Hydro-mechanical Behaviour Research Articles

Effect of sodium occupancy and solute concentration on the swelling pressure and hydraulic conductivity of Boom Clay

The poorly indurated Boom Clay (BC) has been studied as a potential host formation for the geological disposal of radioactive waste including the intermediate-level long-lived bituminized radioactive waste called Eurobitum. In case of leaching of Eurobitum, fluids with altered chemical compositions would be expected to infiltrate locally into the BC. To evaluate the effect of salinity on the Boom Clay hydromechanical behaviour, a series of one-dimensional constant-volume swelling tests were conducted on intact BC samples that were first percolated with mixed sodium-calcium-nitrate solutions ((Na,Ca)NO3) with varying sodium concentrations, resulting in varying Na+ occupancies in the clay. Results showed that the swelling pressure of BC decreased with an increase in the solute concentration and sodium occupancy. Additionally, higher Na+ concentrations led to a faster stabilization of the swelling pressure. The hydraulic conductivity of BC increased with increasing solute concentration and sodium occupancy. Moreover, the sodium occupancy and concentration affected the swelling pressure and hydraulic conductivity in a similar way. This phenomenon could be elucidated by two primary mechanisms: i) the influence of solute concentration on the Diffuse Double Layer (DDL), and ii) the effect of cation type and hydrated radius within the interbasal space of montmorillonite sheets, especially concerning sodium occupancy.

Impact of Inline Polyacrylamide Polymer Flocculation on the Mechanical and Hydrological Properties of Saline Tailings

This study examines the geotechnical and hydro-mechanical behaviour of a model slurry used in high-solids, high-salinity applications, both before and after inline flocculation with an anionic polyacrylamide. Initial evaluations showed untreated tailings (UT) with a water content of 107%, void ratio of 2.6, and dry density of 0.711 t/m3, compared to polymer-amended tailings (PAT) with 53% water content, a void ratio of 1.6, and a dry density of 1.069 t/m3. Post-flocculation consolidometer tests revealed a distinct consolidation mode, with PAT showing 60% less settlement within the first 48 h and achieving 50% more free water drainage. Polymer treatment improved consolidation parameters, yielding a lower compressibility index (Cc of 0.74 vs. 1.05 for raw slurry), a higher coefficient of consolidation (Cv of 0.005 cm2/s for PAT vs. 0.0009 cm2/s for raw slurry), and an increased water retention capacity. Additionally, PAT demonstrated a final void ratio of 0.62 compared to 0.51 for the UT sample and an internal porosity characterised by discrete voids, supporting enhanced stability for long-term rehabilitation. These findings underscore the potential of inline flocculation to improve tailings management in saline conditions.

An extended kinematic hardening constitutive model to include unsaturated behaviour for the prediction of soil deterioration

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.

Strength and dilatancy of an unsaturated expansive soil at high suction levels

A series of suction-controlled triaxial tests was conducted on Nanyang expansive clay to investigate the effects of dry density and suction on dilatancy and strength. The suction of the soil samples was controlled using a vapour equilibrium technique, with four suction levels ranging from 3.29 MPa to 198.14 MPa, where water retention is dominated by adsorption. The experimental results show that the tested soil exhibits a brittle failure mode under high suction, significantly distinguishing the hydro-mechanical behaviour of the soil at high suction from that observed at low suction. This brittle failure mode significantly increases the contribution of suction to peak strength compared to residual strength, causes the soil to fail before reaching the critical state, a phenomenon not observed in soils under high suction, and results in dilatancy caused by damage to the soil particle aggregates rather than particle rearrangement. The dilatancy data obtained from the triaxial tests reveal that significant soil dilatancy occurs during shear after reaching peak strength, with the maximum dilatancy angle increasing with suction and decreasing with confining pressure. However, the initial dry density has a negligible impact on the soil's dilatancy under high suction levels. This observation further supports that, for unsaturated soils under high suction levels, dilatancy is attributed to damage to soil particle aggregates rather than the rearrangement of soil particles.

Conceptual and Applied Aspects of Water Retention Tests on Tailings Using Columns

The water retention capacity of porous materials is crucial in various geotechnical and environmental engineering applications such as slope stability analysis, landfill management, and mining operations. Filtered tailings stacks are considered an alternative to traditional tailings dams. Nevertheless, the mechanical behaviour and stability of the material under different water content conditions are of concern because these stacks can reach considerable heights. The water behaviour in these structures is poorly understood, particularly the effects of the water content on the stability and potential for liquefaction of the stacks. This study aims to investigate the water retention and flow characteristics of compacted iron ore tailings in high columns to better understand their hydromechanical behaviour. The research used 5 m high columns filled with iron ore tailings from the Quadrilátero Ferrífero region in Minas Gerais, Brazil. The columns were prepared in layers, compacted, and instrumented with moisture content sensors and suction sensors to monitor the water movement during various stages of saturation, drainage, infiltration, and evaporation. The sensors provided consistent data and revealed that the tailings exhibited high drainage capacity. The moisture content and suction profiles were effectively established over time and revealed the dynamic water retention behaviour. The comparison of the data with the theoretical soil water retention curve (SWRC) demonstrated a good correlation which indicates that there was no hysteresis in the material response. The study concludes that the column setup effectively captures the water retention and flow characteristics of compacted tailings and provides valuable insights for the hydromechanical analysis of filtered tailings stacks. These findings can significantly help improve numerical models, calibrate material parameters, and contribute to the safer and more efficient management of tailings storage facilities.

A thermodynamics-based coupled model for multi-phase flowback in deformable dual-porosity shale gas reservoirs

Over the past decades, there has been growing interest in modelling multi-phase flowback in shale gas reservoirs during hydraulic fracturing, primarily due to the significant risk of groundwater pollution. However, the lack of fully coupled simulations and a unified theoretical model has hindered the study of coupled adsorptive hydro-mechanical behaviour and its influence on fluid flowback prediction. In this paper, we have extended the non-equilibrium thermodynamics-based Mixture Coupling Theory to derive the constitutive relationship and governing equation for adsorptive multi-phase fluid flows in deformable dual-porosity media. Helmholtz free energy density has been used to establish the relationship between solids and fluids. The interaction of adsorptive fluids in the pore and fracture space has been fully considered, leading to a new form of constitutive equation, which obtained the molecular adsorptive effect on the rock by introducing adsorption entropy production. The proposed governing equations determine the fully coupled evolution of solid deformation and multi-phase flow, with the consideration of adsorption as well as pore and fracture porosity change. Numerical simulation is then performed to study the flowback of a real shale gas well and the influence of coupled behaviour on model prediction. The simulation shows that the flowback flux is mainly by the fracture (more than 99.9 %) and the overall cumulative volume is 2427.1 m3 within 225 h which accounts for about 5.3 % of the total injected fracturing fluid, and the sensitivity analysis reveals that the manual fracture porosity is the most sensitive factor to the cumulative flowback. This work provides new insights into the flowback process of shale reservoirs in a fully coupled view and the proposed theoretical tool should contribute to the modelling of other adsorptive multi-phase flow problem in deformable dual-porosity media such as carbon storage and coalbed methane production.

Hydromechanical behaviour of a slope reinforced by grass roots under rainfall conditions

Soil bioengineering using vegetation has been considered an environmentally friendly solution to improve slope stability. Although several studies have demonstrated the contribution of vegetation to slope stability, a gap in understanding the mechanisms of grass root–soil interactions under rainfall conditions remains. This study investigates the effects of the roots of vetiver grass (Chrysopogon zizanioides) on the hydromechanical behaviour of an unsaturated soil slope using the centrifuge modelling technique. The changes in pore water pressure and slope deformation were monitored during the test. The monitored data were subsequently back-analysed and interpreted using seepage–stability analyses. In addition, this study focused on evaluating the effect of roots on slope stability, considering safety and pore water pressure during rainfall. Results revealed that the vetiver roots remarkably affected the initial suction of the slope by increasing the soil's air-entry value. The increased suction and the additional cohesion provided by the roots enhanced slope stability under rainfall conditions.

Hydro-mechanical behaviour of straw fiber-reinforced cemented dredged sediment at high water content

Hydro-mechanical behaviour of straw fiber-reinforced cemented dredged sediment at high water content

Hydro-mechanical behaviour of reinforced concrete linings in hydraulic tunnels: steady state

Existing literature covers the behaviour of reinforced concrete (RC) tunnel linings in hydraulic tunnels during filling and steady-state conditions. However, certain aspects remain insufficiently addressed, risking the under-dimensioning of structures, particularly concerning transient behaviour. This paper introduces a comprehensive analytical framework for the design of RC linings in hydraulic tunnels, clarifying conditions that lead to a water-filled gap between the lining and rock mass, significantly impacting system behaviour. The presence or absence of this gap notably affects the number of cracks in the lining, with a substantial increase in cracks when the gap is absent. Contrary to prior assumptions, the paper highlights that the cracking process of the lining is progressive and can stabilize before reaching a fully cracked state, resulting in fewer cracks. Understanding these phases is crucial for setting realistic initial conditions for transient events, particularly concerning lining stiffness. Accurate stiffness determination is essential for evaluating the lining’s response during transients and its fatigue performance. This becomes increasingly significant with the rise of pumped storage schemes for grid stabilization, posing challenges in resistance and fatigue verification of hydraulic tunnel RC linings.

A numerical analysis of Thermo–Hydro–Mechanical behavior in the FE experiment at Mont Terri URL: Investigating capillary effects in bentonite on the disposal system

We investigated thermo–hydro–mechanical (T-H–M) coupled behavior observed during the full-scale heater emplacement experiment at the Mont-Terri underground research laboratory conducted in the Opalinus clay as part of the DECOVALEX-2023 Task C project. Utilizing the OGS-FLAC simulator, we created a three-dimensional model to simulate multiphase flow in the experiment, applying extended Philip and de Vries’ model and incorporating the anisotropic T–H–M properties of the Opalinus clay. The simulation, which included a ventilation process, spanned five years of heating experiments and successfully replicated the measured temperature, pore pressure, displacement, and relative humidity results in bentonite and host rock during the experiment. The analysis revealed that capillary pressure significantly influenced the pore pressure change in the host rock near the tunnel, while thermal pressurization became dominant with increasing distance. Consequently, we conducted a sensitivity analysis on a simplified model to evaluate the effect of capillary pressure on the disposal system. Capillarity is a dominant factor for the multiphase flow depending on the distance from the heat. Variations in capillary pressure were observed depending on the gas entry pressure and water retention model, indicating that the capillarity of unsaturated bentonite could inherently affect the T–H–M results within the disposal system.

Implicit hydromechanical representation of fractures using a continuum approach

Fractures control fluid flow, solute transport, and mechanical deformation in crystalline media. They can be modeled numerically either explicitly or implicitly via an equivalent continuum. The implicit framework implies lower computational cost and complexity. However, upscaling heterogeneous fracture properties for its implicit representation as an equivalent fracture layer remains an open question. In this study, we propose an approach, the Equivalent Fracture Layer (EFL), for the implicit representation of fractures surrounded by low-permeability rock matrix to accurately simulate hydromechanical coupled processes. The approach assimilates fractures as equivalent continua with a manageable scale (≫1 μm) that facilitates spatial discretization, even for large-scale models including multiple fractures. Simulation results demonstrate that a relatively thick equivalent continuum layer (in the order of cm) can represent a fracture (with aperture in the order of μm) and accurately reproduce the hydromechanical behavior (i.e., fluid flow and deformation/stress behavior). There is an upper bound restriction due to the Young's modulus because the equivalent fracture layer should have a lower Young's modulus than that of the surrounding matrix. To validate the approach, we model a hydraulic stimulation carried out at the Bedretto Underground Laboratory for Geosciences and Geoenergies in Switzerland by comparing numerical results against measured data. The method further improves the ability and simplicity of continuum methods to represent fractures in fractured media.

Numerical simulation of rainfall-induced deformations of embankments considering the coupled hydro-mechanical behavior of unsaturated soils

This paper presents numerical simulations of the deformation response of unsaturated embankments subjected to rainfall infiltration using a coupled hydro-mechanical constitutive model for unsaturated soils. The constitutive model accounts for the influence of degree of saturation on the stress–strain behavior and the influence of void ratio on the water retention behavior. The constitutive model was implemented in the finite difference program FLAC, calibrated using triaxial test data, and validated using measurements of wetting-induced deformations of embankment models from centrifuge tests. The validation demonstrates that the constitutive model can capture the coupled hydro-mechanical behavior of unsaturated soils under wetting conditions. Simulations of the hydro-mechanical response of unsaturated embankments subjected to rainfall infiltration indicate that the differential settlement across the top surface of the embankment between the centerline and shoulder increases significantly during rainfall infiltration, which could result in severe damage to overlying transportation infrastructure. As the rainfall infiltration increases, shear strains accumulate and form a potential failure surface at a shallow depth of approximately 2 m from the slope surface. Insights into the hydro-mechanical response of unsaturated embankments subjected to rainfall infiltration gained will be useful for considering climate change effects in design and construction of compacted embankments.

Effect of technological voids on hydro-mechanical behavior of compacted bentonite

Effect of technological voids on hydro-mechanical behavior of compacted bentonite

Quantifying pore-level non-uniformity of pore-water distribution in unsaturated soils

Quantifying the magnitude and distribution of degree of saturation (Sr) in unsaturated soils is crucial to understand the grain-scale hydromechanical behavior, but it has been a major experimental challenge. This study proposes a new method to quantify the pore-water distribution non-uniformity, in terms of Sr, based on three-dimensional (3D) X-ray computed tomography images. The algorithm constructs vectors that consider the 3D spatial distribution of Sr for each REV. A weighted water distribution tensor (G, characterizing the spatial distribution of Sr) was derived to calculate a scalar parameter A that represents the non-uniformity. Application of the algorithm on sand samples demonstrated that the Sr distributions could be highly non-uniform at different drying states. The algorithm captured pore-water transport between the dilated and non-dilated zones of samples subjected to pre-peak shearing. The evolution of A with matric suction and axial strain showed potential in incorporating the pore-water distribution into microstructure-based constitutive models.

Hydrodynamic study of gas–liquid-solid mini-fluidization based on fluorescence PIV

The fluorescence-based tracer particle image velocimetry (fluorescence PIV) was used to obtain the flow regimes transition, bubble size and velocity, liquid and particle flow field characteristics of the 1 ∼ 3 mm GLSMFBs and 93 ∼ 195 μm glass beads. When the bed-to-particle diameter ratio increased, the bubble size and velocity distribution tended to be more concentrated. Bubble rose as straight line and S-shaped spiral trajectory. The divergence and vorticity of the flow field were preliminarily quantified. There showed a close relationship between the macroscopic hydromechanical behaviors such as bubble size and velocity and the mesoscopic behaviors such as the movement and development of vortices. The upward drag force of liquid and bubble on particles mainly affected the particle axial velocity, while the particle radial motion was mainly caused by bubble wake and surface force among particles. The results provide theoretical reference for the design and optimization of GLSMFB reactors.

A Study on the Factors Controlling the Kinematics of a Reactivated and Slow-Moving Landslide in the Eastern Liguria Region (NW Italy) through the Integration of Automatic Geotechnical Sensors

This paper deals with the investigation of factors influencing the movement patterns of a reactivated slow-moving landslide situated in the eastern Liguria region (NW Italy) through the analysis of extensive ground-based hydrological and geotechnical monitoring data. Subsurface horizontal displacement and pore water pressure data were acquired simultaneously by means of automatic sensors positioned at pre-existing and localized failure zones. The joint examination of field measurements enabled us to explore the connections between rain, pore water pressure, and displacements. The results of continuous displacement monitoring showed that the landslide kinematics involved phases of extremely slow movements alternated with periods of relative inactivity. Both stages occurred prevalently at seasonal scale displaying similar durations. The slow-motion phases took place at relatively constant pore water pressure and were ascribed to mechanisms of viscous shear displacements along failure surfaces. Inactive phases entailed no significant deformations, mostly corresponding to prolonged dry periods. The two motion patterns were interrupted by episodic sharp deformations triggered by delayed (preparation periods from 4 to 11 days) rainfall-induced pore water pressure peaks, which were ascribed to sliding mechanisms taking place through rigid-plastic frictional behaviour. During these deformation events, hysteresis relationships between pore water pressure and displacement were found, revealing far more complex hydro-mechanical behaviour.

Comparative study of hydro-mechanical behaviors of compacted bentonite powder and granular bentonite

In the deep geological disposal repository of high-level radioactive waste, buffer/backfill materials typically consist of compacted bentonite block and granular bentonite. As these materials undergo a long-term hydration, it is anticipated that the two forms of bentonite materials (i.e. compacted bentonite powder (CBP) and granular bentonite (GB)) are expected to exhibit differing hydro-mechanical behaviors due to the differences in their structures. This work aims to investigate the differences in swelling pressure and compressibility through a series of swelling pressure tests, compression tests and mercury intrusion porosimetry (MIP) tests. The experimental results demonstrated that swelling pressure curves of the CBP specimens showed higher first peak values and more pronounced collapse than those of the GB specimens at a given dry density, regardless of vapor-water hydration or liquid-water hydration. The final swelling pressures of the two materials were similar at the same dry density, suggesting an independent correlation between swelling pressure and dry density. At the high suction range, the compression curves exhibited an obvious bi-linear pattern for the CBP specimens and a significant nonlinearity for the GB specimens. Meanwhile, the CBP specimens presented higher pre-consolidation pressures and larger compression indices than the GB specimens at a given suction. As suction decreased, the compression curves of the two materials gradually approached each other and their differences were reduced accordingly. After reaching saturation, a good consistency between them was observed whether for final swelling pressure or compressibility. Pore structure analysis revealed that the two materials both presented an initially double structure, and their differences were primarily manifested at the macrostructural level. Eventually, the differences in swelling pressure or compression curves of the two materials were well interpreted by combining microstructural evolutions.

Numerical assessment of the time-dependent behaviour of a tunnel with a yielding support system for a potential repository in claystone

For the disposal of heat-generating radioactive waste and spent fuel in claystone formations, support structures for underground excavations are essential for the operational safety of geological disposal facilities (GDF). Claystone formations of moderate strength, at considerable depths, are characterised by their squeezing, creeping, and, sometimes, swelling behaviour that results in continuous tunnel convergence over time. Under these conditions, a rigid support system can be subjected to very high loads, requiring a considerable thickness and/or high-performance concretes. Therefore, yielding support systems are being currently investigated as a promising alternative for GDF. This work presents a numerical study of the behaviour of a tunnel in a claystone formation with a yielding support system. The use of a compressible mortar between the rock and the lining is assumed, as a means of reducing loads and mitigating the effects of creep deformations. A key aspect of the analyses is that the host rock is characterised by a constitutive model that includes a number of features that are relevant for the satisfactory description of the hydro-mechanical behaviour of stiff clayey materials, such as mechanical anisotropy, creep, strain softening, and its ability to simulate localised deformations through a nonlocal regularisation. An elastoplastic constitutive model was also developed to represent the behaviour of the compressible mortar. Results provide relevant insights into the performance of the adopted yielding support system, particularly regarding the effect of time-dependent deformations and the additional relaxation of the rock on the fractured zone near the excavation.

Hydromechanical modelling of the influence of water saturation on the penetration performance of concrete target

The objective of the present paper is to study numerically the hydro-mechanical behaviour of concrete targets subjected to a rigid projectile, with special attention to the influence of water saturation. An elastoplastic model, encompassing two primary plastic mechanisms (shear and pore collapse), is improved to capture the influence of water saturation. The adopted model is implemented in the finite element code Abaqus/Explicit and validated by simulating two dynamic compression tests at the material point level and two penetration tests at the structure level. Not only a strong influence of water saturation on both volumetric and deviatoric behaviours of concrete is observed at material point level, but also the time-evolutions of projectile velocity, projectile deceleration and penetration depth in concrete targets are also satisfactorily reproduced by the numerical simulations. After that, a series of parametric studies are performed to understand the influence of water saturation on the penetration performance of concrete. It is found that with the presence of pore water, penetration resistance of concrete increases with decreasing water saturation, indicating a higher projectile deceleration and lower penetration depth obtained in saturated targets. Numerical modelling and analysis will enhance our understanding of the vulnerability of concrete infrastructures subjected to near-field detonations or impacts.

Assessment of factors and mechanism contributing to groundwater depressurisation due to longwall mining

Assessment of mining impact on groundwater is one of critical considerations for longwall extension and sustainability, however usually constrained by limited data availability, hydrogeological variation, and the complex coupled hydro-mechanical behaviour. This paper aims to determine the factors and mechanism of groundwater depressurisation and identify knowledge gaps and methodological limitations for improving groundwater impact assessment. Analysis of dewatering cases in Australian, Chinese, and US coalfields demonstrates that piezometric drawdown can further lead to surface hydrology degradation, while the hydraulic responses vary with longwall parameters and geological conditions. Statistical interpretation of 422 height of fracturing datasets indicates that the groundwater impact positively correlates to panel geometry and depth of cover, and more pronounced in panel interaction and top coal caving cases. In situ stress, rock competency, clay mineral infillings, fault, valley topography, and surface–subsurface water interaction are geological and hydrogeological factors influencing groundwater hydraulics and long-term recovery. The dewatering mechanism involves permeability enhancement and extensive flow through fracture networks, where interconnected fractures provide steep hydraulic gradients and smooth flow pathways draining the overlying water to goaf of lower heads. Future research should improve fracture network identification and interconnectivity quantification, accompanied by description of fluid flow dynamics in the high fracture frequency and large fracture aperture context. The paper recommends a research framework to address the knowledge gaps with continuous data collection and field-scale numerical modelling as key technical support. The paper consolidates the understanding of longwall mining impacting mine hydrology and provides viewpoints that facilitate an improved assessment of groundwater depressurisation.