The morphology of the contact surface between cast-in-place engineering structures and soil is generally random. Previous research focusing on the shear mechanical properties of soil–concrete interfaces has predominantly concentrated on the role of interface roughness by constructing regular concrete surface types, largely neglecting the potential impact of the roughness morphology (i.e., the morphology of the concrete surface). In this study, concrete blocks with the same interface roughness and different roughness morphologies were constructed based on the sand-cone method, including random rough surface, triangular groove surface, rectangular groove surface, trapezoid groove surface, and semicircular groove surface. A series of direct shear tests were conducted on the rough and smooth sand–concrete interfaces, as well as on natural sand. Through these tests, we examined the shear mechanical behavior and strength of the sand–concrete interfaces, and analyzed the underlying shear mechanisms. The results showed that: (i) the interface morphology had little effect on the variation in the shear stress–displacement curve of sand–concrete interfaces, and it had a significant influence on the shear strength of the interfaces; (ii) under the same normal stress, the shear strength of the sand–concrete interfaces with a random rough surface was the greatest, followed by the triangular groove surface, while the shear strength of the rectangular groove surface proved the lowest; (iii) the shear strength of the sand–concrete interfaces with the same roughness was affected by the size of the contact area between the concrete plane and the sand, that is, a larger contact area correlated with a decrease in shear strength. It can be concluded that the shear strength value of a sand–concrete surface with the triangular groove is the closest to the shear strength of a random rough interface. By gaining a deep understanding of the effects of different contact surface morphologies on shear strength and shear behavior, significant insights can be provided for optimizing engineering design and enhancing engineering performance.

A “random-type” sand–concrete interface shear test was developed based on the sand cone method, with a focus on the most commonly encountered triangular contact surface morphology. A “regular-type” triangular interface, matched in roughness to the “random-type”, was meticulously designed. This “regular-type” interface features five distinct triangular groove inclinations: 18°, 33°, 50°, 70°, and 90°. A series of sand–concrete interface direct shear tests were conducted under consistent compaction conditions to investigate the impact of varying compaction densities and triangular groove inclinations on the shear strength at the interface. Particle flow simulations were utilized to examine the morphology of the shear band and the characteristics of particle migration influenced by the triangular contact surface. This analysis is aimed at elucidating the influence of the inclination of the triangular groove on the shear failure mechanism at the sand–concrete interface. The findings indicate that: (1) The morphology of the interface significantly impacts the shear strength of the sand–concrete interface, while the shape of the stress-displacement curve experiences minimal alteration. (2) At smaller inclination angles, particle contact forces are arranged in a wave-like configuration around the sawtooth tip, resulting in a non-uniform stress distribution along the sawtooth slope. However, as the inclination angle grows, the stress concentration at the sawtooth tip diminishes, and the stress distribution across the sawtooth slope becomes more consistent. (3) Particle migration is significantly influenced by the sawtooth’s inclination angle. At lower angles, particles climb the structure’s tip through sliding and rolling. As the angle increases, particle motion shifts to shear, accompanied by a transition in friction from surface friction to internal shear friction. This leads to the formation of a wider shear band and an increase in shear strength.

A method for computing slip-line fields in the case of cohesionless backfills with stress discontinuity was proposed. The potential failure zone is divided into the Rankine zone and the transition zone, and the Rankine zone is rigorously determined using the theory of plastic mechanics. The potential failure zone and the Rankine zone are then further divided into a series of triangular slices. On the basis of the force and moment equilibrium conditions of a typical triangular slice, the recurrence equation of the lateral force is established. Furthermore, the relationship between the failure surface inclination angle and the interslice force inclination angle is established by satisfying the Mohr–Coulomb criterion. An iterative procedure for calculating the lateral force of the triangular slices by changing the failure surface inclination in the transition zone is performed until the interslice force satisfies the stress condition of the transition zone boundary, resulting in a stress discontinuity line if the Rankine zone and the transition zone intersect and the intersection line satisfies the stress characteristics of stress discontinuity. Example studies are performed to verify the present method, which shows that the soil–wall interface friction has the most significant effect on stress discontinuity, and the location of the stress discontinuity line gradually approaches the backfill surface with an increase in retaining wall inclination.