In order to explore the rules for the variation in the adfreeze shear strength at the interface between frozen soil and a pile foundation, and their influencing factors, a measuring system was developed to estimate the freezing strength at the interface by utilizing a pile-pressing method under a cryogenic environment. Experimental results demonstrate that the maximum vertical pressure on the pile top increased significantly with the decrease in temperature under the same moisture content. The shear stress-shear displacement curves, at the bottom part of the interface, presented strain-softening characteristics, while the strain-hardening phenomenon was observed at the upper part of the interface. The strength parameters of the interface decreased with the increase in the pile depth. Moreover, the influence of temperature on the shear strength of the interface was more significant compared with that of the moisture content. The research results can provide references for the construction of pile foundations, structural design optimization, and for frozen damage prevention and treatment in permafrost regions.

Large diameter rigid steel monopile is often used for offshore wind turbines worldwide. China has complex marine environment and large thickness soft clay quite different from Europe and US. The bearing capacity prediction should be further discussed under more specific pile-soil interface shear mechanism. In this paper, large-scale interface shear tests and numerical simulations were conducted, considering different soil properties and cyclic loadings. Pile-soil interface shear strength behaves like a hardening-stable mode, with a peak strength and a slightly higher stable strength. The increase of interface shear stress required sufficient shear displacement. In homogeneous soil, the shear displacement at peak strength is smaller in pile-clay interface with lower peak strength than pile-sand interface. Pile-sand-clay composite interface has the largest shear displacement at peak strength due to the compatibility of deformation, despite its peak strength is between the above two. Cyclic load amplitude and frequency had a comprehensive weaken impact on the interface shear strength, particularly to diminish in pile-clay interface than pile-sand interface. Particle displacements and force chain well illustrated the shear behaviours of pile-soil interface, which is highly consistent with macroscopic mechanical properties. Correction coefficients for pile-soil interface shear strength under cyclic loads are proposed, especially for clay.

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 capabilities compared with traditional grouting methods, validated effectively in field experiments. The softening model and hyperbolic function model conform to the side resistancedisplacement 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.