Conventional plasticity assumes that a yield surface exists and the direction of plastic strain increment (DPSI) is uniquely dependent on the current stress state. Triaxial stress probing tests of yield and plastic flow of sand have been conducted using discrete-element modelling with polyhedral particles resembling the shapes of Toyoura sand. It is found that a yield surface does not exist, but a memory surface (MS) separating two types of distinct sand behaviour can be established. Within the MS, the DPSI is primarily controlled by the stress increment, and the magnitude of plastic strain increment is insensitive to the stress increment direction. When the stress state is on or outside the MS, a much larger plastic strain increment is observed if the stress increment points outside the MS, and the DPSI is dependent on both the current stress state and stress increment. The shape and size of the MS, which can be modelled by the SANISAND yield function, are dependent on the soil density and evolve with plastic strain.
Currently, our understanding of material-scale deterioration resulting from meteorologically induced variations in pore water pressure and its significant impact on infrastructure slopes is limited. To bridge this knowledge gap, we have developed an extended kinematic hardening constitutive model for unsaturated soils that refines our understanding of weather-driven deterioration mechanisms in heterogeneous clay soils. This model has the capability of predicting the irrecoverable degradation of strength and stiffness that has been shown to occur when soils undergo wetting and drying cycles. The model is equipped with a fully coupled and hysteretic water retention curve and a hysteretic loading-collapse curve and has the capability to predict the irrecoverable degradation of strength and stiffness that occurs during cyclic loading of soils. Here, we employ a fully implicit stress integration technique and give particular emphasis to deriving a consistent tangent operator, which includes the linearisation of the retention curve. The proposed algorithm is evaluated for efficiency and performance by simulating various stress and strain-driven triaxial paths, and the accuracy of the integration technique is evaluated through the use of convergence curves.
A constitutive model for cemented granular materials based on modified Cam-clay model has been developed. The model is capable of predicting the stress-strain response of cemented granular materials subject to mechanical and chemical bond degradation. Two simple variables, the void ratio e and cement content Cc are used to evaluate the mechanical and chemical degradation 'history' of cemented granular materials. Simulation results show the effect of confining pressure, and the change from strain softening to hardening behaviour can be successfully reproduced. Moreover, the model also reproduces the changes of mechanical properties such as strength, bulk modulus and shear modulus due to the increase of void ratio and degradation of bond strength caused by chemical dissolution. Comparisons with experimental data show that the model can capture the stress-strain response of cemented geomaterials subjected to chemical degradation at different stress levels.
The soil water retention curve (SWRC) strongly influences the hydro-mechanical properties of unsaturated soils. It plays a decisive role in geotechnical and geo-environmental applications in the vadose zone. This paper advances a novel framework to derive the water retention behavior of multimodal deformable soils based on the pore size distribution (PSD) measurements. The multiple effects of suction on the soil pore structure and total volume during SWRC tests are considered. The complete picture of soil microstructure is quantitatively described by the void ratio (for the pore volume) and a newly defined microstructural state parameter (for pore size distribution) from a probabilistic multimodal PSD model. Assuming a reversible microstructure evolution, a unique PSD surface for wetting and drying links the SWRC and PSD curves in the pore radius-suction-probability space. A closed-form water retention expression is obtained, facilitating the model's implementation in particle applications. The model is validated using the water retention data of four different soil types, showing a strong consistency between the measurement and the reproduced curve. The proposed method provides new insights into the pore structure evolution, the water retention behavior and the relationship between them for multimodal deformable soils.
Linear geotechnical infrastructure undergoes seasonal volumetric changes due to climatic and hydrological cycles, leading to progressive failure of the soil mass and performance deterioration. The magnitude of the seasonal cycles of pore water pressure is expected to be increased by more extreme and frequent wet and dry events, leading to an accelerated deterioration process. Advanced constitutive models for unsaturated soils able to reproduce the observed behaviour are therefore required. A new advanced constitutive model for the hydro-mechanical behaviour of unsaturated soils is formulated and implemented within the framework of elasto-plasticity with internal variables. The effect of recent suction history is incorporated using a kinematic hardening constitutive model augmented by elements of bounding surface plasticity, initially developed for saturated soils, which is further extended to the unsaturated range through the inclusion of a loading collapse curve. The model permits the retention of information on recent stress history, allows prediction of irrecoverable stiffness and strength loss, and replicates the hysteretic response during cyclic loading. The model has been implemented in a constitutive driver using an implicit numerical scheme and its predictive capabilities are demonstrated by performing numerical simulations of a series of laboratory experiments involving complex sequences of isotropic loading, wetting, drying and shearing stages.
Mortars will remain critical in future land wars due to their flexibility and versatility. When mortars are fired continuously, the contact soil is gradually compacted by the mortar base plate, and dynamic research into this process is the basis for innovative mortar design. However, the discontinuity and nonlinearity of soil contact absolutely necessitate the constitutive relationship of soil contact, which is difficult to study. Therefore, this study conducted experimental research and theoretical derivation to establish an accurate dynamic model of the mortar system. First, based on the nonlinear elastic-plastic theory and the stress-strain relationship of soil under cyclic loading, a theoretical analysis method for the constitutive relationship of contact soil under continuous loading was proposed. Second, an experimental and testing system was designed to simulate launch loads, and the stress-strain response of soil under continuous impact loads was obtained experimentally. Subsequently, based on theoretical analysis and experimental data, the stress-strain relationship during the gradual compaction of soil was established using the least squares method. Finally, a constitutive relationship model of the contact soil in the mortar system was established in ABAQUS using the VUMAT subroutine interface, and the calculated results were compared and analyzed with traditional calculation results. The results indicated that studying the constitutive relationship of mortar in contact with soil during continuous firing using this method can improve the accuracy of dynamically modeling mortar systems. Moreover, this study has practical value in the engineering design of mortar systems.
Due to their advantages of high rupture strength and long service life, polymer fibers are often used for soil improvement. However, there is no consensus on how the mixing of discrete polymer fibers affects the stress-strain relationship of clays. In this study, a constitutive relationship of polymer fiber-reinforced clay was established on the basis of the stress-strain relationship between clay and polymer fibers. The elastic-plastic unified hardening (UH) model was employed, and the fiber contribution was introduced based on the UH model. The constitutive relationship of polymer fiber-reinforced clay considers the anisotropic distribution of the discrete fiber orientation and the relative sliding between the fibers and clay matrix. The model was verified by referring to the results of consolidated undrained (CU) and consolidated drained tests of typical polymer fiber-reinforced clays in previous studies. A series of CU tests on rubber fiber-reinforced clay were conducted to validate the model further. The ratio of the simulated results to the experimental results gradually approached 1 with increasing axial strain. The constitutive relationship of polymer fiber-reinforced clay could provide satisfactory results. Polymer fiber mixing increases soil strength and enhances the properties of problematic soils, which makes the problematic soils more valuable for engineering applications. Studies have shown that the fibers in the soil tend to be distributed horizontally after the compaction process. With the anisotropic distribution of fiber orientation considered, the authors established a numerical calculation method for the stress-strain relationship of polymer fiber-reinforced clay. A major objective of this work was to allow the use of computerized numerical analysis methods when performing mechanical analyses of polymer fiber-reinforced clay, which avoids the need to conduct a large number of shear tests. In this study, a series of consolidated undrained tests of rubber fiber-reinforced expansive clay were conducted. With the data collected, the numerical calculation method for the stress-strain relationship of polymer fiber-reinforced clay was verified, and the numerical results agreed with the test results better.
This paper presents an investigation into the suitability of the SANISAND-MS model for the three-dimensional finite-element (3D FE) simulation of cyclic monopile behaviour in sandy soils. In addition to previous work on the subject, the primary focus of this study is to further assess the model's capability to reproduce the accumulation of permanent deflection/tilt under cyclic lateral load histories. To this end, experimental data from the PISA field campaign are employed, particularly those emerged from the medium-scale cyclic tests conducted at the Dunkirk site in France. The methodology adopted herein involves calibrating the SANISAND-MS model's parameters to align with 3D FE simulation of a selected monotonic pile test reported by the PISA team using a bounding surface plasticity model partly similar to SANISAND-MS. Subsequently, the soil parameters governing SANISAND-MS' ratcheting response are calibrated using only minimal information from published PISA field data. While representing the first attempt to simulate the reference data set using a fully ' implicit ' 3D FE approach, this paper offers novel insights into calibrating and using advanced cyclic models for monopile analysis and design - particularly, with regard to the quantitative influence of pile installation effects and sand's microstructural evolution under drained cyclic loading.
Clays exhibit complex mechanical behaviour with significant viscous, nonlinear, and hysteric characteristics, beyond the prediction capacity of the well-known modified cam clay (MCC) model. This paper extends the MCC model to address these important limitations. The proposed family of models is constructed entirely within the hyperplasticity framework deduced from thermodynamic extremal principles. More specifically, the previously developed MCC hyper-viscoplastic model based on the isotache concept is extended to incorporate multiple internal variables and to capture recent loading history, hysteresis, and smooth response of the material. This is achieved by defining an inelastic free energy and an element that implements a bounding surface within hyperplasticity, resulting in pressure dependency in both reversible and irreversible processes with a unique critical state envelope, and only eight material parameters with a readily measurable viscous parameter. A kinematic hardening in the logistic differential form in stress space is derived that enables the proposed model to function effectively across a wide range of stresses. Based on this kinematic hardening rule, the current stress state acts as an asymptotic attractor for the back/shift stresses whose evolution rates are proportional to their current state.
This paper examines the impacts of deposition direction and density of granular soils on their large deformation behavior encapsulated by column collapse processes. We combine a critical state-based, anisotropic constitutive model for sand and material point method (MPM). The constitutive model includes state-and fabric-dependent dilatancy and hardening, thus accounting for the effects of density and deposition direction on the mechanical behavior of soils. The MPM model is first validated against experimental results. We then investigate the fundamental connections between local constitutive properties of soils (e.g., friction and dilation) and global column collapse response. Based on these correlations, this work further studies how material density and deposition direction influence collapse behavior, including run-out distance, residual height, and slope angle of the static region. Results indicate that run-out distance is relatively insensitive to initial soil density but can be significantly altered by the deposition direction of soils. Peak run-out distance is observed as the deposition direction is aligned with sliding band formed within the granular column. Moreover, a higher density or a smaller inclination of deposit direction leads to a greater residual height and a steeper slope of the static region. The mechanisms of above effects from soil properties perspectives are discussed.