Bin-Hua Xu

Soft soils subjected to cyclic loading under undrained conditions can experience either undrained yielding (i.e. plastic yielding that has reached the critical state) or the transition to a fluid-like state caused by the upward migration of fine particles and moisture. The latter changes the internal soil fabric and pore structure, resulting in a non-uniform change in the soil state. This paper presents a theoretical framework that incorporates both empirical and critical state-based constitutive approaches to determine the evolution of excess pore water pressure, localised void ratio changes and spatial state redistribution within soil subjected to undrained cyclic loading. The proposed framework is verified against two sets of experimental data obtained from a large-scale one-dimensional cylindrical apparatus and the cyclic triaxial apparatus. A liquidity index (IL) criterion is introduced to identify any potential for the occurrence of localised fluidisation as a function of the number of loading cycles and depth, while critical state principles are also used to assess undrained yielding in triaxial tests. The proposed framework can identify the onset of undrained yielding, early softening-induced potential fluidisation and the attainment of stable states with increasing number of loading cycles. The model output for various cyclic loading conditions provides a valuable insight to improve the future design of rail infrastructure built over a soft subgrade.

When piles are installed in soft clay, negative skin friction may develop when surrounding soil settles more than piles. This study examines the effectiveness of prefabricated vertical drains (PVDs) prior to pile installation in reducing the downdrag forces under cyclic loading. Model tests on a coupled pile–PVD system were conducted using a dynamic consolidation apparatus. Cyclic vertical stresses were applied to vary from 25 kPa to peak values of 65–95 kPa at loading frequencies of 1 and 5 Hz. Preconsolidation using PVDs was carried out to achieve degrees of preconsolidation (DOPC) between 40% and 100% under a surcharge load of 65 kPa before pile installation. The results demonstrate that preconsolidation to approximately 60%–80% DOPC markedly increases the soil stiffness, thereby limiting the relative displacement at the pile–soil interface. Lower loading frequency and higher cyclic stress amplitudes result in greater excess pore-water pressure (EPWP) generation, larger settlements, and stronger downdrag force mobilisation. Increasing the cyclic stress amplitude from 40 to 70 kPa amplifies the magnitudes of vertical strain (~30%), EPWP buildup, and corresponding downdrag forces. PVDs could significantly enhance drainage and accelerate EPWP dissipation during cyclic loading, leading to deeper neutral planes. Overall, the findings provide guidance on optimising the timing of pile installation and PVD-based consolidation.

The consolidation behaviour of soft clays is significantly influenced by loading patterns, soil inhomogeneity, and boundary drainage conditions. Despite their significance, a notable gap exists in the availability of rigorous analytical solutions capable of integrating these factors into the consolidation analysis of inhomogeneous soft clay. This paper presents a comprehensive analysis of the consolidation behaviour of inhomogeneous soft clay subjected to impeded drainage conditions under time-dependent loading. The study employs the spectral method to derive solutions for specific distributions of inhomogeneous soil property distributions and loading patterns. The accuracy of the proposed method is validated through comparisons with previous analytical solutions, finite element method (FEM) solutions, laboratory tests and case studies, demonstrating improved predictive capabilities. A parametric study based on the proposed solution indicates that the dimensionless factors related to drainage capacity ( R_t and R_b) adversely impact soil consolidation, with smaller values leading to increased divergence from calculations assuming fully pervious conditions. Notably, solutions assuming fully pervious conditions can maintain considerable accuracy within a 5% error range only when R_t and R_b exceed a certain threshold (e.g., R_t > R_b/(0.049 R_b − 0.426) in this study). Otherwise, impeded drainage conditions must be considered for accurate consolidation calculations.

The nonlinear variation of soil compressibility and permeability with void ratio (i.e., e-log σ′ and e-log k) has been included in the consolidation theory to accurately predict the behavior of soft soil stabilized by vertical drains. However, most current nonlinear consolidation models incorporating the coupled radial-vertical flow are based on some simplified assumptions, while including some features such as the complex implementation of multilayered computations, time-dependent loading and stress distribution with depth. This study hence introduces a novel approach where the spectral method is used to analyze the nonlinear consolidation behavior of multilayered soil associated with coupled vertical-radial drainage. In addition, time- and depth-dependent stress and soil properties at each soil layer are incorporated into the proposed model. Subsequently, the solution is verified against experimental and field data with comparison to previous analytical solutions. The results show greater accuracy of the proposed method in predicting in-situ soil behavior. A parametric study based on the proposed solution indicates that the ratio between the compression and permeability indices (ω = C_c/C_k) has a great impact on the consolidation rate, i.e., the greater the ω, the smaller the consolidation rate. Increasing the load increment ratio and the absolute difference between unity and ω (i.e., |ω − 1|) can exacerbate prediction error if the conventional simplified methods are used.

This paper summarizes the current treatments and countermeasures for liquefiable foundations, and divides the existing anti-liquefaction countermeasures into two categories. One of the ideas is proceeding from the properties of liquefiable foundation soils, by the means of improvement for the soil’s qualities to enhance the capacity of soil’s anti-liquefaction in the early stage. The other idea is considering from the stress conditions of liquefiable foundation soils, and to reduce the liquefaction-induced disasters by changing the stress conditions of the soil. The advantages and disadvantages of various anti-liquefaction measures were analysed by verifying the effectiveness of field applications of anti-liquefaction measures against ground liquefaction hazards, and the applicable conditions of various anti-liquefaction measures were classified. This paper provides experience for resisting soil liquefaction disasters.