An appropriate interface constitutive model is crucial to the simulation of soil-structure interface behavior. Currently, most models are only capable of describing the mechanical properties of rough interface. However, they are unable to simultaneously account for the effects of surface roughness and particle breakage. This study proposes an elastoplastic interface constitutive model considering the effects of normal stress, relative density, particle breakage, and surface roughness. It describes the variations of critical void ratio and critical stress ratio with normalized surface roughness by exponential functions. Change in critical void ratio caused by particle breakage is denoted by input work. An expression of the critical state line and a modified dilatancy function are derived based on the state-dependent dilatancy theory, uniformly describing the influences of relative density, particle breakage, normal stress, and surface roughness. The yield and hardening functions are introduced by including the plastic shear displacement as the hardening parameter based on the Mohr-Coulomb criterion. Finally, experimental data from the literature are utilized to validate the accuracy of the proposed model for various materials under different conditions.

The physical and mechanical properties of the soil-structure interface under the freeze-thaw condition are complex, making empirical shear strength models poorly applicable. This study employs integrated machine learning algorithms to model the shear behavior of frozen-thawed silty clay and the structure interface. A series of direct shear tests have been conducted under high normal stress and freeze-thaw conditions using an improved direct shear test system (DRS-1). The test data obtained were used to train and validate a classification and regression tree (CART)-based integrated model. Through cross-validation, the model's optimal hyperparameters were determined on the training set, and its performance was then verified on the test set. The results indicated that the proposed integrated learning models closely match the experimental data. The accuracy of the CART-based model on the training set is R2 = 0.994, while the accuracy on the test set is R2 = 0.763. High pressure and freeze-thaw temperature were identified as key factors influencing the trend of shear stress-strain curves. The CART-based model offers a scientific basis for predicting the shear behavior of the frozen-thawed soil-structure interface.

Energy piles are highly favored for their excellent, low energy consumption in providing heating for public residences. The temperature field changes the activity of the diffuse double electric layer (DEL) on the particle surface, thereby altering the distribution of the stress field in the soil and ultimately affecting the mechanical properties of the interface between the energy pile and the soil. Therefore, studying the influence of water content on the mechanical behavior of the soil-structure interface in the temperature field is crucial for energy pile safety. This study used a modified temperature-controlled direct shear apparatus to obtain the influence of water content and temperature on the shear behavior of the soil-structure interface. Then, the test results were analyzed and discussed. Finally, three results were obtained: (1) The water content of bentonite (wbent) had a significant impact on the shear stress-shear displacement curve of the soil-structure interface; when the wbent was less than the wp of the bentonite, the tau-l curve exhibited a softening response, then displayed a hardening response. (2) The shear strength of the soil-structure interface gradually decreased with the increase of wbent. (3) The shear strength of the soil-structure interface increased with increasing temperature under various wbent and vertical loads.

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 simulation of the soil-structure interface (SSI) under cyclic loading is critically important in geotechnical engineering. Numerous studies have been conducted to explore the cyclic behaviors exhibited at the SSI. However, existing model evaluations primarily rely on direct comparisons between experiments and simulations, with limited analysis focused on specific behaviors like accumulated normal displacement and stress degradation under cyclic loading. This study proposes and adapts six SSI models, including three nonlinear incremental models and three elastoplastic models. These models incorporate nonlinear shear modulus, critical state theory, and particle breakage effects to enhance their capability to capture SSI behaviors. Utilizing optimization-based calibration for a fair comparison, the model parameters are fine-tuned based on the experimental data. Comprehensive assessments including global comparisons and specific behaviors like accumulated normal displacement and stress degradation are carried out to evaluate the models' performance. The results indicate that all models effectively replicate the typical behaviors of SSI systems. By incorporating the particle breakage effect, the models can represent both the reversible and irreversible normal displacements under cyclic loading with better performance. The irreversible normal displacement remains stable and is solely influenced by the soil properties rather than the stress level. Moreover, the models successfully capture the stress degradation under constant normal stiffness caused by the irreversible normal displacement.

Shear behaviour of soil-structure interfaces greatly affects the performance of geotechnical structures. The soil-structure interfaces in geothermal structures (e.g., energy pile and energy wall) are often subjected to varying temperature and suction conditions. However, there is no constitutive model to simulate the coupled effects of suction and temperature on the shear behaviour of soil-structure interfaces. In this study, a thermo-mechanical model was newly developed based on the bounding surface plasticity framework to predict the thermo-mechanical behaviour of saturated and unsaturated interfaces. A power function was used to calculate the degree of saturation at the interface and improve the evaluation of suction effects on interface shear strength. A linear relationship between temperature and interface critical state friction angle was proposed to incorporate thermal effects. New equations were also proposed to describe the critical state lines (CSLs) in the void ratio versus stress plane (e-lns sigma(n)*) and to model the shearing-induced deformation at various temperatures and suctions. The experimental data from different interfaces in the literature were used to evaluate the model capability. Comparisons between measured and computed results suggest that this model can well capture the coupled effects of temperature, suction, and net normal stress on the shear behaviour of interfaces.

Pile foundations are frequently subjected to dynamic loads, necessitating a thorough investigation of cyclic shear characteristics at pile-soil interfaces. To investigate the influence of soil moisture content and concrete surface roughness on the cyclic shear characteristics of interfaces, a series of cyclic shear tests were conducted using a large-scale indoor direct shear apparatus. The effects of three normal stresses (100, 200, and 300 kPa), four moisture content levels (14%, 19%, 24%, and 29%), and five concrete surface joint roughness coefficients (0.4, 5.8, 9.5, 12.8, and 16.7) on interface shear stress and volumetric strain behavior of residual soil were analyzed. Numerical simulations were employed to analyze the microstructural changes in particles. The results show that the water content has a significant effect on the interface stress-displacement curve. It shows a cyclic hardening type at low water content and a cyclic softening type at high water content. There is a critical roughness on the concrete surface. After exceeding this value, the shear strength of the interface is no longer improved. The number of force chains in the soil increases with the increase of the number of cycles and roughness. The increase of the number of particles in the force chain leads to the increase of the instability of the force chain structure. Therefore, most of the force chains are composed of three particles. The main direction of the normal and tangential contact force anisotropy is closely related to the shear direction. The main direction will deflect with the shear direction, and the deflection angle is about 35 degrees.

A statistical damage model is proposed to investigate the impacts of the interface roughness and the shear area of the soil on shear deformation characteristics at the soil- structure interface. Assuming that the damage to the soil shear plane and the soil-structure interface follows Weibull distribution damage theory, the model introduces an equivalent initial damage factor that considers the effect of interface roughness. Results indicate that the interface roughness and soil shear area considerably influence the shear stress-displacement relation at the soil-structure interface. Under the same normal stress, the strengths of the soil-structure interface and the soil shear plane approach with increasing interface roughness and soil shear area. Furthermore, the total damage to the soil shear plane during sample loading is due to load-induced damage, whereas the total damage to the soil-structure interface comprises the equivalent initial damage, load-induced damage, and coupled damage arising from the interactions between the equivalent initial damage and load-induced damage. The feasibility of the model was verified by comparing the theoretical and experimental results, which indicate that the model can well predict the shear deformation behavior of the soil-structure interface under different soil shear areas and interface roughnesses.

Soil -structure interfaces in shallow foundations, embankments and other geo-systems are usually unsaturated, which has great influences to the performance of geo-structures. However, researches on unsaturated soilstructure interfaces are still kept in very limited numbers, particularly for the theoretical part. This paper proposes a state -dependent model for unsaturated soil -structure interfaces based on two independent stress state variables: net stress and suction. To consider the effects of initial state, stress level, and suction, a state -dependent dilatancy and suction -induced relocation of critical state line is introduced, and a rigorous, exhaustive and schematic calibration procedure of the proposed model is presented. Thereafter, simulations of direct shear tests on sand -steel, sand-geotextile, silt -steel and soil-cement interfaces with different initial states, boundary conditions and suctions are carried out. Results show that, shear strengths of interfaces increase with initial density, normal stiffness and suction, and the strain softening and dilatancy behavior are significantly enhanced by initial density and suction, while normal stiffness makes a contrary contribution. More importantly, calculations of the proposed model are fairly consistent with measurements, indicating such features of strain softening, stateand suction -dependent dilatancy, as well as the improved peak and critical shear strengths, are well captured by the model.

Recent studies focused on the shear behaviour of clay-structure interfaces have shown the importance of the shearing rate on the strength of these interfaces. In normally-consolidated clays, increasing the shearing rate results in a decrease in the interface strength, while the trend is opposite in heavily overconsolidated clays. While analytical and empirical interpretation methods indicate that the generation of shear-induced excess pore pressures are responsible for the aforementioned trends, experiments with pore water pressure measurements at the clay-structure interface are rare. In this paper, we first describe a modified interface shear box testing setup that is equipped with a pore water pressure sensor. For this equipment, the fully rough structural surface was manufactured with a port at the centre of the clay-surface interface to measure the pore water pressure. We present the results of undrained clay-structure interface tests on normally consolidated ( NC) and overconsolidated (OC) specimens of kaolin clay. The results agree with the expectations, where the NC specimens generate excess pore pressures with greater magnitudes and heavily OC specimens generate negative excess pore pressures. Measurements of the pore water pressures allow calculating vertical effective stresses, which can be used to investigate the effective stress paths followed by the clay-structure interface during the tests. This paper also provides a comparison of the measured values of beta and adhesion factors with previously published results and relationships used for the design of deep foundations.