Abstract:  In Southwestern China, soil-rock mixtures characterized by loose structure, high porosity, and low strength are widely distributed. Tunneling through such strata often leads to, challenges such as poor stability, excavation collapse, and structural failure,posing serious risks to construction and operation. Based on a metro tunnel section in Chongqing, this study investigates the mechanical responses during tunnel excavation using physical model tests. It reveals the evolution of surrounding rock displacement, stress distribution, and surface settlement, and compares the effects of three excavation methods such as full-face, top-and-bottom heading, and core-soil-reserved methods. The displacement of the surrounding rock during excavation progresses through four stages, namely, undisturbed, slow deformation, abrupt change, and stable equilibrium. Deformation is most significant at the tunnel crown, where vertical displacement at 0.2 times the tunnel diameter above the crown is 1.96 times the horizontal displacement at the same distance from the haunch. Radial stress variations exhibit a zigzag trend. Stress-relief influence weakens from crown to shoulder to haunch, while stress-drop magnitude ranks shoulder, haunch, crown. Surface settlement also follows a four-stage pattern: insensitive, slow deformation, accelerated deformation, and stabilization. The curvature of surface monitoring points along the tunnel axis increases gradually as the excavation face approaches the monitoring section. The influence of different excavation methods on the disturbance of surrounding rock is decreased in the order of full section method, upper and lower bench method and reserved core soil method. The reserved core soil method can effectively reduce the deformation of surrounding rock, surface settlement and surface influence range.

Key words: soil-rock mixture, similar model test, tunnel excavation, surrounding rock deformation, surface settlement