The soil strength of soft clay is influenced by strain rate effect. Models considering strain rate effect always ignore the impact of loading rate on pore pressure and have poor applicability to 3D engineering problems. Based on the classic inelastic core boundary surface model, a logarithmic rate function representing the strain rate effect of soft soil was introduced to the hardening law. A new parameter H was added to adjust the plastic modulus while another new parameter mu is introduced to account for the strain rate effect. A rate-effect boundary surface constitutive model suitable for saturated clay was subsequently proposed. Combined with the implicit integral numerical algorithm and stress-permeability coupling analysis, the innovative model was implemented in the finite element software and validated by comparing with the results of triaxial tests. By analysing the rate-effect of 11 types of soft soil, a formula to calculate the rate parameter was derived. The developed model was used to calculate the undrained vertical bearing capacity and sliding resistance of a movable subsea mudmat. The mudmat frictional coefficient from soil undrained to partial drained and finally undrained state was obtained and compared with those from the Modified Cam-Clay model. Identical results were obtained without considering the rate effect. When considering the strain rate effect on the improvement of soil strength, the friction resistance coefficient initially decreases and then increases with the decrease of the sliding speed, eventually stabilising after reaching the limit value. The rate-effect on the friction resistance coefficient is most prominent under undrained conditions with high sliding speeds. The soil strain rate effect is suggested to be considered in the design of the subsea mudmat avoid underestimating the friction resistance.

Accurate characterization of soil dynamic response is paramount for geotechnical and protective engineering. However, the impact properties of unsaturated cohesive soil have not been well characterized due to lack of sufficient research. For this purpose, impact tests using the Split Hopkinson Pressure Bar (SHPB) were elaborately designed to investigate the dynamic stress-strain response of unsaturated clay with strain rates of 204 similar to 590 s(-1). As the strain rate increased up to 500 s(-1), a lock-up behavior was observed in the plastic flow stage, which can be explained as the breakage and rearrangement of soil gains under a high level of stress. Further, the strain rate dependency of the dynamic strength was quantitatively characterized by the Cowper Symonds (CS) model and the CS coefficients were determined to be the intercept of 55 and slope of 0.8 in the double logarithmic scale of Dynamic Increase Factor (DIF) and strain rate space. Furthermore, the SHPB test was reproduced using a modified Material Particle Method (MPM), which involves an improved dynamic constitutive model for unsaturated soil considering the strain rate effect. The simulated stress-strain curves basically agree with the experimental results, indicating the feasibility of MPM for investigating the dynamic properties of unsaturated soil under SHPB impact loading.

The evaluation of thermo-hydro-mechanical (THM) coupling response of clayey soils has emerged as an imperative research focus within thermal-related geotechnical engineering. Clays will exhibit nonlinear physical and mechanical behavior when subjected to variations in effective stress and temperature. Additionally, temperature gradient within soils can induce additional pore water migration, thereby resulting in a significant thermo-osmosis effect. Indeed, thermal consolidation of clayey soils constitutes a complicated THM coupling issue, whereas the theoretical investigation into it currently remains insufficiently developed. In this context, a one-dimensional mathematical model for the nonlinear thermal consolidation of saturated clay is proposed, which comprehensively incorporates the crucial THM coupling characteristics under the combined effects of heating and mechanical loading. In current model, the interaction between nonlinear consolidation and heat transfer process is captured. Heat transfer within saturated clay is investigated by accounting for the conduction, advection, and thermomechanical dispersion. The resulting governing equations and numerical solutions are derived through assuming impeded drainage boundaries. Then, the reasonability of current model is validated by degradation and simulation analysis. Subsequently, an in-depth assessment is carried out to investigate the influence of crucial parameters on the nonlinear consolidation behavior. The results indicate that increasing the temperature can significantly promote the consolidation process of saturated clay, the dissipation rate of excess pore water pressure (EPWP) is accelerated by a maximum of approximately 15%. Moreover, the dissipation rate of EPWP also increases with the increment of pre-consolidation pressure, while the corresponding settlement decreases. Finally, the consolidation performance is remarkably impacted by thermo-osmosis and neglecting this process will generate a substantial departure from engineering practice.

Long-term traffic loadings will induce strong vibrations in the saturated ground, and it probably produces excessive settlements of saturated ground and even various distresses (such as cracks and leakage) of the tunnel structure. To better understand the long-term cyclic deformation behaviors of saturated clay subjected to cyclic traffic loading, a series of cyclic undrained hollow cylinder apparatus tests were performed on Shanghai saturated clay. The secondary cyclic compression stage of permanent axial strain, energy dissipation, and damping ratio are employed to identify the distinct shakedown ranges of saturated clay. Moreover, attempts are made to establish a link between the permanent deformation behavior invoked by different levels of dynamic stress and a kinematic yielding framework. The cyclic test results of Shanghai clay can be classified as plastic shakedown, plastic creep, and incremental collapse, and Y-2 and Y-3 yield limits are interpreted as threshold cyclic dynamic stress to divide the shakedown ranges. Additionally, the effective cyclic dynamic stress ratio can better identify the shakedown ranges of saturated clay. Eventually, a criterion is recommended to identify distinct shakedown ranges of saturated clay. The findings will contribute to the safe design of the transport infrastructure in saturated ground.

The prediction of time-dependent behavior of axial capacity for jacked piles are essential for coastal pile engineering. This study develops a numerical model to simulate the entire process of pile installation, soil consolidation, and loading, incorporating soil-pile interaction effects on excess pore pressure and effective stress distribution in the surrounding soil, which influence the bearing performance of jacked piles in saturated clay. The well consistency between the predictions from the presented approach and the experimental measurement data validate the applicability of the proposed model. The mechanism of set-up effects on the pile axial capacity is elucidated through the evolution of excess pore pressure. A parametric study is performed to assess the influence of the permeability coefficient (k) and length-to-diameter (L/De) ratio on the axial capacity of jacked piles. The findings demonstrate that the proposed model accurately predicts the set-up effects of jacked piles. Specifically, the permeability coefficient primarily impacts the rate of capacity increase, while the axial capacity exhibits a significant rise with an increase in L/De. The derived empirical formula can reasonably guide the design of the axial bearing capacity of piles in saturated clay.

Freeze-thaw cycles (FTC) cause significant changes in the physical and mechanical properties of soil, leading to structural alterations that can seriously threaten the safety and longevity of engineering structures. To investigate the consolidation characteristics of soils subjected to FTC, 18 sets of consolidation compression tests were carried out with saturated clay. Using a modified consolidation apparatus, the changes in pore-water pressure (PWP) and strain during consolidation were measured, with a focus on the effects of dry density and the number of FTC. The results show that although the overall patterns of PWP and strain during consolidation are similar before and after FTC, variations in dry density and the number of FTC lead to significant differences in the measured values. Specifically, PWP decreases while soil deformation increases with an increasing number of FTC cycles, even across different dry density conditions. The most pronounced changes in PWP and strain occur during the first 1-3 FTC cycles, with some samples showing continued significant changes up to 3-5 cycles. However, beyond five FTC, the increments in PWP and strain become considerably smaller. Meanwhile, an approximate linear relationship was observed between the peak PWP and steady-state strain values during graded loading, with this linearity decreasing as dry density increases. In addition, the Burgers model was modified based on the measured dissipation pattern of PWP to overcome the shortcomings of the traditional Burgers model. The modified Burgers model provides a more accurate representation of the soil's deformation process following FTC compared to the traditional model. This study can provide theoretical guidance for predicting the deformation of soils after freeze-thaw cycles.

The unfrozen water content is crucial to soil's physical and mechanical properties. Soils on the Qinghai-Tibet Plateau are frequently subjected to freeze-thaw (F-T) cycles. The quantitative relationship between F-T effects and the unfrozen water content of soils requires further investigation. This study employs a nuclear magnetic resonance (NMR) scanner with a temperature-control module to measure the unfrozen water content of silty clay during multiple F-T cycles. The soil freezing characteristic curves (SFCC) of silty clay are derived from the T2 (transverse relaxation time) distribution curves based on NMR measurements. Two distinct T2 cutoff values are used to classify three types of water in soils: bound water, capillary water, and bulk water. The impact of F-T cycles on the evolution of unfrozen water content as temperatures decrease has been analyzed. The testing results indicate that the SFCC of silty clay can be segmented into three stages: super-cooling, fast-declining, and stable. As the number of F-T cycles increases, capillary water content decreases while bulk water content increases during the super-cooling stage. The damage coefficient, derived from pore volume measurements, increases sharply during the first four F-T cycles before stabilizing gradually. Additionally, there is a negative linear correlation between the damage coefficient and the initial capillary water content, and a positive linear correlation with the initial bulk water content. This study offers valuable insights for the quantitative analysis of unfrozen water content in seasonally frozen regions and serves as an essential guide for geotechnical construction projects in cold areas.

Extreme variations in weather patterns have become increasingly common across the Southern Great Plains of the United States. The soil layer in the active zone above the groundwater table is often subjected to moisture variations due to seasonal weather changes that will influence the behavior of soils, including their strength and stiffness parameters. Designing a pile foundation in seismic-prone areas without considering the moisture changes in soil interacting with piles may adversely impact the seismic performance of the piles. The main aim of this study is to investigate the role of soil moisture conditions and suction caused by soil-atmospheric interactions on the dynamic behavior of the pile foundations interacting with clayey soils. This study uses a stand-alone finite element computer code called DYPAC (Dynamic Piles Analysis Code) developed using the Beams on Nonlinear Winkler Foundation (BNWF) approach. The influence of soil suction is incorporated into the p-y curves and free-field soil displacements using site response analyses by employing the concept of apparent cohesion. To perform nonlinear site response analyses, DEEPSOIL software V6.1 is utilized. The variation in soil suction with depth along the pile is considered using unsaturated seepage analysis performed by employing the commercial software PLAXIS LE Groundwater for three different clayey soils with plasticity ranging from low to medium to high. The analyses were performed using actual past daily recorded weather data for a testbed that experienced significant back-to-back flash droughts in 2022. This study found that extreme weather events like flash droughts can significantly affect the soil suction and seismic performance of the piles interacting with the unsaturated clayey soils.

Under the framework of Biot porous media theory, the fractional order Kelvin model is used to describe the rheological effect of soil skeleton, considering the coupling effect of pore pressure dissipation and skeleton rheology. By establishing a spatiotemporal analytical function for periodic cyclic loads, a three-dimensional axisymmetric dynamic consolidation control equation for a half space saturated clay foundation is constructed in a cylindrical coordinate system. The analytical solution of the control equation in the transformed domain is derived using Hankel-Laplace joint transformation and tensor operations, followed by numerical inversion to acquire the spatiotemporal solution of the physical field. By analyzing numerical examples, the dynamic consolidation characteristics of a saturated clay foundation under cyclic loading are studied. The results indicate that the settlement rate of saturated clay is slower during primary consolidation but faster during secondary consolidation. With cyclic loading, the soil's cumulative settlement development accelerates as the rheological properties of the soil skeleton strengthen. The amplitude of soil displacement fluctuations decreases as the order of viscosity increases, and the more significant the order of viscosity, the more pronounced the displacement hysteresis becomes. The rheological properties of the soil skeleton lead to a lag in pore pressure response compared to effective stress, resulting in horizontal movement of the spiral curve between pore pressure and effective stress under cyclic loading. In the unloading stage of cyclic loads, due to the decrease of normal stress with the decrease of external load, but the increase of shear stress, the soil undergoes shear dilation phenomenon, resulting in negative pore pressure in the soil.

In this paper, by introducing a new yielding mechanism based on the widely acknowledged double-structure theory, the original UH model for unsaturated soils is extended to capture the behaviour of expansive clays. A novel expansion potential is further established to evaluate the effect of overconsolidation on the volume change of unsaturated expansive clays during wetting. With only one additional parameter, the proposed model can describe the behaviour of both wetting-collapse and wetting-induced swelling for unsaturated clays. Comparisons between model predictions and test results show a good agreement which verifies the capability of the proposed model in charactering the features of unsaturated expansive clays under various stress histories and stress paths.