Macro/meso-evolution of shear behavior of sand-steel interface in pipe jacking via digital image correlation (DIC) technology

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 (ω = 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.