Suction caisson, characterized by convenient installation and precise positioning, is becoming increasingly prevalent. Over prolonged service, a significant seepage field forms around the caisson, particularly in sandy seabed, altering the contact stress at the caisson-soil interface and causing change in the interface shear strength. Given these interface contact properties, a series of cyclic shear tests are performed, incorporating the effect of pore water pressure. Test results indicate that the interface shear strength depends on normal stress, while the interface friction angle is only minimally influenced. Drawing from the findings of the cyclic shear tests, a cyclic t-z model is established to simulate the seepage-influenced caisson-soil interface shear behavior, which is also validated at the soil unit scale through interface shear tests and at the suction caisson model scale by centrifuge tests. It is further employed to forecast the evolution of skirt wall friction for a cyclic uplifting suction caisson, showcasing the capability in capturing the foundation failure under high-amplitude cyclic loading.

The critical normalized roughness (Rcr) serves as a pivotal metric for distinguishing the roughness of the soil-- structure interface. The accurate determination of Rcr is highly important in both research and engineering applications related to the mechanical properties of the interface. However, research on methods for determining Rcr are scare, and the theoretical methods are especially rare. This paper aims to establish a theoretical calculation method of critical normalized roughness Rcr. Using tribology theory, the existence of Rcr was corroborated through the analysis of both single-particle sliding and whole-soil sliding mechanisms. A theoretical formula was subsequently established for the computation of Rcr. A comparison between the theoretical calculations and experimental results revealed that the proposed formula is applicable to both scenarios involving particle breakage and scenarios lacking particle breakage at the interface. Compared with scenarios without particle breakage, the theoretical formula exhibits a superior predictive capacity for cases involving particle breakage. The proposed theoretical calculation method in this paper provides a novel approach and perspective for determining the critical normalized roughness Rcr.

Foundation elements with rough (textured) surfaces mobilize larger interface shear resistance than ones with conventional smooth or random rough surfaces when sheared against soils under monotonic loading. The overall performance of foundation elements such as piles supporting offshore wind turbines, suction caissons supporting tidal energy converters, soil nails, and soil anchors installed in cohesive soils could be enhanced through utilizing rough (textured) surfaces to resist applied static and/or cyclic loading. This paper describes the shear behavior of smooth and rough (textured) surfaces in kaolinite clay and kaolinite clay-sand mixture soils under static and cyclic axial loading. The experimental investigation presented herein consists of a series of interface shear tests performed on 3D printed rough (textured) surfaces and a 3D printed smooth reference surface utilizing the Cyclic Interface Shear Test system. The paper includes a description of the interface testing system components, cohesive soil specimens' preparation procedure, smooth and rough (textured) surfaces details, testing procedure, and results of static and cyclic tests. Test results indicate that kaolinite clay-sand mixture soil mobilized larger static and post-cyclic interface shear resistance and volume contraction relative to kaolinite clay soil when sheared against the smooth reference surface. When tested against rough (textured) surfaces with variable asperity height, larger shear resistance was mobilized and larger soil dilation greater than that mobilized by the reference untextured surface in both soils. The results also indicate rough (textured) surfaces exhibited a prevalent frictional anisotropy increases with asperity angle and height in cohesive soils, the surfaces mobilized larger shear resistance and volume change in one direction (i.e., against the asperity right-angled side) than the other direction (i.e., along the asperity inclined side).

The Cyclic Interface Shear Test (CIST) device was recently developed to evaluate the response of soil-structure interfaces subjected to monotonic or cyclic loading. Numerical models of the CIST have not been documented. Such simulations may be beneficial to help guide the design of experiments, interpret results, and inform the development of further experimental device modifications. In the present paper, a series of interface shear tests utilizing the CIST system on a cohesive soil under monotonic loadings were simulated using a proposed three-dimensional model in the commercial finite element analysis software ABAQUS/Standard. Comparisons of simulations with experimental results are presented for the Mohr-Coulomb and hypoplasticity models for cohesive soils. It is found that (i) the clay-based hypoplasticity model outperformed the simpler Mohr-Coulomb model in terms of predicting the interface shear stress evolution and the soil volume change and (ii) the clay-based hypoplasticity model allows for identification of trends in shear response as a function of normal confining pressures at the soil-structure interface (e.g. soil-structure interface shear zone thickness). Neither of these capabilities have previously been documented or experimentally validated for cohesive soil-structure interface simulations using clay-based hypoplasticity models.

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.

In geotechnical engineering, it is of great importance to study the mechanical properties of the interface between geomaterials and structure. In order to understand the shear behavior of the interface between soil-rock mixture and structure, the discrete element method is used to simulate the interface shear test of soil-rock mixture and rough structural plane. Two confining pressures and three roughness are considered. In terms of mechanical properties, macro-mechanical behavior and micro-mechanical behavior are analyzed. In terms of deformation, in order to identify the local area that plays a critical role in the interface, three methods are used to quantitatively describe the shear band thickness, including localized deformation, new contact, and particle displacement. Particle displacement is a physical quantity that can be directly observed in laboratory tests, while localized deformation must be calculated and analyzed. And new contact is a quantity that can be captured by discrete element simulation. Different analysis methods of shear band thickness are suitable for analyzing different problems by different test methods. The results show that the thickness of shear band increases with the increase of structural plane roughness. The mechanism of this phenomenon is explained by the microscopic particle inlay and the active and passive earth pressure on both sides of the structural plane unit.