This work presents a simplified method for the nonlinear analysis of the load-displacement response of piles in multi-layered soils. As a starting step, a new interface model based on the disturbed state concept (DSC) is put forth to simulate the interface shear stress-displacement relationship by considering the nonlinear hardening-softening behaviour. In the new model, input parameters can be conveniently calibrated using conventional interface shear tests or on-site tests. The good agreement between predictions and experimental data from interface direct shear tests validated the performance of the proposed DSC model. The DSC model performed better in terms of predictions when compared to the hyperbolic one. Next, the soil-structure interface model and bearing capacity theory are coupled to provide a theoretical framework for the analysis of pile load-transfer in saturated and unsaturated multi-layered soils, where the DSC model is employed to represent base resistance as well as skin friction. This work also discusses the profile of steady-state in-situ matric suction, soil-water characteristic curve, and pore-water pressure of unsaturated soils. The proposed method has the advantage of being used in practice as it is simple to obtain input parameters from laboratory tests, as well as Standard Penetration or Cone Penetration Tests. The proposed framework is finally applied to the analysis of five welldocumented case studies. The proposed approach and the static load test results from the field measurements are found to be in satisfactory agreement, indicating that the proposed method performs well. The proposed method is suggested to be utilised for preliminary analysis, planning a suitable programme of loading tests, as well as optimizing the pile design by back analysis of the load test results.

Strength characteristics of graded gravels are essential in the construction of roadway and railway substructures. Traditional constitutive models, primarily nonlinear elastic and plastic types, fall short in accurately capturing the strain-softening properties of such materials. To address this limitation, the current study introduces a statistical damage model designed to outline the stress-strain behavior of densely compacted graded gravels in transport infrastructures. Utilizing medium-sized triaxial tests, the model examines variations in strength and deformation parameters in relation to compaction levels and incorporates a unique damage-softening index (DSI) along with a threshold axial strain to improve accuracy. The study establishes that the DSI and threshold axial strain effectively regulate stress-strain relations in the postpeak segment, the model's statistical parameters and threshold axial strain can be precisely determined through the introduction of DSI, and the model closely aligns with experimental data across multiple compaction levels. These findings are especially relevant for engineering design in the context of roadway and railway construction and indicate potential for further refinement, such as the incorporation of loading rate considerations.

The strain softening characteristics of carbonate red clay materials will have a significant impact on the safety and economy of engineering construction. To study such stress-strain characteristics and influencing factors, this paper takes the native carbonate red clay in Guiyang as the research object and conducts triaxial consolidated undrained tests and scanning electron microscopy under different moisture content and confining pressure conditions. The test results show that the stress-strain curve of the native red clay in Guiyang exhibits a peak-shaped variation and that the strain softening characteristics are jointly influenced by moisture content and confining pressure. With increasing confining pressure, the consolidation time of the specimen becomes longer, and the drainage volume increases. The volume change of the specimen during shearing follows the pattern of shearing contraction and then shearing expansion. After the shear test, the pore diameter of the specimen decreases, and numerous crack-like pores appear. A double-logarithmic strain softening model is established based on the characteristic stress-strain curves (DL model). This model includes three parameters that can be determined based on the test curve. And it is used to simulate the stress-strain curve of the native red clay in Guiyang; the results show that the DL is a new nonlinear model that can be applied to strain-softening soils.