The mechanical behavior of structured soils is influenced by both inter-particle bonding and fabric arrangements. Existing constitutive models primarily account for soil structure through fabric arrangements. In this study, we first present experimental investigations on intact loess samples, including isotropic compression (IC), conventional consolidation undrained (CU), and consolidation drained (CD) triaxial tests, which reveal the complex structural properties of the soil. Next, we employ the work done by strain energy to comprehensively account for soil structure, incorporating both inter-particle bonding and fabric arrangements. Subsequently, a new strain work constitutive model for structured soils is presented within the critical state framework. Specifically, a linear decreasing function between strain power and mean effective stress is introduced to capture structural degradation, and a new hardening rule is derived from the relationship between strain work and mean effective stress. Compared to traditional structured soil models, the proposed model offers clear physical meaning, and its parameters are easily obtainable. The model's simulation results are validated against experimental data, demonstrating its ability to capture key mechanical and deformation characteristics, such as strain softening under CU conditions and strain hardening under CD conditions. Finally, we compare our model with the structured cam clay (SCC) model, and the results show that our model provides a better fit to the experimental data, further confirming its accuracy and effectiveness.

The geotechnical characterization of residual soils is a complex matter and is not always successful because current interpretation methodologies dedicated to sedimentary soils do not adequately respond to the behavior of this type of soils. The problem has been under scope by several Portuguese and international institutions. The work carried out in the experimental Site of the Polytechnic Institute of Guarda (IPG) since 2003, constituted by residual soils and decomposed rocks of the local granite massif, is highlighted herein. The work was strongly supported by MOTA-ENGIL (Portuguese construction company) and the Laboratory of Math Engineering (LEMA, Polytechnic Institute of Porto). The characterization of the test site and the respective research work is presented. The research work involved interpretation of in situ tests (SDMT, SCPTu, PMT, SPT, DPSH, and geophysical tests), tests in controlled chambers (DMT, geophysical, and suction tests), and laboratory tests (oedometric tests, direct shear tests, and triaxial tests with several stress paths). The tests were performed on natural structured soils, artificially cemented mixtures, and unstructured soils. Advanced math and statistical analysis were applied in the development of new correlations to obtain geotechnical parameters representative of these soils. Furthermore, the work also allowed to recognize the physical characteristics of the materials and better understand their mechanical behavior.

Introduction Many theories of consolidation for soils have been proposed in the past, but most of them have ignored the structural characteristics of clay, yet the natural layered soils are widely distributed around the world.Methods A theoretical model is established to analyze the one-dimensional consolidation behavior of layered soils, in which a time-dependent drainage boundary and the structural characteristics of the soil are taken into account. Using the integral transform and characteristic function methods, the analytical solution is derived, the effectiveness of which is evaluated against the degradation of solutions and the numerical results calculated using the finite element method.Results and discussion Finally, the influences of interface parameter, soil permeability coefficient and soil compressibility on consolidation behaviors are discussed. Results show that in structured soils, early dissipation of excess pore water pressure and consolidation rates are predominantly influenced by interface parameters, permeability, and volume compression coefficients. Higher values of these parameters accelerate early stages of consolidation, which is especially evident in the upper soil layers. Over time, the distinct effects of interface and permeability coefficients on consolidation diminish. Higher volume compression coefficients, while initially beneficial, eventually slow down the consolidation process, indicating an interaction with the ongoing soil structural changes.

To capture the influence of loading rate on the deformation process of structured soft clay, applying the approach used in the unified hardening model (UH model) to describe time effects, an equivalent time term is introduced into the yield function (t) of the structured UH model, and then a structured UH model for soft clay considering loading rate is extended. In the equivalent time term, the internal variable R of the original structured UH model is transformed into Rt to account for time-driven strain. The presented structured UH model considering time effects comprises a total of 8 parameters, all of which can be determined through routine soil tests. Comparisons between model predictions and experimental data demonstrate that the presented model is qualified to reflect the influence of loading rate on structured soft clays reasonably.

Triaxial tests are performed for remolded, artificially isotropic, and anisotropic structured samples under undrained conditions at confining pressures of 25, 100, and 200 kPa. Based on these test results, a binary-medium constitutive model is formulated based on homogenization theory and a breakage mechanism to describe the behaviors of structured soils. In this model, the binary-medium material is idealized as a representative volume element (RVE) composed of bonded elements, whose mechanical behaviors are expressed by the linearly elastic model, and frictional elements, whose mechanical behaviors are described by the double-yield surfaces constitutive model. The parameters of the bonded and frictional elements are determined from the test results of structured and remolded samples, respectively. The expressions for the breakage ratio and local stress coefficient matrix are introduced, and their parameters are provided. The computed results are compared with the test results, demonstrating that the model can reflect the main deformation features of structured soil relatively well, including the influence of anisotropy, gradual damage to particle bonding, and pore development.

Computational modelling has been widely used in the geotechnical field to represent soil behaviour in real problems. Most commercial programmes use the Finite Element Method (FEM) to compute stresses and deformations distributions in soils, as observed in Plaxis software. In this method, choosing an appropriate constitutive model is fundamental to obtaining accurate results. However, most models built in commercial software do not consider effects such as overconsolidation and structure, observed for natural soils with structure. The Structured Sub-Loading Cam Clay (SSLCC) model is recommended to represent this behaviour. This article aims to describe a practical methodology to code the SSLCC model to represent the behaviour of structured soils. The model was implemented in Plaxis by the User-Defined Soil Model feature and validated with experimental data of consolidation and drained and undrained triaxial tests performed for different soils. The model implemented presents good performance and can be used in the FEM interface. The methodology described can also be used to introduce any constitutive model in Plaxis.

Structured soils exhibit significantly different mechanical behaviors than reconstituted soils because of the influence of their structure. A theoretical study of the structured soils is carried out in this paper. A newly defined variable-relative structure degree-was used to quantify the integrity of the soil structure during compression based on intrinsic compression curves of the intact structured soils. Also, a new volume change equation for structured soils was developed by using effective stress and relative structure degree as variables. The volume change equation provides the interpretation of the nonlinear compression curve of structured soils in the space of void ratio against logarithmic mean effective stress. The proposed approach for structured soils was extended to the triaxial stress state by introducing equivalent, current, and normal yield surfaces, so that the influence of stress history and soil structure could be considered in the model. The characteristics of the proposed model were illustrated through simulations of the influence of soil structure and stress history. The proposed model was validated by making comparisons between experimental data and model predictions.