Surface settlement caused by twin-parallel shield tunnelling in sandy cobble strata

City metro tunnels are usually constructed as twin-parallel tunnels and their adjacent construction may lead to surface deformation, affecting the surface environment and the safety of the tunnels. Due to its strong dispersion, sandy cobble strata can be easily disturbed by shield tunneling. Based on the project of the Chengdu Metro Line 1, field and model tests were carried out to study the surface settlement caused by shield tunneling in sandy cobble strata by measuring surface settlement curves, ground loss ratios and construction influence zones. The discrete element method (DEM) was used to study the factors affecting the formation of ground arches in sandy cobble strata at the microscopic level. Results show that the shape of the surface settlement curve in sandy cobble strata is different from that in soft soil. The buried depth and clear spacing of the two tunnels had a significant impact on the formation of ground arches.