Experimental Investigation of Shear Behavior at the Pile–Soil Interface for Distributed Post-grouted Piles in Cohesive Soil

ABSTRACT Post-grouting techniques in pile foundations effectively improve the pile–soil interface conditions. Because of the complex mechanics of the pile–soil interface after grouting, the influence of grouting pressure and grouting amount on the shear performance under different depths of confinement are not yet fully understood. This paper employs a self-developed shear apparatus to conduct large-scale direct shear tests on the pile–soil interface, simulating the characteristics of distributed post-grouting with small spacing and multiple grout injections and systematically investigating the shear mechanisms of the pile–soil interface under various grouting amounts, grouting pressures, and normal pressure conditions. The results indicate that normal stress and grouting amount have a greater impact on the ultimate shear stress of the interface than grouting pressure. Under constant normal stress and grouting pressure, increasing grouting amount significantly enhances the shear strength and initial shear modulus of the interface. However, the improvement in mechanical properties with increased grouting amount diminishes as normal stress increases. Under different grouting and loading conditions, the cohesion and friction angle of the grouted pile–soil interface are significantly enhanced to varying degrees. The grout improves the mechanical properties of the interface by enhancing the effective cohesion, with the coefficient of shear strength enhancement ranging from 1.72 to 1.92. Furthermore, distributed post-grouted piles exhibit superior load-bearing deformation capabilities compared with traditional grouting methods, validated effectively in field experiments. The softening model and hyperbolic function model conform to the side resistance-displacement curves before grouting, whereas the hyperbolic function model better fits the side resistance-displacement curves after grouting. The research findings reveal the shear mechanisms of distributed post-grouted pile–soil interfaces, providing important references for theoretical studies on the bearing capacity of post-grouted piles.