Ground movement induced by tunnelling in shallow loess strata

Abstract

Ground movement frequently leads to disasters, including ground subsidence, damage to adjacent structures, and instability of surrounding rock, these failure modes critically challenge safety assurance in tunnel construction. This paper conducts a ground movement monitoring on the Luochuan Tunnel, systematically tracking surface settlement, layered settlement and horizontal displacement of deep soil, and support structure deformations during the construction of a shallow-buried large-span loess tunnel, and the mechanism of ground movement was explored and analyzed. Furthermore, integrated with numerical simulations, the characteristic patterns of ground movement influenced by surface load effects was investigated. The research findings indicate that the shape of the surface settlement trough exhibits a “narrow and steep” distribution, which transforms into a “wide and gentle” distribution under the influence of surface load, with the settlement value increasing from 187 mm to 303 mm. The deformation of the ground and supporting structure exhibits characteristics of minor horizontal movement and significant vertical movement. The layered settlement initially increases and then decreases with depth. The horizontal displacement along the tunnel axis experiences a process of rebounding deformation after increasing in the opposite direction of the tunnelling, and the horizontal displacement along the tunnel transverse converges towards the center. Under the influence of surface load, the ground movement increases, with a significant surge in the deformation rate during the construction of the middle and lower benches. The settlement rate of arch exceeds that of ground surface, and volume loss increases from 2.51 to 3.81 with depth. Under the influence of surface load and rainfall, differential settlement decreases from 142 mm to 3 mm, while volume loss increases to a range of 5.77 to 5.80, and the ground deformation mode shifts from progressive loosening to global settlement. Under the surface load coverage, the settlement contour of the ground exhibits a vertical profile, diminishing from the tunnel centrical line towards the periphery. The surrounding rock is primarily characterized by progressively expanding shear failures, with a tensile-shear composite failure occurring above the arch crown, and the failure range stabilizes within 15 m from the tunnel centrical line. These findings can contribute to effective deformation control in loess tunnels.