The mechanical behaviour of soil subject to shear loading or deformation is typically considered either completely drained or undrained. Under certain conditions, these drained and undrained scenarios can represent boundaries on the allowed volumetric strain. There is growing interest in exploring the response under intermediate conditions where partial drainage is allowed, particularly in the development of new approaches to mitigate the risk of liquefaction induced failure and the design of off-shore structures. This study uses the discrete element method (DEM) to investigate the effect of partial drainage conditions on the mechanical behaviour of spherical assemblies. Samples with different interparticle friction values are isotropically compressed and then subjected to undrained, drained, and partially drained triaxial shearing. The partially drained conditions are simulated in the DEM samples by applying a controlled volumetric strain that is a fraction of the drained volumetric strain. Results on loose samples indicate that allowing drainage enhances peak shear resistance and can also prevent liquefaction. Moreover, dense samples show a substantial increase in shear resistance when small changes in drainage and volumetric strain take place. The peak stress ratio and the stress ratio at the phase transformation point are insensitive to the drainage level. There is a linear correlation between the state parameter and the drainage level at the peak stress ratio and the phase transformation point. This observation could be used to trace partially drained stress-paths and could also aid the development of uncoupled constitutive models that account for drainage effects.

In slopes and embankments, soil elements are often anisotropically loaded and the sustained stress ratio SR may vary a lot. The understanding of the influence of SR on the small-strain shear modulus G0 of sands prior to failure is a practical concern that remains inadequately understood in the existing literature. This study aims to address this knowledge gap through a meticulously designed experimental program. The testing program encompasses three quartz sands with differing particle shapes and a diverse set of principal stress ratios produced via drained triaxial compression. By employing bender elements embedded within the apparatus, elastic shear waves are generated, enabling the measurement of G0 from isotropic stress states to anisotropic stress states. A careful evaluation and comparison of existing anisotropic G0 models in the literature is also conducted, and the potential limitations when subjected to elevated SR levels are noted. A new, unified model is proposed to effectively characterize G0 of different sands subjected to a wide range of triaxial compression states and it is validated using literature data.

The contraction behavior of monotonically expanded cavities is intriguing as it offers insights into certain geotechnical scenarios, especially for pressuremeter tests, where the unloading data is equally informative as the loading data. Despite many solutions for cavity expansion, attempts for the analyses of cavity contraction from an expanded state were rarely made. To extend previous solutions to include contraction, this paper presents a novel semianalytical solution for drained contraction of spherical and cylindrical cavities from an initially expanded state in soils characterized by a unified state parameter model for clay and sand (CASM). Given the nonself-similar nature of the contraction after expansion problems, the hybrid Eulerian-Lagrangian (HEL) approach is employed to derive distributions and evolutions of stresses and strains around the cavities during the unloading process. Combined with the previous expansion solution, the complete loading-unloading cavity pressure curves and stress paths at the cavity wall are presented and verified against numerical simulations. Following validation through comparisons with calibration chamber pressuremeter tests conducted in Stockton Beach sand, a new method for the interpretation of pressuremeter testing data is developed based on the proposed solution. This method demonstrates its capability in the back-calculation of the effective horizontal stresses and state parameters for four distinct types of sands.

There are many geotechnical applications involving dams, embankments and slopes where the presence of an initial static shear stress prior to the cyclic loadings plays an important role. The current paper presents the experimental results gathered from undrained cyclic simple shear tests carried out on non-plastic silty sand with fines content in the range 0-30% with the consideration of sustained static shear stress ratio (alpha). Two distinct parameters, namely the conventional state parameter Psi, and the equivalent state parameter Psi*, are introduced in the context of critical state soil mechanics (CSSM) framework to predict failure mode and undrained cyclic resistance (CRR) of investigated soils. It is proved that the failure patterns for silty sands are related to (a) the initial states of soils (Psi or Psi*) and (b) the combination of initial shear stress with respect to cyclic loading amplitude. At each alpha, the CRR-Psi (or Psi*) correlation can be well represented by an exponential trend which is practically unique for both clean sands and silty sands up to a threshold fines content (f thre congruent to 24.5%). Varying alpha from low to high levels simply brings about a clockwise rotation of the CRR-Psi (or Psi*) curves around a point. This CRR-Psi (or Psi*) platform thus provides an effective methodology for investigating the impact of initial shear stress on the cyclic strength of both clean sands and silty sands. The methodology for estimating Psi (or Psi*) state parameters from in-situ cone penetration tests in silty sands is also discussed.

Unsaturated soil-continuum interfaces determine the behavior of structures such as friction piles embedded in unsaturated soil and retaining walls with unsaturated backfill. Developing accurate constitutive models for these interfaces is essential for innovative computational design and analysis in geotechnical engineering. This study presents a rigorous mathematical derivation to quantify the bonding effect at interparticle and pore space contact zones, introducing a bonding variable and a state parameter based on the ratio between current and critical void ratios. The novelty lies in introduction of a hydraulic-mechanical (HM) coupling constitutive model for unsaturated soil-continuum interfaces, incorporating the bonding effect and critical-state concept. Despite the 16 parameters in proposed model, all are methodically calibrated using suction-controlled interface shear tests. Effectiveness of the model is demonstrated through predictions in interface shear tests involving unsaturated Toyoura sand, smooth/rough steel, and published tests with unsaturated Minco silt and steel. The model's applicability is extended to diverse conditions, including structural stiffness, interface thickness, counterface roughness, and drying- wetting (D-W) cycles. This model provides a unified framework for analyzing volume changes and stress-displacement responses in unsaturated soil-continuum interfaces, providing valuable insights for design of pile foundations in unsaturated soils.

This study presents a novel formulation for incorporating anisotropy into the generalized plasticity constitutive model. Generalized plasticity is a hierarchical framework allowing for extensibility, in order to encompass new phenomena and improve its predictive capabilities. Anisotropy formulation is based experimentally on the phase transformation state and considers explicitly the direction of the maximum principal stress and the magnitude of the intermediate principal stress, through an anisotropy state variable that contributes to the state parameter. Additionally, the model incorporates the fabric using an evolving fabric variable that reflects initial fabric due to sample preparation method for granular soils. The formulation is simple and introduces three constitutive parameters, allowing for straightforward implementation into the constitutive model and direct application in finite element analysis. The model is validated with undrained triaxial tests conducted on Toyoura sand, covering a wide range of initial conditions with a unique set of constitutive parameters, and yielding overall satisfactory results despite some limitations.