The shear stress-shear displacement relationship and shear strength parameters at the pipe-soil interface are crucial for calculating jacking force. To investigate these properties, a series of tests were performed using a large-scale direct shear device to examine the shear mechanics of the steel-sand interface under various conditions. The effects of particle size, normal stress, and slurry concentration on shear performance were analyzed macroscopically. Additionally, the evolution of interface micro-behavior was studied using discrete element software PFC 2D. The experimental results indicate that the particle size of the sand has a significant impact on the shear stress-shear displacement curve of the interface, with smaller particle sizes requiring greater shear stress to achieve stability during shear. The strain-softening degree of sand is affected by normal stress. The shear stress-shear displacement curve is more significantly affected by particle size with the increase of normal stress. By considering different slurry concentrations, it is observed that both the shear stress and the sliding friction coefficient reached a minimum value at a concentration of 14%. The numerical simulation results indicate that particle motion causes changes in the distribution of particle structures. The distribution of particle force chains is relatively dispersed before shear. Particles move vigorously toward the shear interface, and force chains primarily concentrate on the shear interface during shear. Shear stress is transmitted through particle movement, and particle displacement causes shear dilation within the contact zone. Particles essentially cease moving toward the shear interface, and the force chains no longer change once the shear band is formed.

The shear behavior of the pipe-soil interface determines the frictional resistance of pipe jacking. In the interfacial direct shear tests of well-graded dense sand against steel pipe under both unlubricated and lubricated scenarios, the shear stress initially exhibits hardening followed by softening. The shear band forms in the hardening stage, and significant morphology of the shear band varies in the softening stage. Eventually, the shear band exhibits a bell-shaped distribution in the pattern of horizontal displacement influenced by boundary conditions and fabric anisotropy. Coarse particles exhibit greater displacement and more intense softening due to larger initial void ratios and rotational radius, while specimens with more fine particles possess smaller maximum vertical displacement away from the interface and larger critical interface friction angle. Increased normal stress restricts particle displacement, resulting in larger shear displacement at peak state, more severe particle breakage, reduced shear band thickness, and increased peak interface friction angle. The shear stress reaches the critical stage earlier with bentonite slurry (omega = 6 %) due to reduced dilatancy and particle breakage. When the slurry concentration exceeds 14 %, overall sliding of particle displacement occurs instead of the layered distribution with increased vertical particle movement and noticeable stress softening. Continuous accumulation of irreversible dilation might induce forward movement of overlying soil. Moreover, excessive slurry concentration increases hardening and interfacial friction coefficient.