Abstract: To investigate the differences in the evolution of passive soil arching between sand and compacted clay and to develop a corresponding analytical method, a series of trapdoor model tests were conducted in combination with digital image correlation (DIC) and discrete element simulations. The effects of relative density and fill height of sand, as well as dry density and initial water content of compacted clay, were investigated. The results show that sand is mainly characterized by continuous deformation, whereas compacted clay exhibits discontinuous failure involving the simultaneous development of shear and tensile cracks. Increases in relative density, fill height, and dry density facilitate stress transfer within the soil, whereas an increase in initial water content reduces the stress-transfer capacity of compacted clay and lowers its maximum stress ratio. The two soil types also exhibit distinct characteristics in porosity evolution and force-chain reorganization. Based on the experimental and numerical results, a deformation-dependent theoretical model for passive soil arching was established by relating the strength parameters and dilatancy angle to shear strain, in which the strain-softening parameter characterizes the rate of post-peak strength degradation. The proposed model can reasonably describe the stress ratio–normalized trapdoor displacement relationships of sand and compacted clay.