Hydro-mechanical Response Research Articles

Insights into the hydromechanical rock mass responses during in-situ rock mass stimulations

Insights into the hydromechanical rock mass responses during in-situ rock mass stimulations

Effects of repeated hydraulic loads on microstructure and hydraulic behaviour of a compacted clayey silt

Soils used in earth construction projects are mostly unsaturated, and they undergo frequent drying–wetting cycles (repeated hydraulic loads) due to changes in climatic conditions or variations of the ground water level, particularly at shallow depths. After compaction, changes in water content can significantly influence the hydromechanical response of the construction material, which therefore must be assessed for repeated hydraulic loads. This research investigates the effect of such loads on the microstructure and hydraulic behaviour of a silty soil, typically used in the construction of embankments and dykes, with the aim of providing a better understanding of the consequences of drying–wetting cycles on the response of the material over time. Experimental tests were performed to study the impact of drying–wetting cycles on the water retention, hydraulic conductivity, and fabric of compacted specimens. Fabric changes are documented to take place even without significant volumetric strains, promoting an irreversible increase in the hydraulic conductivity and a reduction in the capacity to retain water compared to the as-compacted soil. The fabric changes are interpreted and quantified by means of a hydromechanical model, which accounts for the evolving pore-size distribution at different structural levels. The proposed model reproduces quite well the microstructural observations, together with the evolution of the water retention behaviour and hydraulic conductivity.

Hydromechanical reactivation of natural discontinuities: mesoscale experimental observations and DEM modeling

Fracture interaction mechanisms and reactivation of natural discontinuities under fluid pressurization conditions can represent critical issues in risk assessment of caprock integrity. A field injection test, carried out in a damage fault zone at the decameter scale, i.e., mesoscale, has been studied using a distinct element model. Given the complex structural nature of the damage fault zone hydraulically loaded, the contribution of fracture sets on the bulk permeability has been investigated. It has been shown that their orientation for a given in situ stress field plays a major role. Based on these results, a simpler model with a fluid-driven fracture intersecting a second fracture has been set up to perform a sensitivity analysis. It is in presence of a minimum differential stress value with a minimum angle with the maximum principal stress that the second fracture could be both, hydraulically and mechanically reactivated. Results also showed that in the vicinity of the fluid-driven fracture, a natural fracture will offer contrasted hydromechanical responses on each side of the intersection depending on the stress conditions and its orientation with respect to the stress field. In this case, we show that a hydromechanical decoupling can occur along the same plane. These results provide insights into fracture-controlled permeability of fault zones depending on the properties of the fractures and their hydromechanical interactions for a given in situ stress field.

Modeling dynamic responses of heterogeneous seabed with embedded pipeline through multiresolution random field and coupled hydromechanical simulations

In this work, novel multiresolution random field models are integrated with coupled hydromechanical finite element simulations to model dynamic responses of heterogeneous seabed with embedded pipeline subjected to nonlinear wave-induced loadings. The heterogeneous properties of the seabed are characterized and modeled as spatially correlated random variables. A conditional sequential simulation technique is implemented and multiresolution random field realizations of heterogeneous properties are generated. The effects of spatially correlated seabed properties on the characteristic hydromechanical responses are analyzed in details through validated hydromechanical finite element simulations. Statistics and associated uncertainties of system response are obtained with recourse to Monte Carlo simulations. The relations between the inherent heterogeneities and uncertainties in the system responses are derived. Findings of this study are shown to have important implications for probabilistic engineering designs.

Hydro-mechanical analysis of pipe armed by nanoparticles based on numerical methods

Fluid influence on the hydro-mechanical response of pipes armed by nanoparticles is investigated here. Mori-Tanaka approach is utilised for calculating the structure equivalent properties assuming agglomeration influences. The famous Navier-Stokes relation is applied for deriving the fluid force. According to the Reddy theory of shell, the equations of motion are obtained utilising principal of Hamilton and method of energy. The critical velocity of fluid and frequency of system are presented using numerical approach and the influences of nanoparticles volume fractions and agglomeration as well as pipe geometrical variables are assumed. Results present that enhancing the nanoparticles volume fractions, the critical velocity of fluid and frequency of the structure are enhanced.

Coupled thermo-hydro-mechanical behaviour of a deep clay

An experimental study on Ypresian clays –one of the potential deep formations in Belgium for the geological disposal of heat-emitting radioactive waste– was undertaken to systematically study its thermal properties and coupled hydro-mechanical response during fast heating pulse tests. An accurate characterisation of the thermal properties is required for assessing the near-field perturbations around disposal galleries that the sedimentary host rock formation will undergo. A new experimental cell adapted to apply the high in situ stresses and with thermal flux sensors was used to directly measure the thermal conductivity at different sample orientations (heat flux orthogonal and parallel to bedding planes). A clear influence of the degree of saturation – despite being close to saturation – and anisotropic features on thermal conductivity have been detected. The study was complemented by performing fast heating pulse tests under constant volume on a new and fully-instrumented axisymmetric cell. The cell allowed recording the pore pressure build-up and dissipation along a heating pulse and under water-undrained conditions.

Fault slippage and its permeability evolution during supercritical CO2 fracturing in layered formation

Understanding the hydromechanical responses of faults during supercritical CO2 fracturing is important for reservoir management and the design of energy extraction systems. As small faults are widespread in Chang 7 member of the Yanchang Formation, Ordos Basin, China, supercritical CO2 fracturing operation has the potential to reactive these undetected small faults and leads to unfavorable fracking fluid migrate. In this work, we examined the role of fault slippage and permeability evolution along a small fault connecting the pay zone and the confining formation during the whole process of fracturing and production. A coupled hydromechanical model conceptualized from actual engineering results was introduced to address the main concerns of this work, including, (1) whether the existence of a undetected small fault would effectively constrain the hydraulic fracture height evolution, (2) what the magnitude of the induced microseismic events would be and (3) whether the permeability change along the fault plane would affect the vertical conductivity of the confining formation and thus increase the risk for the fracturing fluid to leak. Our results have shown that the initial hydrofracture formed at the perforation and propagated upward, once it merged with the fault surface, the existence of an undetected small fault would effectively constrain the hydraulic fracture height evolution. As fracturing continued, further slippage spread from the permeability increase zone of high permeability to shallower levels, and the extent of this zone was dependent on the magnitude of the fault slippage. At the end of extraction, the slip velocity decreases gradually to zero and the fault slippage finally reaches stabilization. In general, undetected small faults in targeted reservoir may not be the source of large earthquakes. The induced microseismic events could be considered as the sources of acoustic emission events detected while monitoring the fracturing fluid front. Due to the limited fault slippage and lower initial permeability, the CO2 fracturing operation near undetected small faults could not conduct preferential pathway for upward CO2 leakage or contaminate overlying shallower potable aquifers.

Hydro-mechanical analysis of pipe armed by nanoparticles based on numerical methods

Fluid influence on the hydro-mechanical response of pipes armed by nanoparticles is investigated here. Mori-Tanaka approach is utilised for calculating the structure equivalent properties assuming agglomeration influences. The famous Navier-Stokes relation is applied for deriving the fluid force. According to the Reddy theory of shell, the equations of motion are obtained utilising principal of Hamilton and method of energy. The critical velocity of fluid and frequency of system are presented using numerical approach and the influences of nanoparticles volume fractions and agglomeration as well as pipe geometrical variables are assumed. Results present that enhancing the nanoparticles volume fractions, the critical velocity of fluid and frequency of the structure are enhanced.

Internal erosion in dike‐on‐foundation modeled by a coupled hydromechanical approach

SummaryOne of the major causes of instability in geotechnical structures such as dikes or earth dams is internal erosion, an insidious process that occurs over a long period of time. Research on this topic is still fairly new and much more needs to be understood in order to solve the problems posed by this phenomenon. This paper proposes a hydromechanical model based on porous continuous medium theory to assess how internal erosion impacts the safety of earthen structures. The saturated soil is considered as a mixture of four interacting constituents: soil skeleton, erodible fines, fluidized fine particles, and fluid. The detachment and transport of the fine particles are described by a mass exchange model between the solid and the fluid phases. An elastoplastic constitutive model for sand‐silt mixtures has been developed to monitor the effect of the evolution of both porosity and fines content induced by internal erosion upon the behavior of the soil skeleton. The model has been numerically solved with the finite element method. It has then been applied to the specific case study of a dike foundation subjected to internal erosion induced by the presence of a karstic cavity beneath the alluvium layer. The numerical results show the onset of erosion, the time‐space evolution of the eroded zone, and the hydromechanical response of the soil constituting the dike, all of which highlights the effects of the cavity location, the erosion rate, and the fines content.

An evolutive bounding surface plasticity model for early-age cemented tailings backfill under cyclic loading

Cemented tailings backfill (CTB), an evolutive man-made soil or porous medium, is extensively utilized in underground mines as a sustainable solution for the disposal of tailings (mine waste) and/or ground control. At the early stages of curing, CTB is commonly saturated and weakly cemented, and thus subjected to the risk of seismically-induced liquefaction caused by mine blasts, rock bursts and earthquakes. The resulting intrusion of liquefied backfill mass into the mine tunnels may give rise to significant casualties and loss of production. In the present study, an evolutive bounding surface plasticity model is developed to capture the hydro-mechanical response of hydrating CTB under cyclic loading. Specifically, the changes of model components (yield and plastic potential functions, etc.) due to evolution of material properties are incorporated into the prototype bounding surface model. Then, the material property changes induced by binder hydration are determined and coupled to a binder hydration model that quantifies the curing process of CTB. Therefore, the developed model can be employed to assess the cyclic response of hydrating CTB at any given time of interest during its early age of curing when seismically induced liquefaction is of primary concern. The simulated results show good agreement with the experimental data for both monotonic and cyclic loadings. The proposed model can therefore provide more insight into the behavior of early-age CTB under seismic loading, and contribute to designing cost-effective CTB structures and assessing the risk of liquefaction.

A Simplified Approach to Assess the Soil Saturation Degree and Stability of a Representative Slope Affected by Shallow Landslides in Oltrepò Pavese (Italy)

The identification of the triggering mechanism of rainfall-induced, shallow landslides requires a complete understanding of the hydro-mechanical response of soil, which can be represented through the trends of the degree of soil saturation. In this paper, multiple annual cycles of soil saturation obtained through field monitoring were used to validate an empirical model based on climate data. Both field measurements and model outputs were used to conduct simplified slope stability analysis to evaluate the model chain capability in predicting the temporal occurrence of shallow failures. Field data were collected on a testsite slope located in Oltrepò Pavese (Northern Italy), where a shallow landslide occurred during the monitoring period. The experimental trends of the degree of saturation at various depths in the soil profile were compared with the calculated values and showed good agreement. Landslide triggering is reached when the soil is completely saturated. Both measured and modeled trends of soil saturation correctly identified the triggering time of the shallow landslide and the depth of the sliding surface, 1.0 m below the ground surface, in the test slope. The obtained results indicated the possibility of extending this approach for theassessment of the initiation time and the depth of shallow landslides, particularly for preliminary susceptibility evaluations, based on widely available climate data.

Some aspects of the hydro-mechanical behaviour of Callovo-Oxfordian (COx) claystone around a gallery parallel to the principal horizontal minor stress

The Meuse/Haute-Marne Underground Research Laboratory (MHM URL) is an outstanding scientific tool that provides reliable data on the behaviour of Callovo-Oxfordian (COx) claystone. More than 10,000 sensors have been installed in the argillaceous formation to measure rock properties and behaviour under several configurations in order to assess coupled processes. Since 2004, data have been acquired continuously through several measurement campaigns –mainly in galleries at the main level, −490 m (clay unit level –, i.e. UA level), but also in a gallery at −445 m [silty carbonate unit level (USC)]. This experiment has increased understanding of the hydro-mechanical behaviour of claystone and highlight various phenomena. The research program carried out in the MHM URL galleries also is dedicated to technology improvement and demonstration issues of several disposal systems.1 At MHM URL, the principal horizontal major stress (σH) is oriented at NE150° , and the principal vertical stress (σv) is nearly equal to the principal horizontal minor stress (σh) that is oriented NE065° . The ratio σH∕σh is close to 1.3 and varies with depth and the rheological characteristics of the respective layers. This paper presents the hydro-mechanical coupling (i.e. interaction between the interstitial water and the solid skeleton) during and after excavation work. The influence of the in-situ stress fields on several types of support was studied for a gallery oriented according to the principal minor horizontal stress (σh). This paper presents an analysis of measurements recorded during and after the construction of an experimental gallery, referred to as the GER, from 2013 to 2015. The GER experimental gallery is an 83-m-long tunnel, in which four different supports were tested: sliding arches; shotcrete; compressible wedges; and poured concrete. Sliding arches, shotcrete and compressible wedges constitute the initial supports that were installed during excavation. The poured concrete was set up about 9 months after the section excavation as a final support, first at the bottom and then at the vault of the gallery. Hydrogeological and geotechnical measurements help to describe the hydro-mechanical response of the surrounding rock mass. Experimental results highlight the anisotropy of the loading from the surrounding rock mass and the influence of the stiffness/flexibility of the support.

Effect of Fault Architecture and Permeability Evolution on Response to Fluid Injection

AbstractInjection‐induced seismicity is thought to be due primarily to increase in fluid pore pressure, which reduces the shear strength of a nearby fault. We address the modeling and prediction of the hydromechanical response due to fluid injection, mainly as wastewater disposal. We consider the full poroelastic effects, including the changes in porosity and permeability of the medium due to changes in local volumetric strains. Our results consider effects of the fault architecture (low‐permeability fault core and anisotropic high‐permeability damage zones) on the pressure diffusion and the fault poroelastic response. We show that the high‐permeable damage zone, the poroelastic response, and the permeability evolution can accelerate the pore pressure diffusion process during and after wastewater injection. By studying a geologically based model of the Guy‐Greenbrier fault and of the earthquake sequence induced along it in Arkansas, United States, from October 2010 to July 2011, we show that the existence of highly permeable damage zones facilitates the pressure diffusion and results in a sharp increase in pore pressure at levels much deeper than the injection wells, while the anisotropic permeability in the damage zone can act as a barrier to cross‐fault fluid flow. Furthermore, by computing the change ΔCFS of Coulomb failure stress, our simulations show that ΔCFS increases starting from the top of the Guy‐Greenbrier fault and then propagates toward greater depth and toward the southwest direction, which is consistent with the seismicity migration.

An updated Lagrangian LBM–DEM–FEM coupling model for dual-permeability fissured porous media with embedded discontinuities

Many engineering applications and geological processes involve embedded discontinuities in porous media across multiple length scales (e.g. rock joints, grain boundaries, deformation bands and faults). Understanding the multiscale path-dependent hydro-mechanical responses of these interfaces across length scales is of ultimate importance for applications such as CO2 sequestration, hydraulic fracture and earthquake rupture dynamics. While there exist mathematical frameworks such as extended finite element and assumed strain to replicate the kinematics of the interfaces, modeling the cyclic hydro-mechanical constitutive responses of the interfaces remains a difficult task. This paper presents a semi-data-driven multiscale approach that obtains both the traction-separation law and the aperture-porosity–permeability relation from micro-mechanical simulations performed on representative elementary volumes in the finite deformation range. To speed up multiscale simulations, the incremental constitutive updates of the mechanical responses are obtained from discrete element simulations at the representative elementary volume whereas the hydraulic responses are generated from a neural network trained with data from lattice Boltzmann simulations. These responses are then linked to a macroscopic dual-permeability model. This approach allows one to bypass the need of deriving multi-physical phenomenological laws for complex loading paths. More importantly, it enables the capturing of the evolving anisotropy of the permeabilities of the macro- and micro-pores. A set of numerical experiments are used to demonstrate the robustness of the proposed model.

Stress-dependent shear wave splitting and permeability in fractured porous rock

It is well known that shear wave propagates slower across than parallel to a fracture, and as a result, a travelling shear wave splits into two directions when it encounters a fracture. Shear wave splitting and permeability of porous rock core samples having single fracture were experimentally investigated using a high-pressure triaxial cell, which can measure seismic shear wave velocities in two directions mutually perpendicular to the sample axis in addition to the longitudinal compressive wave velocity. A single fracture was created in the samples using a modified Brazilian split test device, where the cylindrical sample edges were loaded on two diametrically opposite lines by sharp guillotines along the sample length. Based on tilt tests and fracture surface profilometry, the method of artificially induced tensile fracture in the sample was found to create repeatable fracture surfaces and morphologies. Seismic velocities of the fractured samples were determined under different levels of stress confinement and fracture shear displacement or mismatch. The effective confining stress was varied from 0.5 MPa to 55 MPa, while the fractures were mismatched by 0 mm, 0.45 mm and 1 mm. The degree of matching of the fracture surfaces in the core samples was evaluated using the joint matching coefficient (JMC). Shear wave splitting, as measured by the difference in the magnitudes of shear wave velocities parallel (VS1) and perpendicular (VS2) to the fracture, is found to be insensitive to the degree of mismatching of the fracture joint surfaces at 2 MPa, and decreased and approached zero as the effective stress was increased. Simple models for the stress- and JMC-dependent shear wave splitting and fractured rock permeability were developed based on the experimental observations. The effects of the joint wall compressive strength (JCS), JMC and stress on the stress dependency of joint aperture were discussed in terms of hydro-mechanical response. Finally, a useful relationship between fractured rock permeability and shear wave splitting was found after normalization by using JMC.

Injection‐Induced Shear Slip and Permeability Enhancement in Granite Fractures

AbstractShear stimulation, so‐called hydroshearing, is believed to reactivate critical or near‐critical preexisting fractures by pressurized fluid injection, causing them to slip and dilate for economic production from geothermal and unconventional petroleum reservoirs. However, little or no experiments have been conducted to directly probe the coupled hydromechanical responses and permeability evolution during fracture shearing. In this paper, we present the results of novel injection‐induced shear tests on cylindrical granite samples each containing a single tensile or saw cut fracture. The shear test on rough fractures demonstrates that retainable permeability enhancement can be achieved through dilatant shear slip. The characteristic stick‐slip behavior of the fracture during shearing is also observed. Fracture aperture controlled by effective normal stress dominates the fluid flow in stick state, while in slip state, the major contributor to production increase is the irreversible normal dilation caused by fracture shear slip. The experimental data show that depending on the fracture roughness, the fracture slip process includes two quasi‐static slip intervals with a slip velocity of ~10−9 to ~10−6 m/s and a dynamic slip interval with a slip velocity of ~10−7 to ~10−5 m/s. The fracture slip correlates well with the associated stress relaxation: a faster fracture slip induces a quicker stress relaxation, and vice versa. The influence of surface roughness on the hydromechanical responses during fracture shearing and the shear‐induced asperity degradation are also studied. The various experimental observations on rough and smooth fractures clearly indicate that the key component to shear stimulation is fracture self‐propping by asperities.

Effect of road salts on the hydro-mechanical behavior of unsaturated collapsible soils

Abstract Hydro-mechanical behavior of unsaturated collapsible soils can be drastically affected by saline infiltrations. Saline infiltrations into the underlain soil layers of the urban transportation and lifelines due to deicing or existence of traffic pollutants has been a common issue of concern for years in many countries. Additionally, many reports have been published so far, indicating the construction of roads and railways on unsaturated collapsible soils. In this research, hydro-mechanical characteristics of an unsaturated collapsible loessial soil diluted with saturated solutions of three different salts that are frequently involved in transportation infrastructure (namely NaCl, CaCl2 and KCl) are investigated by conducting scanning electron microscopy (SEM), filter paper, uniaxial compression and oedometer tests. Obtained test results have been analyzed by simultaneous consideration of the matric and osmotic suctions in the soil texture and the applied stresses. Test results revealed that the soil texture and hydro-mechanical responses of the soil are considerably affected by the salt type, saline degree of saturation and applied loading paths. Results also imply that there is a critical saline degree of saturation (corresponded to each loading path and each mixing salt type), at which the magnitude and modality of the osmotic and matric suctions within the soil fabric are changed. In addition to experimental studies, empirical constitutive models are presented in this paper to predict the changes in strength, stiffness and yield stress of the tested collapsible soil subjected to different road salts. Comparisons show very good agreement between the laboratory test results and the model predictions. The achievements presented in this research can be directly implemented for better prediction of the behavior of unsaturated collapsible soils subjected to salinity. In addition, the result can be implemented in a suitable stabilization plan when the soil and salt involved in transportation infrastructures.

Investigation of the hydro-mechanical behaviour of a pellet/powder MX80 bentonite mixture using an infiltration column

Non-compacted pellet/powder bentonite mixtures are considered as candidate sealing plug materials in geological disposals of radioactive waste. Due to its nature, this mixture is characterized by a heterogeneous porosity network, which is responsible for its complex hydro-mechanical (HM) behaviour. The French Institute of Radioprotection and Nuclear Safety (IRSN) has investigated this mixture within the SEALEX project. Both in situ large-scale and laboratory small-scale experiments were carried out. This paper presents the results of a small-scale mock-up test at 1/10th scale of the in situ experiments, in which the pellet/powder mixture was saturated from both sides (top and bottom of the specimen). Both swelling pressure and relative humidity were monitored at several positions of the specimen. Different responses from the sensors were found, depending on the local porosity as well as the evolution of the hydration front. The HM response of the mixture is strongly conditioned by the initial pellet/powder distribution, which depends on the protocol followed for the specimen preparation. After 800-day hydration, an anisotropy was found between the axial and the radial swelling pressures, due to the presence of larger void at the top of the sample and the friction at the cell wall. The sample was still heterogeneous after 800-day hydration mainly due to the initial heterogeneous porosity distribution, combined with the effect of friction and the non-saturation of the mixture. The evolution of injected water with time revealed that the sample was not full saturated after 800-day hydration.

Experimental study of the hydro-mechanical response of Opalinus Clay – Part 2: Influence of the stress path on the pore pressure response

Triaxial tests on back-saturated specimens using different controlled stress paths were conducted to study the influence of the stress path on the hydro-mechanical behavior of a clay shale. Opalinus Clay, a clay shale chosen for the disposal of high-level nuclear waste in Switzerland, was utilized. Three different stress paths approximating a tunnel excavation were applied: a two-dimensional stress path with isotropic initial stress conditions (i.e. pure shear compression), a two-dimensional stress path with anisotropic initial stress conditions, and a three-dimensional stress path. The influence of the stress path was investigated with respect to the specimens' geometry. Two end members were tested: specimens where the axial load was applied parallel to bedding (P-specimens) and specimens where the load was applied normal to bedding (S-specimens).Differences in the hydro-mechanical response were identified with respect to the specimens' geometry but also to the stress paths. When subjected to the two-dimensional stress paths, positive excess pore pressure was measured for S-specimens at low differential stresses whereas P-specimens showed negative excess pore pressure. The magnitudes of excess pore pressure were higher for the anisotropic two-dimensional stress path. Dilation associated with yielding was observed for all specimens. The application of the three-dimensional stress path revealed pore pressure responses that were conceptually comparable to the pore pressure response observed in situ. Pore pressure increase in front of the tunnel face can be related to poroelastic response of the rock mass to a change in differential and mean stress. Again, differences between P- and S-specimens were identified. Decrease in pore pressure before the tunnel face can be related to the dilation associated with yielding.No difference in peak strength between P- and S-specimens and the different stress paths was found. Nevertheless, it was shown that the state of failure is affected by the transversal isotropy as S-specimens fail at lower effective stresses compared to P-specimens.

Experimental study of the hydro-mechanical response of Opalinus Clay – Part 1: Pore pressure response and effective geomechanical properties under consideration of confinement and anisotropy

Consolidated drained and undrained tests with pore pressure measurements were conducted on back-saturated specimens of a clay shale to characterize the influence of confinement and anisotropy on the pore pressure response and effective geomechanical properties (i.e. first-loading E-modulus, stress at onset of dilation and peak strength). Opalinus Clay, a clay shale chosen as host rock for high level nuclear waste in Switzerland was utilized. The result showed that there is a dependency on the confinement of Skempton's pore pressure parameter A and B, the stress at the onset of dilation, and the first-loading E-modulus. Additionally, a change in behavior of the material was observed at effective consolidation stresses between 5 and 8 MPa. The specimens at lower effective consolidation stresses (i.e. heavily overconsolidated specimens) showed a dilatant behavior in the pre-peak region and a significant post-failure stress drop. Specimens consolidated at higher effective consolidation stresses (i.e. slightly overconsolidated to normally consolidated specimens) showed compaction from initial loading until post-peak and a brittle-ductile post-failure behavior. These observations were manifested in pore pressure response curves, effective stress paths and stress-strain curves. Furthermore, they could be correlated to a non-linear appearance of the peak strength failure envelope. An explanation for the non-linear failure envelope related to the dilatant structure of the material is suggested.