Face Stability of Shield Tunnel in Sandy Cobble Stratum with Continuum-Based Discrete Element Method

The instability mechanism of shield tunnel excavation working face in a sandy cobble stratum was explored, and the CDEM (continuum-based discrete element method) was adopted to establish a quasi-continuous numerical model. Triaxial compression tests of sand-cobble soil were carried out numerically to analyse its macro–mesoscopic mechanical characteristics. Based on the features of " particle flow”, " soil arch effect”, and " over-excavation”, a 2D dynamic discrete element model was established to simulate the tunnel working face over-excavation with plane triangular blocks. The progressive instability mechanisms of the tunnel face (such as soil arch effect and cavity zone) were studied. The results show that the macroscopic stress-strain curve of the sandy cobble soil triaxial compression test can be divided into linear elastic, elastoplastic, and ideal plastic stages. The cobble enhances the soil structure as coarse granular particles. The contact surface of the cobble and sand weakens the macroscopic strength. The Hopper flow calibration test with the soil arching effect as the calibration criterion can effectively obtain the friction angle transformation relationships of discrete plane triangles of different scales. The arching effect is notably ahead of the face, develops gradually with over-excavation, and finally dissipates gradually after reaching the limit state. Void regions start from the bottom of the screw conveyor, develop forward and upward gradually with over-excavation and finally cut through the load-bearing arch, reaching the ground surface and forming water-drop shaped void regions. Three types of control standards for over-excavation are proposed based on different strategies.