Thermo-hydro-mechanical Research Articles

Shear Banding and Cracking in Unsaturated Porous Media through a Nonlocal THM Meshfree Paradigm

The mechanical behavior of unsaturated porous media under non-isothermal conditions plays a vital role in geo-hazards and geo-energy engineering (e.g., landslides triggered by fire and geothermal energy harvest and foundations). Temperature increase can trigger localized failure and cracking in unsaturated porous media. This article investigates the shear banding and cracking in unsaturated porous media under non-isothermal conditions through a thermo–hydro–mechanical (THM) periporomechanics (PPM) paradigm. PPM is a nonlocal formulation of classical poromechanics using integral equations, which is robust in simulating continuous and discontinuous deformation in porous media. As a new contribution, we formulate a nonlocal THM constitutive model for unsaturated porous media in the PPM paradigm in this study. The THM meshfree paradigm is implemented through an explicit Lagrangian meshfree algorithm. The return mapping algorithm is used to implement the nonlocal THM constitutive model numerically. Numerical examples are presented to assess the capability of the proposed THM mesh-free paradigm for modeling shear banding and cracking in unsaturated porous media under non-isothermal conditions. The numerical results are examined to study the effect of temperature variations on the formation of shear banding and cracking in unsaturated porous media.

Numerical investigation into the thermo-hydro-mechanical behavior in unsaturated soils considering hypergravity effect

Unsaturated soils are commonly found in coastal regions and marine sediments, where understanding the thermo-hydro-mechanical (THM) behavior, especially regarding phase transition, is crucial for related practical engineering applications. Centrifugal model offers an effective means to reflect the prototype with smaller scale and shorter duration under hypergravity conditions. Present studies on similarity based on governing equation analysis of the THM coupled responses are mainly focused on saturated rather than unsaturated soils, nor involved with different boundary conditions. This paper theoretically derives the similarity of THM behavior of unsaturated soils between the centrifugal model and prototype, and numerically assesses the similarity considering Dirichlet / Neumann condition based on the open-source FEM code OpenGeoSys. In addition, numerical results shows that phase transition between liquid water and water vapor will occur under temperature variation, and pore pressure difference rather than initial pore pressure causes the difference of THM coupled responses. Significant pore pressure variation will be amplified with increase of gas diffusion coefficient, and large air entry pressure corresponds to high liquid saturation and obvious phase transition.

Measurement and Modeling of Thermo-Hydro-Mechanical Behaviors of Frozen Clays: Frost Susceptibility and Compressibility

The risk of geohazards associated with frozen subgrades is well recognized, but a comprehensive framework to evaluate frost susceptibility from microstructural characteristics to macroscopic thermo-hydro-mechanical (THM) behaviors has not been established. This study aims to propose a simple framework for quantitatively assessing frost susceptibility and compressibility in frozen soils. A systematic THM model was devised to predict heat transfer, soil freezing characteristics, and stress states in frozen soils. Constant freezing experiments and oedometer compression tests were performed on bentonite clays under varying temperatures (−5°C, −10°C, and −20°C) and stress levels to validate the proposed model. Additionally, soil electrical conductivity measurements were employed to assess the temperature- and stress-dependent volumetric and mechanical properties of frozen soils. The model used Fourier’s law to compute the transient soil temperature profile and estimated the volume change and stress states based on the soil freezing characteristic curve. Experimental results showed that frost heave of bentonite reached between 9.0% and 26.6% of axial strain, which was largely predicted by the proposed model. It also demonstrated that the frost heave was mainly attributed to the fusion of the porewater. Additionally, the preconsolidation pressure of frozen soils exhibited a rapid increasing trend with decreasing temperature, which was explained by the temperature-dependent ice morphology in the soil interpore. Furthermore, the findings also demonstrated a remarkable sensitivity in the electrical conductivity in response to the soil temperature during the frost heave process and the stress state under the loading or unloading path.

Numerical Study of the Thermo-Hydro-Mechanical Coupling Impacts of Shallow Geothermal Borehole Groups in Fractured Rock Mass on Geological Environment

Fractured rock mass is extensively distributed in Karst topography regions, and its geological environment is different from that of the quaternary strata. In this study, the influences on geological environment induced by the construction and operation of a large-scale borehole group of ground source heat pumps are analyzed by a thermo-hydro-mechanical (THM) coupling numerical model. It was found that groundwater is redirected as the boreholes can function as channels to the surface, and the flow velocity in the upstream of borehole group is higher than those downstream. This change in groundwater flow enhances heat transfer in the upstream boreholes but may disturb the original groundwater system and impact the local geological environment. Heat accumulation is more likely to occur downstream. The geo-stress concentration appears in the borehole area, mainly due to exaction and increasing with the depth. On the fracture plane, tensile stress and maximum shear stress simultaneously occur on the upstream of boreholes, inducing the possibility of fracturing or the expansion of existing fractures. There is a slight uplift displacement on the surface after the construction of boreholes. The correlations of the above THM phenomena are discussed and analyzed. From the modeling results, it is suggested that the consolidation of backfills can minimize the environmental disturbances in terms of groundwater redirection, thermal accumulation, occurrence of tensile stress, and possible fracturing. This study provides support for the assessment of impacts on geological environments resulting from shallow geothermal development and layout optimization of ground heat exchangers in engineering practices.

Modelling Thermo-hydro-mechanical (THM) Effect on the Hydro-mechanical Properties of Granite in Disposal System Using an Improved Meso-structure-Based DEM Model

Modelling Thermo-hydro-mechanical (THM) Effect on the Hydro-mechanical Properties of Granite in Disposal System Using an Improved Meso-structure-Based DEM Model

The stability issue of fractured rock mass slope under the influences of freeze–thaw cycle

Freeze–thaw failure of frozen rock slope often occurs during engineering construction and mining in cold area, which poses a great threat to engineering construction and people's life safety. The properties of rock mass in cold region will change with the periodic change of temperature, which makes it difficult to accurately evaluate the stability of slope under the action of freeze–thaw cycle by conventional methods. Based on field investigation and literature review, this paper discusses the characteristics of frozen rock mass and the failure mechanism of frozen rock slope, and gives the types and failure modes of frozen rock slope. Then, the research status of frozen rock slope is analyzed. It is pointed out that the failure of frozen rock slope is the result of thermo-hydro-mechanical (THM) coupling. It is considered that freeze–thaw cycle, rainfall infiltration and fracture propagation have significant effects on the stability of frozen rock slope, and numerical simulation is used to demonstrate. The research shows that the safety factor of frozen rock slope changes dynamically with the surface temperature, and the safety factor of slope decreases year by year with the increase of freeze–thaw cycles, and the fracture expansion will significantly reduce the safety factor. Based on the above knowledge, a time-varying evaluation method of frozen rock slope stability based on THM coupling theory is proposed. This paper can deepen scholars' understanding of rock fracture slope in cold area and promote related research work.

Thermo-hydro-mechanical (THM) analysis of rock-ice avalanches: Considering the effects of particle breakage in the sliding zone

The high-speed and long-distance sliding characteristics of large-scale rock-ice avalanches usually make them a catastrophic event, which seriously threatens the safety of human beings and infrastructure. Obviously, apart from the lubrication of ice and water, the fragmentation of particles in the slip zone is also an important contribution to the hypermobility of rock-ice avalanches. Up to now, existing researches have not taken into account the impact of particle breakage on rock-ice landslides. In this paper, a thermal-hydro-mechanical (THM) model for the high-speed rock-ice avalanches is developed, which mainly considers the particle breakage due to sliding shear deformation and the ice melting caused by frictional heat. The results of the model analysis applied to the Chamoli avalanche indicate that particle fragmentation has a significant effect on landslide sliding. When the particle breakage is considered, the simulated velocity and displacement of landslide are closer to the existing research results. Both thermal pressurization and particle crushing are two important mechanisms that lead to a high-speed sliding of rock-ice landslide, and these two mechanisms affect each other. It is demonstrated from a set of sensitivity analyses that the initial grain size distribution, fractal dimension and the thickness of the slide mass have clear impacts on the evolution of the granular crushing index, which thus affects the characteristics of the landslide eventually. In particular, the sensitivity analysis of the slide thickness once again demonstrates the importance of considering particle fragmentation in the analysis of landslide sliding.

Study on thermal‐hydro‐mechanical coupling and stability evolution of loess slope during freeze–thaw process

AbstractDisasters occurring at loess slopes in seasonal frozen regions are closely related to changes in the thermo‐hydro‐mechanical (THM) state in loess by freeze–thaw (FT) action. Current research on FT‐induced soil slope failure focuses on frozen stagnant water effects, while the intrinsic connection between the FT‐induced stagnant water effect and soil strength deterioration remains unclear. In this study, by taking the FT‐induced loess slope failure as an example, field surveys, boreholes, exploratory wells, and 3D topographic mapping were used to reveal the landslide features and stratigraphic information; Furthermore, the temporal and spatial variation of water and heat in loess slope was revealed by on‐site monitoring data; A THM coupled model of frozen soil was established using COMSOL Multiphysics simulation software to reconstruct the frozen stagnant water process of shallow loess slope, as well as the influence of THM field on loess landslide. The results show that the effects of FT in the seasonally frozen region occurred in the shallow layer of the loess slope. The water‐ice phase transition during FT process broke the phase equilibrium of loess. Numerical calculations and field monitoring indicated a continuous migration of water to the freezing front, creating a water‐enriched zone inside the loess. Both the impact of the frozen stagnant water and changes in the stress field led to the degradation of loess structure and reduced the strength properties, thus threatening the stability of the loess slope. The study results can contribute to an in‐depth understanding of the mechanism underlying FT loess landslides in seasonal frozen regions, and provide a scientific basis for the evaluation and prevention of FT landslides.

THM model of rock tunnels in cold regions and numerical simulation

The freezing damage of rock tunnels in cold region involves ice-water phase change and complicated interaction of Thermo–Hydro–Mechanical (THM) field. Taking the fractured rock mass of cold region tunnels as research subject, the THM coupling model of cold region tunnels was established, which is based on the seepage mechanics, heat transfer theory, damage mechanics and equivalent continuum theory. This model could reflect the anisotropic properties of deformation, water migration and heat transfer caused by the initial fracture of rock mass. The construction and operation processes of a rock tunnel in cold region were simulated, and results were compared with the measured value and predecessor’s achievements. It shows that proposed model could reflect the anisotropic property of surrounding rock and the simulated deformation and stress are not symmetrical. Compared with the literature, the calculated results in this paper are closer to the measured values. The insulating layer has a significant effect on the stress of the supporting structures. The maximum tension stress of the lining is 4.5 times as that without insulating layer, and the lining will be destroyed for the overlarge tension stress.

Coupled Thermo-Hydro-Mechanical Processes in Fractured Rocks: Some Past Scientific Highlights and Future Research Directions

Coupled thermo-hydro-mechanical (THM) processes in fractured rocks have been a topic of intense scientific research for more than 30 years. The present paper takes a look into the past and highlights some scientific advances which are of an unusual “out-of-the-box” nature, and then looks forward and discusses possible directions of future research in this interesting field of study. Concerning future research directions, we see a trend from a focus on coupled THM processes in single fractures or a few interacting fractures, to the study of coupled THM behavior in complex fracture network systems where the fractures act collectively giving rise to local stress concentration points and points of large pressure gradients. Three examples of future research directions are presented. First is an effort towards identifying characterizing parameters of a fracture network that play a direct controlling role in major coupled THM phenomena (such as induced seismicity and flow channeling), rather than parameters of stochastic distributions of fractures in the network. The second example of research direction is accounting for the heterogeneity and hierarchy of fractures in a fault or fracture zone which has been associated with major THM events in a number of geo-energy projects. The third example is at the opposite end of the first; here it is recognized that in some cases, the coupled THM processes in fractured rocks may be controlled dominantly by only a few key bridges. Identification, characterization, and evaluation of these key bridges should be one of the important research directions in the coming days.

Evolution of the mixed mode FPZ and fracture roughness of Beishan granite after coupled thermo-hydro-mechanical treatment

Deep surrounding rocks contain a network of longitudinally intersecting fissures. Exploration of their fracture process zone (FPZ) size evolution during mixed loading and fracture surface roughness after failure has always been a hot topic, especially for coupled thermo–hydro–mechanical (THM) treatment. This study takes Beishan granite as the research object. The fracture surface roughness and FPZ length were obtained using Grasselli statistical and mixed strain–displacement methods. Finally, an electron microscope was employed to observe the fracture surface after failure at different temperatures and loading angles. Results reveal a notable temperature and load dependence of the fracture surface roughness and FPZ length evolution. In the low-temperature stage (25°C–300°C), the mineral primarily exhibited transgranular fractures and the average fracture surface roughness showed a slightly decrease with increasing temperature. In the high-temperature stage (500℃–650℃), the degree of internal damage to the prepared specimens substantially increased, with intergranular fractures being the dominant fracture mode, and rapid roughness growth occurred. Under the same temperature condition, the roughness value was in the order of mode I > mixed mode > mode II. The FPZ length increased, stabilized, and decreased as the load increased. In the low-temperature stage, the FPZ length reached its maximum value at the maximum load, whereas in the high-temperature stage, the FPZ length was fully developed before the peak load. Under the same loading angle condition, the FPZ lengths corresponding to the three loading modes followed the order of mode I < mixed mode < mode II.

A fully coupled thermo-hydro-mechanical elastoplastic damage model for fractured rock

A detailed and fully coupled thermo-hydro-mechanical (THM) model for fractured rock is presented. This model distinguishes itself by harmoniously integrating elastoplastic material behaviour with a continuum damage mechanics framework. Solid matrix displacement, pore and fissure water pressures and temperature of the solid are introduced as the primary nodal variables. The ingenuity of this research is embedded in the intricate coupling of THM processes with plastic deformation and damage mechanics in a double porous medium, a venture that significantly broadens the remit of existing methodologies. The model is implemented using finite element method (FEM) and validation is achieved by comparing the FEM results against existing literature numerical outcomes describing linear and elastoplastic continuum damage behaviour of fractured rock. The model also exhibits an extraordinary proficiency in reproducing experimental triaxial test results, using THM components conjoined with elastoplastic bounding surface aspects and inherent hardening effects. It is imperative, nonetheless, to underscore the model's sensitivity to certain material properties, inclusive of strength parameters, leakage coefficients, and permeability attributes. This fully coupled THM model provides a comprehensive and sophisticated tool for investigating the behaviour of fractured rock under various loading conditions. It can help us better understand the physics of fractured rock behaviour and contribute to the development of more accurate and reliable models for engineering applications, such as CO2 injection.

Development of a Scalable Parallel Compositional Simulator for Thermo-Hydromechanical Coupling in Fractured Rocks Using an Embedded Discrete Fracture Model

Summary Numerical simulation of thermo-hydromechanical (THM) coupling in practical complex fractured rocks is an essential but challenging issue for the evaluation and optimization of underground energy production. In this study, we present our work on a scalable parallel compositional simulator for THM coupling, which is suitable for massive 3D polygonal fractures. In addition, we also present the improvements, parallel implementation, and optimization of an embedded discrete fracture model (EDFM). A unified cell-centered grid system based on the finite volume method (FVM) is used for all governing equations, and an extended stencil is adopted for mechanical equations to resolve the low-resolution defect of the traditional FVM. The deformation of both matrix rock and fractures is considered. A sequential fully implicit (SFI) method is adopted to solve THM coupling. This simulator is validated against three analytical solution models. Finally, we apply the simulator to two cases including a multilayered shale gas reservoir with massive natural fractures and a fractured geothermal model using CO2 as a working fluid. We also test the performance and parallel scalability on 1,024 CPU cores with up to 50 million matrix gridblocks and 5.5 million fracture gridblocks. The results show that this simulator can efficiently solve the THM coupling problem in practical massive fractures.

Freezing-thawing response of sand-kaolin mixtures in oedometric conditions

The freezing and thawing cycles applied to soils can often be causes of irreversible instabilities, uplift, or subsidence. These phenomena can be both natural (e.g. permafrost regions) or artificially induced (e.g. artificial ground freezing (AGF), as an excavation earth support technique for tunnelling). When the soil temperature falls below 0°C, the liquid water changes phase turning into ice. The freezing process generally implies an expansion of the soil element due to (i) the lower density of the migration of the ice and (ii) the pore water toward the frozen front. The complex interaction between the solid grains, the water and the ice formation during freezing determines the soil’s overall thermo-hydromechanical (THM) behaviour. Sandy mixtures with different percentages of kaolin were tested on an oedometer developed at the Geotechnical Laboratory of Universita degli Studi di Roma Tor Vergata, working at temperatures below zero. The samples were compressed under five different vertical stresses (50, 100, 200, 400 and 800 kPa, respectively), and then a freezing and thawing cycle was applied by steps to a minimum temperature of -20°C. The experimental results regarding temperature evolution, vertical displacements, and liquid water amount monitored are discussed and interpreted through a micro-to-macro approach. The data analysis revealed interesting outcomes that characterise mechanical and hydraulic hysteresis during freezing and thawing processes. A deeper understanding of the coupled phenomena that occur during the freezing and thawing cycles is supposed to contribute to developing a constitutive model that can reproduce the irreversible volumetric response of frozen soils.

Effect of thermo-mechanical treatment with different initial moisture content on bamboo cell walls

Thermo-hydro-mechanical (THM) treatment is an effective way to improve the performance of bamboo. However, to date, research on the impact of moisture content (MC) on THM treatment has been relatively little. This study examined the microstructure and physical and chemical properties of bamboo treated with THM under different initial moisture content conditions. The interaction and influencing relationships among the three were explored to provide a reference for THM in the bamboo industry. According to the results of the Three-point Bending test and Nanoindentation test, it is indicated that humidity has a relatively minor impact on the mechanical performance and micro-mechanical properties of compressed samples. However, Scanning Electron Microscope (SEM) and Mercury Intrusion Porosimetry (MIP) tests have shown that an increase in moisture content effectively improves the uneven deformation of bamboo cell walls during compression, thereby reducing their porosity. However, appropriate moisture content in the experiment affected the softening effect of bamboo and caused slight cracking of the fiber tube during compression. With respect to Fourier Transform Infrared Spectrometer (FTIR) spectroscopy, differences in the initial moisture content can also affect the chemical composition. The X-ray diffraction (XRD) analysis has also shown that the crystallinity of the treated samples was higher than that of the untreated samples. Moisture content has a significant impact on the hygroscopicity and dimensional stability of compressed samples by influencing changes in the chemical composition, microstructure, and crystallinity during thermo-hydro-mechanical treatments. In this experiment, bamboo with a moisture content of 12% had a pronounced softening effect during the hot-pressing process while exhibiting the dimensional stability and low moisture absorption required.

Thermo-Hydro-Mechanical Modeling of Brine Migration in a Heated Borehole Test in Bedded Salt

AbstractThis research paper focuses on the thermo-hydro-mechanical (THM) modeling of brine migration in a heated borehole test conducted as part of the ongoing Brine Availability Test in Salt (BATS) at the Waste Isolation Pilot Plant (WIPP) in New Mexico. It is a component of the international collaboration project DECOVALEX-2023 (DEvelopment of COupled models and their VALidation against EXperiments), which aims to understand the THM processes governing brine flow in heated rock salt repositories through collaborative analysis by multiple research teams. Using the TOUGH–FLAC simulator, THM simulations were performed and compared with data from the BATS phase 1a. This experiment involved two identical horizontal-borehole arrays, one heated and one serving as a control, both equipped with sensor arrays. Analysis of measurements revealed water flow rate surges during heater power transitions, with the highest jump observed during cooling. Acoustic emission activity exhibited distinct patterns in response to heater power changes, suggesting that damage to rock salt is particularly pronounced during the cooling phase. The THM simulations successfully captured these phenomena, highlighting the significance of thermal effects, brine migration, and mechanical behavior in predicting brine availability in heated and damaged rock salt. Our modeling also revealed the critical interplay between heating and cooling-induced damage and its influence on flow properties, particularly affecting brine inflow estimation. Notably, we found that cooling-induced brine inflow spikes result from increased permeability due to tensile dilatancy. These findings have important implications for the development of robust containment strategies and enhance our understanding of the complex processes involved in repository performance.

A thermomechanical model for argillaceous hard soils–weak rocks: application to THM simulation of deep excavations in claystone

The paper presents the enhancement of an existing constitutive model for argillaceous hard soils–weak rocks to incorporate non-isothermal conditions, to be used in coupled thermo-hydro-mechanical (THM) simulations of underground excavations subjected to temperature variations within the context of deep geological nuclear waste disposal. The proposed thermo-elastoplastic extension accounts for the effect of temperature on the yield and plastic potential functions and on the elastic stiffness. The resulting model is validated through the simulation of relevant non-isothermal laboratory tests reported in the literature. The model is then applied to the coupled THM simulation of an in situ heating test conducted at the Meuse/Haute-Marne underground research laboratory in Bure, France, excavated in the Callovo-Oxfordian claystone. Results show that the incorporation of thermal effects into the constitutive description of the host rock plays a significant role in the behaviour of the excavation when subjected to thermal loading, particularly in the evolution of the excavation fractured zone.

On the coupled thermo-hydro-mechanical behaviors of layered porous media by the transformed differential quadrature method

In this paper, the thermo-hydro-mechanical (THM) problem of layered porous media is investigated by using the transformed differential quadrature method (TDQM). Starting from the THM governing equations in cylindrical coordinates, the partial differential equations in the transformed domain are obtained by using the Hankel integral transform. Then, the temporal domain and the spatial domain along depth are discretized according to the proposed TDQM, and the partial differential equations in the transformed domain are converted into algebraic equations at sampling points. By introducing load conditions, boundary conditions and continuity conditions, the matrix equation to solve the coupled THM problem of layered porous media is obtained. Finally, case studies are carried out to verify the TDQM solution, and to further discuss the influences of thermal and hydraulic parameters including the linear expansion coefficient of fluid and skeleton, permeable coefficient of fluid and porosity on the THM behaviors of layered porous media.

A coupled cryogenic thermo-hydro-mechanical model for frozen medium: Theory and implementation in FDEM

This paper presents the development of a coupled modeling approach to simulate cryogenic thermo-hydro-mechanical (THM) processes associated with a freezing medium, which is then implemented in the combined finite-discrete element method code (FDEM) for multi-physics simulation. The governing equations are deduced based on energy and mass conservation, and static equilibrium equations, considering water/ice phase change, where the strong couplings between multi-fields are supplemented by critical coupling parameters (e.g. unfrozen water content, permeability, and thermal conductivity). The proposed model is validated against laboratory and field experiments. Results show that the cryogenic THM model can well predict the evolution of strongly coupled processes observed in frozen media (e.g. heat transfer, water migration, and frost heave deformation), while also capturing, as emergent properties of the model, important phenomena (e.g. latent heat, cryogenic suction, ice expansion and distinct three-zone distribution) caused by water/ice phase change at laboratory and field scales, which are difficult to be all revealed by existing THM models. The novel modeling framework presents a gateway to further understanding and predicting the multi-physical coupling behavior of frozen media in cold regions.

Thermo-hydro-mechanical (THM) coupled simulation of the land subsidence due to aquifer thermal energy storage (ATES) system in soft soils

Aquifer thermal energy storage (ATES) system has received attention for heating or cooling buildings. However, it is well known that land subsidence becomes a major environmental concern for ATES projects. Yet, the effect of temperature on land subsidence has received practically no attention in the past. This paper presents a thermo-hydro-mechanical (THM) coupled numerical study on an ATES system in Shanghai, China. Four water wells were installed for seasonal heating and cooling of an agriculture greenhouse. The target aquifer at a depth of 74–104.5 m consisted of alternating layers of sand and silty sand and was covered with clay. Groundwater level, temperature, and land subsidence data from 2015 to 2017 were collected using field monitoring instruments. Constrained by data, we constructed a field scale three-dimensional (3D) model using TOUGH (Transport of Unsaturated Groundwater and Heat) and FLAC3D (Fast Lagrangian Analysis of Continua) equipped with a thermo-elastoplastic constitutive model. The effectiveness of the numerical model was validated by field data. The model was used to reproduce groundwater flow, heat transfer, and mechanical responses in porous media over three years and capture the thermo- and pressure-induced land subsidence. The results show that the maximum thermo-induced land subsidence accounts for about 60% of the total subsidence. The thermo-induced subsidence is slightly greater in winter than that in summer, and more pronounced near the cold well area than the hot well area. This study provides some valuable guidelines for controlling land subsidence caused by ATES systems installed in soft soils.