before and behind the tunnel face, but it tends to be stable sooner. Both the experimental and calculated pressure arches exhibit pointed-arch shapes. Their formation is of great significance to maintaining the tunnel stability and reducing the stress of support structure.