Abstract: A model experiment of passive soil with limited width behind a rigid retaining wall was carried out using non-cohesive coarse sand, with the soil images during the experiment collected through a high-speed camera. In addition, the soil deformation and shear strain were analyzed based on the digital image correlation method to reveal the slip surface characteristics in 3 movement modes: translational mode (T mode), rotate-about-top mode (RT mode), rotate-about-base mode (RB mode). The results show that: the boundary value of width to height ratio (B/H) of soil with finite passive width is, B/H≤2.0 for T mode, B/H≤1.6 for RT mode and B/H≤1.1 for RB mode. In T mode, when 1.6≤B/H≤2.0, the passive slip surface is composed of two discontinuous fracture surfaces, the lower and upper fracture surfaces start from the heel of the moving retaining wall and the top of the fixed retaining wall respectively; when 1.10≤B/H≤1.35, the slip surface is a curved surface, the starting point and the end point are the heel of the moving retaining wall and the top of the fixed retaining wall respectively; when B/H≤0.75, the slip surface is a "reflective broken line" composed of multiple straight lines, and the horizontal angle between the sliding line starting from the moving retaining wall and the fixed retaining wall is obviously different. In RT mode, the passive slip surface is a curved surface which passes through the H/4 point, the heel of the moving retaining wall and the top of the fixed retaining wall; in RB mode, the passive slip surface starts from the top of the fixed retaining wall and extends to the middle of the moving retaining wall, and the height of extension point gradually increases with the decrease of B/H.

Key words: model experiment, limited width soil, passive soil, different movement modes, slip surface, non-cohesive coarse sand