Tunnel instability failure characteristics and water insulation course thickness influenced by water-rich sand layer

ObjectiveThis study aims to deeply investigate the instability failure mechanism of tunnel surrounding rock in the geological conditions of water-rich sand layers, in order to provide theoretical support for the design optimization and safe construction of related projects.MethodRelying on a typical subway section tunnel project in a coastal city, a 3D calculation model was established by using FLAC3D numerical simulation software. The stress distribution, plastic zone development, and deformation characteristics of tunnel surrounding rock in different working conditions were simulated and analyzed by systematically varying key parameters, e.g., water insulation course thickness and soil cohesion. Based on simulation results, the failure mode of surrounding rock was further investigated. A theoretical method for predicting water insulation course thickness was proposed.ResultThe plastic zone of tunnel surrounding rock shows X-shaped distribution characteristic influenced by water-rich sand layers. The plastic zone is isolated from water-rich sand layer when the water insulation course thickness is large or the soil cohesion is high. It does not form a through connection. The key parts of surrounding rock failure are concentrated on the tunnel roof. The water insulation course thickness is positively correlated with the tunnel burial depth. It is negatively correlated with the cohesion, unit weight, internal friction angle of soil at water insulation course, and tunnel initial support force.ConclusionThis study clarifies the influences of water insulation course thickness and soil strength parameters on tunnel surrounding rock stability at water-rich sand layer. The proposed method for predicting water insulation course thickness comprehensively considers geological conditions and engineering measures. It would provide theoretical basis and engineering reference for determining reasonable overburden thickness, evaluating construction risks, and formulating targeted support schemes in tunnel projects in similar geological environments.