The majority of existing effective stress-based constitutive models approach thermal effects through the temperature dependency of surface tension and its effects on the soil-water retention curve (SWRC) and effective stress. Experimental tests and theoretical studies, however, suggest that the temperature effect on surface tension alone is not sufficient to properly explain thermal-induced changes in the effective stress and SWRC. This study focuses on the temperature-dependent elastoplastic behavior of low plasticity unsaturated soils by developing a set of constitutive-level relations that incorporate temperature-dependent SWRC and effective stress models. These models account for the effect of temperature on the enthalpy, contact angle, and surface tension. The application of the presented constitutive relations was demonstrated and validated for low plasticity soils, specifically incorporating temperature effects into the hardening modulus, specific volume change, yield stress of the modified Cam-Clay model, and stress-strain relationships. The proposed relationships are incorporated in any effective stress-based constitutive model for modeling temperature dependency of elastoplastic response in low plasticity unsaturated soils. Employing these relationships can enhance the numerical simulation of low plasticity unsaturated soils under thermo-mechanical or other coupled processes involving temperature-dependent conditions.

As temporary support in geotechnical and tunneling scenarios, frozen soil bodies are often subjected to varying stress states during different construction stages and techniques and, thus, exhibit stepwise loading and unloading, leading to multi-stage creep. However, experimental and numerical investigations on frozen soil creep behavior have focused primarily on monotonic loading, i.e., single-stage creep. This study expands an existing experimental database on stepwise loaded creep and introduces a unique test series focusing on the uniaxial creep behavior of frozen sand under stepwise unloading and load-unload cycles. Here, similar to stepwise loaded creep, the minimum creep rate is found to remain mostly independent of the loading history, while the corresponding frozen soil lifetime depends on the latter. In contrast to equivalent single-stage creep scenarios, the lifetime becomes longer for stepwise loaded creep and shorter for stepwise unloaded creep. To consider multi-stage creep in the geotechnical design of frozen soil bodies, based on our experimental database and literature data, we test the ability of two versions of an advanced constitutive model to capture the frozen soil creep behavior under varying stress states. Comparison of the extended version, called EVPFROZEN, with the original highlights the advantages of EVPFROZEN in consistently capturing the creep rate evolution and the practically important frozen soil lifetime under complex loading histories. Combining the insights from the novel experimental database with testing and validation of the advanced constitutive model EVPFROZEN advances the efficient and sustainable design of frozen soil bodies in geotechnical applications under multi-stage loading conditions.

In geotechnical engineering, the precise identification of essential soil parameters from sensing and experimental data is vital for the accuracy of constitutive and finite element models. However, the complexity of sophisticated soil models often makes this task challenging. Traditional optimization methods that rely on gradient information often fall short in this class of problems, due to their struggle with black box models lacking clear gradient pathways. Gradient-free methods, though circumventing the need for direct gradient data, can still miss out on integrating previous insights when faced with new information. To tackle these issues, our study presents a cutting-edge method inspired by the mechanisms underlying AlphaZero, DeepMind's acclaimed algorithm that excels in mastering complex strategic games through autonomous learning. By adopting a comparable selflearning technique, our approach reinvents the task of parameter identification of advanced geotechnical models as a strategic game. It draws a parallel between optimizing model parameters and the complex task of developing victorious chess tactics. This method utilizes a blend of deep learning for initial estimations and Monte Carlo Tree Search (MCTS) for finer adjustments, promoting a self-enhancing calibration process. Such an approach paves the way for a more self-reliant and intelligent parameter identification methodology from sensing and experimental data. The outcomes of our study demonstrate the robustness and versatility of this approach across various geotechnical models, ranging from the parameter identification of sophisticated constitutive models to more complex applications involving inverse analyses using finite element models that include interactions between mechanical sensing devices and unsaturated soils.

Numerous geotechnical applications are significantly influenced by changes of moisture conditions, such as energy geostructures, nuclear waste disposal storage, embankments, landslides, and pavements. Additionally, the escalating impacts of climate change have started to amplify the influence of severe seasonal variations on the performance of foundations. These scenarios induce thermo-hydro-mechanical loads in the soil that can also vary in a cyclic manner. Robust constitutive numerical models are essential to analyze such behaviors. This article proposes an extended hypoplastic constitutive model capable of predicting the behavior of partially saturated fine-grained soils under monotonic and cyclic loading. The proposed model was developed through a hierarchical procedure that integrates existing features for accounting large strain behavior, asymptotic states, and small strain stiffness effects, and considers the dependency of strain accumulation rate on the number of cycles. To achieve this, the earlier formulation by Wong and Ma & scaron;& iacute;n (CG 61:355-369, 2014) was enhanced with the Improvement of the intergranular strain (ISI) concept proposed by Duque et al. (AG 15:3593-3604, 2020), extended with a new modification to predict the increase in soil stiffness with suction under cyclic loading. Furthermore, the water retention curve was modified with a new formulation proposed by Svoboda et al. (AG 18:3193-3211, 2023), which reproduces the nonlinear dependency of the degree of saturation on suction. The model's capabilities were examined using experimental results on a completely decomposed tuff subjected to monotonic and cyclic loading under different saturation ranges. The comparison between experimental measurements and numerical predictions suggests that the model reasonably captures the monotonic and cyclic behavior of fine-grained soil under partially saturated conditions. Some limitations of the extended model are as well remarked.

In this work, we develop a micro-mechanics inspired constitutive model for lightly cemented granular materials, whose internal variables have a clear physical interpretation. It builds on our previous work, which was able to predict several aspects of the behavior of bonded geomaterials ( e.g. , macroscopic stress-strain responses, localization patterns) but did not explicitly account for porosity nor could it predict dilatancy at low mean stress levels. In the present work, we extend the original formulation by Tengattini (2015), formulating a model that is able to predict the material response across a broad range of mean stress while maintaining the same number of parameters as the original model, all of which have a clear physical interpretation, which helps guide their calibration.

Due to the increasing need to find new alternative energy sources, more attention has been given to the development of energy geostructures, which not only serve as foundations, but also employ the geothermal properties of soils for heating and cooling structures, inducing mechanical and thermal loads. Additionally, the up -growing effects of climate change are influencing the performance of foundations due to the increase in temperature and seasonal variations. The previously mentioned examples correspond to scenarios where soils are subjected to thermo-hydro-mechanical loading, which can vary cyclically. To predict this behavior, in this article a coupled thermo-hydro-mechanical hypoplastic model for partially saturated fine-grained soils that accounts for both monotonic and cyclic loading is presented. The proposed constitutive model is capable of reproducing temperature and suction effects at large strains and asymptotic states. Additionally, coupled effects are predicted by incorporating a Water Retention Curve (WRC) that depends on temperature and void ratio. Small strain stiffness effects are captured based on the Improvement of the Intergranular Strain concept (ISI), modified to include the influence of temperature under cyclic loading, as well as a temperature dependent secant shear modulus formulation at very small strains. The capabilities of the constitutive model were evaluated through element tests simulations of monotonic and cyclic mechanical loading tests under temperature- and suction- controlled conditions, as well as heating/cooling experiments at constant stress. The proposed constitutive model shows accurate predictions when compare to experimental data. Nevertheless, some limitations have been encountered and further discussed.

This keynote lecture discusses the results of a long lasting experimental research, devoted to the investigation of clay microstructure and its evolution upon loading. Micro-scale analyses, involving scanning electron microscopy, image processing, mercury intrusion porosimetry and swelling paths to test the clay bonding, are presented on clays subjected to different loading paths, with the purpose of providing experimental evidence of the processes at the micro-scale which underlie the clay response at the macro-scale. Data from the literature on clays of different classes, either soft or stiff, are compared to original results on two stiff clays, Pappadai and Lucera clay, both in their natural state and after reconstitution in the laboratory. The results presented herein allow building a conceptual model of the evolution of clay microstructure upon different loading paths, providing microstructural insights into the macro-behaviour described by constitutive laws and advising their mathematical formalization in the framework of either continuum mechanics or micro-mechanics. For editorial purposes, the research results are presented in two parts. The first part, presented in this paper, concerns the results for reconstituted clays, whereas a second part, concerning the corresponding natural clays, is discussed in a second companion paper.

In this paper, a state-dependent, bounding surface plasticity model that simulates the behavior of unsaturated granular soils is presented. An unsaturated, soil mechanics-compatible elastoplastic response is adopted in which no part of the response occurs in a purely elastic fashion. To create an appropriate hydro-mechanical coupling, a newer generation stress framework, consisting of the Bishop-type effective stress and a second stress variable, is used in conjunction with a soil-water characteristic curve function. Details regarding the model development, parameter estimation, and assessment of the model's predictive capabilities are outlined. With a single set of parameter values, the model realistically simulates the main features that characterize the shear and volumetric behavior of unsaturated granular soils over a wide range of matric suction, density, and net confining pressure.

During undrained cyclic loading, clayey soils experience substantial stiffness and strength degradation when subjected to shear amplitudes exceeding a critical threshold. This paper presents an enhanced bounding surface rate-independent plasticity model, an evolution of the previous SANICLAY model, tailored to capture this specific behavior during cyclic loading. A distinguishing feature of the proposed model is the introduction of an activation mechanism. This mechanism triggers degradation modeling based on the applied cyclic shear amplitude. To measure this amplitude, the activation mechanism incorporates a novel state variable that serves as a proxy for the applied cyclic stress. The effectiveness of the proposed model is demonstrated by comparing it to experimental data from various materials subjected to cyclic shearing under undrained conditions. The study encompasses a broad range of constant strain or stress amplitudes. Compared to the reference model, the proposed model exhibits improved predictive accuracy for the stress-strain response of clays at small amplitudes of cyclic loading and large number of cycles. Furthermore, it accounts for strength degradation due to cyclic loading.

While hypoplastic models have demonstrated accurate predictions of sand behavior under monotonic loading, their accuracy diminishes when applied to cyclic loading conditions. To address this limitation, the intergranular strain approach is used as an extension to the model. The current investigation focuses on the analysis of two variants: the original Intergranular Strain (IS) approach proposed by Niemunis and Herle (1997) and the Intergranular Strain Anisotropy (ISA) by Fuentes et al. (2019). Although both models have the same objective, they present distinct mathematical structures and therefore different repercussions on the simulations. In this study, sand Hypoplasticity is enhanced with IS and ISA, and employed to simulate a series of experimental tests conducted on Fontainebleau sand. These tests encompass isotropic compression, drained monotonic triaxial, and undrained cyclic triaxial tests, while considering different initial densities and test characteristics. Furthermore, the calibrated models were applied to simulate a series of centrifuge tests, involving a pile embedded in the same sand, which is subjected to various episodes of monotonic and cyclic lateral loading. A comparison and discussion of the similarities and differences in elemental and finite element predictions, arising from the two intergranular strain formulations is presented.