Hydro-mechanical Coupled Model Research Articles

Coupled hydro-mechanical modelling of soil–vegetation–atmosphere interaction in natural clay slopes

Soil–vegetation–atmosphere interaction has long been known to induce significant pore pressure variations at shallow depths and associated superficial slope movements. Recent findings suggest that the effect of this interaction may also extend to large depths in natural clay slopes. Multiple examples of weather-induced deep landslide mechanisms can be found in the Southern Apennines (Italy), where slopes are often formed of fissured clays. The relationship between the activity of these landslides and the hydro-mechanical processes due to soil–vegetation–atmosphere interaction was investigated herein by means of a two-dimensional coupled hydro-mechanical finite element analysis. A constitutive model capable of simulating the behaviour of highly overconsolidated clays, in both saturated and unsaturated states, was adopted in the analysis, in conjunction with a boundary condition capable of reproducing the combined effects of rainfall infiltration, evapo-transpiration, and run-off. The results of the analysis corroborate the connection between weather conditions, pore pressure variations, and slope movements in natural clay slopes. The importance of adequately reproducing the geological history of a natural slope to define its current state is also demonstrated.

Verification of a DDA-based hydro-mechanical model and its application to dam foundation stability analysis

There are many studies concerning hydro-mechanical coupling model based on discontinuous deformation analysis (DDA). However, verification of the coupling model and its application to practical engineering are rarely found. In this paper, we first introduced the distributed load sub-matrix of water pressure and described the calculation procedures of hydro-mechanical coupling analysis in the framework of DDA. Then, the correctness of the coupling model was verified respectively through analyses of buoyancy, deformation of a block under uniform water pressure and hydraulic fracturing. Finally, the model was applied to evaluate the sliding stability of a gravity dam foundation in China. The failure process and stability factor were calculated by two different stability evaluation methods (the strength reduction method and the overloading method), and the obtained stability factors were compared with those solved by the commonly used limit equilibrium analysis methods. The strength reduction method revealed that the rock strata of dam foundation slide with different displacements along several weak interlayers of different depths, whereas the overloading method revealed that the rock strata slide mainly along a special interlayer beneath the anti-sliding concrete stud, which was designed to be near the dam heel. The DDA method clearly shows the process of rock block failure and profoundly reveals the difference in sliding mode caused by using different stability evaluation methods.

A bounding surface model for unsaturated soils considering the microscopic pore structure and interparticle bonding effect due to water menisci

The interparticle bonding effect due to water menisci plays an important role in the hydromechanical coupling properties of unsaturated soils. This paper presents an unsaturated hydromechanical coupling model that considers the influence of matric suction, degree of saturation, and microscopic pore structure on the interparticle bonding effect. The enhanced effective stress and bonding variable are selected as constitutive variables. The bonding variable is correlated with the ratio between unsaturated void ratio and saturated void ratio. The deformation characteristics of unsaturated soils are described based on the bounding surface plasticity theory. A soil–water characteristic model that considers deformation and hydraulic hysteresis is integrated into the constitutive model to achieve hydromechanical coupling. The proposed model can effectively describe the hydromechanical coupling characteristics and the meniscus bonding force of unsaturated bimodal structure soils; the model parameters can be easily obtained through routine experiments. The experimental results of unsaturated isotropic compression, the wetting/drying cycle, and unsaturated triaxial shear tests are used to validate the capability of the proposed model.

Coupled hydro-mechanical modelling of dilatancy controlled gas flow and gas induced fracturing in saturated claystone

Large quantities of gas can be produced during the lifespan of a deep geological repository (DGR) due to several processes that may affect the integrity of the host rock. Therefore, understanding dilatancy controlled gas flow in saturated claystone is important for assessing the safety of a DGR with argillaceous formations as the potential host rock. In this paper, a coupled hydro-mechanical (HM) model that incorporates double porosity poroelasticity is developed to simulate the gas migration process in saturated claystone. The model accounts for the HM behavior of both the porous medium (which represents the matrix) and the fractured medium (which represents the fractures). Double effective stress principles for each medium are derived from the first law of thermodynamics. The volumetric strains of the matrix and fractures, which are work-conjugated to the respective effective stress level, are explicitly included in the mass balance equations. The developed model is successfully evaluated against three gas injection tests on claystone at the laboratory scale. The main experimental observations, i.e., the development of gas preferential pathways, volume dilation, and gas induced fracturing, are well captured.

On the effective stress coefficient of single rough rock fractures

The effective stress coefficient is critical to accurately predict the hydromechanical coupling behavior of single water-bearing rock fractures. The physical meaning of the effective stress coefficient in a rough rock fracture is understood as the ratio of the nominal fracture surface occupied by water to the total fracture surface area. To overcome the difficulty of measuring contact ratio changes in rough rock fractures under normal loading and water pressure, a new effective stress coefficient model for single rough water-bearing fractures is proposed in terms of two mechanical parameters, initial normal stiffness and maximum normal closure. By incorporating the new effective stress coefficient model into the Barton-Bandis constitutive model, a hydromechanical coupling model was built and verified by laboratory and in-situ experimental data. The results indicate that the effective stress coefficient is less than 1 for single rough rock fractures and decreases with increasing difference between normal stress and water pressure. When the normal stress is close to or considerably higher than the fracture water pressure, both the newly built model and Terzaghi's model obtain similar normal displacement. However, for moderate normal stress, the normal displacements predicted by the newly built model and Terzaghi's model differ significantly. The implications of the newly built model in subsurface engineering are discussed.

A Fully Coupled Hydromechanical Model for CO2Sequestration in Coal Seam: the Roles of Multiphase Flow and Gas Dynamic Diffusion on Fluid Transfer and Coal Behavior

CO2sequestration in coal seam has proved to be an effective way for reducing air pollution caused by greenhouse gases. A study on the rules of fluid transfer and reliability of CO2storage during gas injection is necessary for the engineering application. However, the clarification of multifield coupling in long-term CO2sequestration is the difficulty to solve the aforementioned problem. Previous investigations on the coupled model for CO2storage in coal seam were not exactly comprehensive; for example, the multiphase flow in the fracture and the nonlinear behavior of gas diffusion were generally neglected. In this paper, a new multistage pore model of the coal matrix and the corresponding dynamic diffusion model were adopted. Meanwhile, the CO2-induced coal softening and the CO2-water two-phase flow in coal fracture were also taken into account. Subsequently, all the mentioned mechanisms and interactions were embedded into the coupled hydromechanical model, and this new fully coupled model was well verified by a set of experimental data. Additionally, through the model application for long-term CO2sequestration, we found that the stored CO2molecules are mainly in an adsorbed state at the early injection stage, while with the continuous injection of gas, the stored CO2molecules are mainly in a free state. Finally, the roles of multiphase flow and gas dynamic diffusion on fluid transfer and coal behavior were analyzed. The results showed that the impact of multiphase flow is principally embodied in the area adjacent to the injection well and the coal seam with lower initial water saturation is more reliable for CO2sequestration, while the impact of gas dynamic diffusion is principally embodied in the area far away from the injection well, and it is safer for CO2sequestration in coal seam with greater attenuation coefficient of CO2diffusion.

Coupled hydro-mechanical modelling of a 1995 Hong Kong landslide

The paper deals with the modelling of the instability mechanism induced by rainfall in an unsaturated cut-slope. A large-sized landslide occurred in 1995 in Hong Kong (the so-called “Fei Tsui Road landslide”). It was here analysed because it was characterized by unusual dimensions and very large runout distance for the study area. The slope failure was attributed to a decrease in soil shear strength due to the rise of a perched water table above a weak kaolin-rich layer, together with the loss of suction caused by water infiltration during a heavy rainfall event. The hydro-mechanical coupled analyses made through the commercial software Plaxis 2D aimed to investigate the relations between the hydrological variables (i.e., rainfall infiltration, suction, saturation) and the slope response in terms of changes in soil resistance and soil plastic deformations. The study demonstrates that the evaluation of the hydro-mechanical coupling effects on the hydraulic slope response as well as on the stability of the whole slope is a crucial issue to well capture the mechanical behaviour of the unsaturated cut-slope. Different failure scenarios have been also considered in order to match the field observations and to back-analyse the initial condition of the slope before landslide.

Coupled hydro-mechanical analysis for grout penetration in fractured rocks using the finite-discrete element method

Grouting is a widely used geotechnical engineering method to improve the strength and reduce the hydraulic conductivity of rock masses. The grouting process is a typical coupled hydro-mechanical (HM) problem, which should be analyzed by considering the mutual interaction between the grout flow and the rock mass. In this paper, a coupled HM model (Y-grouting) is presented to study the grouting process using the finite-discrete element method (FDEM). This Y-grouting can well explain some typical phenomena observed during grouting operations, which are difficult to be modelled by pure hydraulic analysis. The dilation process experienced by the fracture during grout injection clearly illustrates the necessity of considering the HM coupling effect. The effect of the in-situ stress conditions on the anisotropic grout penetration is properly modelled with the Y-grouting that, together with the stress interaction between neighbor fractures, explains why finer fractures are more difficult to be grouted and whether increasing the grouting pressure could be an effective way to improve the penetration in fine fractures. Ultimately, the results demonstrate importance of considering the HM coupling when simulating grouting and assessing the grouting efficiency.

On the implementation of a hydro-mechanical coupling model in the numerical manifold method

By using the MC system (mathematical cover system) and the PC system (physical cover system), the NMM (numerical manifold method) is able to cope with continuum and discontinuum problems in the same framework. In this study, a HM (hydro-mechanical) coupling model which is based on the “cubic law” is incorporated into the NNM for the purpose of modeling multiple crack propagation problems driven by water flow. A series of numerical examples have been conducted to evaluate the performance of the proposed numerical model. The results show that the proposed numerical model can not only simulate the processes of multiple crack propagation driven by fluid flow, but also can carry out hydraulic analysis of fissured rock networks with high accuracy.

A fully coupled simple model for unsaturated soils

SummaryDifferent phenomena influence the strength and volumetric behavior of unsaturated soils. Among the most important are suction hardening, hydraulic hysteresis, and the influence of volumetric strain on the soil‐water retention curves. Fully coupled hydro‐mechanical models require including all three phenomena in their constitutive relationships. Among these phenomena, suction hardening is the most influencing as it determines the apparent preconsolidation stress, the position of the loading‐collapse yield surface, and the shift of both the isotropic consolidation and the critical state lines. In this paper, a fully coupled hydro mechanical model is presented. It is based on the modified Cam‐Clay model but includes a yield surface with anisotropic hardening that takes account of the shift of the critical state line with suction. For highly overconsolidated materials, the sub‐loading surface concept has been included in order to increase the precision of the model for these materials. The shift of the retention curves produced by volumetric strains is simulated using a hydraulic model based on the grain and current pore size distribution of the soil.

Quantifying the induced fracture slip and casing deformation in hydraulically fracturing shale gas wells

Hydraulic fracturing induces the shear failure of natural fracture which contributes to complex fracture network. The magnitude of fracture slip potentially causes some undesirable consequences including wellbore instability, casing deformation and fault reactivation. Therefore, it is important to predict the fracture slip induced by hydraulic fracturing for a safe and efficient stimulation. In this paper, we used a 2D hydro-mechanical coupled model to predict the injection-induced slip. The hydraulic and natural fractures are embedded in formations with cohesive zone model. The fracture propagation, rock deformation and wellbore slip displacement are captured in the 2D model. The wellbore slip displacement is input into a small-scale 3D mechanical model of casing in slip rock to simulate casing behavior. Casing curvature is also introduced to assess casing integrity. Results indicate that the rock deforms asymmetrically with respect to the wellbore after the shear failure of natural fracture. Particularly, there is a shear slip along the natural fracture. The simulation result shows that the casing failure mechanism is shear deformation induced by the fracture slip during hydraulic fracturing. The curvature of deformed casing is larger than that of directional well trajectory. The predicted results are validated by the logging and operation data from the field. This work provides a novel quantitative method for predicting fracture slip and evaluating well integrity during hydraulic fracturing.

Numerical Analysis of Surrounding Rock Stability in Super-Large Section Tunnel Based on Hydro-Mechanical Coupling Model

The groundwater seepage has an important effect on the stability of tunnel surrounding rock. Considering the effect of seepage, a hydro-mechanical coupling mathematical model is developed based on stress deformation equation of rock mass, flow equation of porous media and relation equation of hydro-mechanical coupling. In connection with the leakage situation of Jiangshuiquan Tunnel with super-large section, a geometry model of tunnel construction is built by COMSOL software platform. Furthermore, the hydro-mechanical coupling model is embedded in the PDE module of the software for numerical simulating the construction process of super-large section tunnel. Consequently, the surrounding rock displacement deformation under is discussed and verified by the data measured in field. Besides, the distribution characteristics of surrounding rock stress, seepage pressure and velocity are deeply analyzed. Results indicate that the displacement calculated by hydro-mechanical coupling model is well consistent with the site monitoring data and is greatly more than that of uncoupling model. After tunnel excavation, stress concentration phenomenon occurs around the cavern, especially at the arch foot part of the outer side wall, so the parts should be viewed as the key supporting part. The groundwater flows into the cavern and the surrounding pore water pressure is unloaded. A large hydraulic gradient occurs in the vault of the tunnel, prone to incur major engineering accidents; while the hydraulic gradient at vault is relatively small and usually has little harmful to engineering safety. The conclusion may provide guidelines for similar tunnel engineering construction.

Hydraulic fracturing modeling using the enriched numerical manifold method

Recent attempts to solve rock mechanics problems using the numerical manifold method (NMM) have been regarded as fruitful. In this paper, a coupled hydro-mechanical (HM) model is incorporated into the enriched NMM to simulate fluid driven fracturing in rocks. In this HM model, a “cubic law” is employed to model fluid flow through fractures. Several benchmark problems are investigated to verify the coupled HM model. The simulation results agree well with the analytical and experimental results, indicating that the coupled HM model is able to simulate the hydraulic fracturing process reliably and correctly.

The Unsaturated Hydromechanical Coupling Model of Rock Slope Considering Rainfall Infiltration Using DDA

Water flow and hydromechanical coupling process in fractured rocks is more different from that in general porous media because of heterogeneous spatial fractures and possible fracture-dominated flow; a saturated-unsaturated hydromechanical coupling model using a discontinuous deformation analysis (DDA) similar to FEM and DEM was employed to analyze water movement in saturated-unsaturated deformed rocks, in which the Van-Genuchten model differently treated the rock and fractures permeable properties to describe the constitutive relationships. The calibrating results for the dam foundation indicated the validation and feasibility of the proposed model and are also in good agreement with the calculations based on DEM still demonstrating its superiority. And then, the rainfall infiltration in a reservoir rock slope was detailedly investigated to describe the water pressure on the fault surface and inside the rocks, displacement, and stress distribution under hydromechanical coupling conditions and uncoupling conditions. It was observed that greater rainfall intensity and longer rainfall time resulted in lower stability of the rock slope, and larger difference was very obvious between the hydromechanical coupling condition and uncoupling condition, demonstrating that rainfall intensity, rainfall time, and hydromechanical coupling effect had great influence on the saturated-unsaturated water flow behavior and mechanical response of the fractured rock slopes.

Deformation Behavior between Hydraulic and Natural Fractures Using Fully Coupled Hydromechanical Model with XFEM

There has been a growing consensus that preexisting natural fractures play an important role during stimulation. A novel fully coupled hydromechanical model using extended finite element method is proposed. This directly coupled scheme avoids the cumbersome process during calculating the fluid pressure in complicated fracture networks and translating into an equivalent nodal force. Numerical examples are presented to simulate the hydraulic fracture propagation paths for simultaneous multifracture treatments with properly using the stress shadow effects for horizontal wells and to reveal the deformation response and interaction mechanism between hydraulic induced fracture and nonintersected natural fractures at orthotropic and nonorthotropic angles. With the stress shadow effects, the induced hydraulic flexural fracture deflecting to wellbore rather than transverse fracture would be formed during the progress of simultaneous fracturing for a horizontal well. The coupled hydromechanical simulation reveals that the adjacent section to the intersection is opened and the others are closed for orthogonal natural fracture, while the nonorthogonal natural fracture is activated near the intersection firstly and along the whole section with increasing perturbed stresses. The results imply that the induced hydraulic fracture tends to cross orthotropic natural fracture, while it is prior to being arrested by the nonorthotropic natural fracture.

Coupled infiltration model of unsaturated porous media for steady rainfall

Unsaturated soil slopes introduce complex hydro-mechanical coupled processes which greatly alter matric suction distribution of an unsaturated soil during rainfall. In order to investigate matric suction and volume change of unsaturated soils, a two-dimensional hydro-mechanical coupled infiltration model (YS-Slope) is developed by incorporating the hydraulic and mechanical characteristics of unsaturated soils, such as the soil-water characteristic curve, permeability function, shear strength, and porosity. Special attention is given to the porosity-dependent permeability function of unsaturated soils. In addition, in order to highlight effectiveness of YS-Slope and coupling effects of hydro-mechanical processes on the infiltration behavior of unsaturated soils, a series of infiltration analyses for a soil column under various soil properties is conducted and their results are compared with those of commercial software, GEO-SLOPE (2012). The results of the numerical analyses show good agreement with data from the analytical solution and laboratory tests, which indicates that the proposed model is appropriate for use in the simulatfion o the infiltration of rainwater into deformable soils. The transient seepage and rainwater flow in deformable soils are influenced by the volume change of the soil. The change in matric suction on a slope due to rainfall infiltration influences change in effective stress while the effective stress alters seepage processes according to hydraulic properties. The results indicate that hydro-mechanical coupled behavior of soils has a positive effect on the stability of unsaturated soil slops during rainfall.

Three-Dimensional Hydromechanical Model of Hydraulic Fracturing with Arbitrarily Discrete Fracture Networks using Finite-Discrete Element Method

AbstractThis study presents a hydromechanical model, finite-discrete element method with fluid flow in three dimensions (FDEM-flow3D), that can simulate three-dimensional hydraulic fracturing of jointed rock mass with complex fracture networks. By taking full advantage of a unique topological connection between joint elements and solid elements in three-dimensional combined finite-discrete element method (FEMDEM) together with the cubic law, the authors built a three-dimensional fluid flow model. In addition, a connectivity search algorithm for arbitrarily complex three-dimensional fracture networks is proposed, which can be used to search the connectivity of arbitrarily complex three-dimensional fracture networks. Combining the connectivity search algorithm and the mechanical calculations of three-dimensional FEMDEM, the authors built the three-dimensional hydromechanical coupling model FDEM-flow3D, which directly implements hydromechanical coupling and can simulate fluid-driven fracturing in rock with a...

Coupled Hydro-Mechanical Model of Bentonite Hydration and Swelling

This paper deals with the modelling of coupled hydro-mechanical processes at the buffer and host rock interface (bentonite and granite) in the context of the safe disposal of spent nuclear fuel. Granite, as one of the barriers, includes fractures which are the source for hydration of bentonite and its subsequent swelling. It affects the mechanical behaviour and possibly the stability of the whole system. A non-linear solution for the stress-deformation problem with swelling was developed. This solution is coupled with the non-linear diffusion problem (for unsaturated flow). The swelling is defined using a coefficient dependent on water content according to literature data, with the effective Young's modulus decreasing close to zero corresponding to the plastic state. Results confirm the expected non-uniform saturation, swelling, and stresses in bentonite and small contribution to a fracture displacement.

FDEM-flow3D: A 3D hydro-mechanical coupled model considering the pore seepage of rock matrix for simulating three-dimensional hydraulic fracturing

A hydro-mechanical coupled model that can simultaneously consider the pore seepage of a rock matrix and the fracture seepage is proposed to simulate three-dimensional hydraulic fracturing. This model appropriately takes into account the fluid leak-off into the surrounding rock matrix from the fracture. Several examples are given to validate the seepage algorithms and the coupled model. The results suggest that this model can solve problems involving pore seepage and fracture seepage through simple pure fracture seepage. Moreover, it can reproduce the fluid pressure distribution and the crack initiation and propagation and consider the fluid loss during hydraulic fracturing.

Model for simulating hydromechanical responses in aquifers to induced hydraulic stresses: Laboratory investigation and model validation

The environmental consequences of the hydraulic fracturing of shale beds have raised widespread concern, especially with regard to groundwater. For example, the hydraulic pressure and permeability in an upper aquifer may be affected by a flow injection process owing to the interaction between the fluids and the deformation of porous media. To experimentally investigate the hydromechanical (HM) effect of the induced hydraulic stresses, a column device was developed. The acquired experimental data was used to develop a coupled HM numerical model, which was used to further investigate the upper aquifer properties. A multiphysics simulator was used to implement the relationship between the permeability and the volumetric strain, and the permeability of the numerical model was calibrated by an error analysis. The results of the HM model were compared with those of a single flow model, and the variations of the hydraulic pressure and permeability with the injection pressure were also analyzed. It was found that the coupled HM model was more sensitive to changes in the permeability compared to the single flow model, and that the former produced better numerical results. The hydraulic pressure in the upper aquifer was determined to decrease with increasing height and the largest change was 1.13 times the initial value. Furthermore, the largest permeability change was observed to correspond to an injection pressure of 40 kPa and was 3.33E-4 times the initial value. This maybe neglected. A higher injection pressure was further found to increase the permeability, which also generally increased with increasing height.