Experimental Study on the Shear Mechanical Properties of the Grout–Soil Interface Considering Grout Permeation

The grout–soil (GS) interface is a weak point for ground anchors, which affects the anchorage performance by load-transfer processes. In order to investigate the mechanical properties of the GS interface, direct shear tests were performed on the GS interface under constant normal load (CNL) conditions using a visual shear container. The variations of interfacial shear stress and dilatancy characteristics were analyzed, considering the effects of relative densities (Dr) of sand and soil specimen height. Moreover, the failure evolution mechanism of the GS interface during the shear process was revealed using the digital image correlation (DIC) method. A finite-slip coupled (FSC) constitutive model was developed to characterize the shear behavior of the GS interface, which was then validated by the experimental results. The results show that under a CNL condition, the peak strength φ′ exhibits a linear decrease with the increase of Dr, while the residual strength φ′ first increases and then decreases. Both peak and residual strengths show a linear reduction as the soil specimen height increases. The shear deformation at the GS interface primary occurs within a narrow zone near the soil, with a thickness of approximately 6 times the median particle diameter (d50). The findings deepen the understanding of the shear mechanical behavior at the GS interface, providing valuable insights for the development of a comprehensive evaluation framework for cast-in-place geotechnical structures, including but not limited to ground anchors.