Advances in Laboratory Testing and Modelling of Soils and Shales (ATMSS)

The paper studies the hydromechanical behaviour of an argillaceous rock from the Colombian Andes. The influence of total suction on some physical/mechanical properties is analyzed on an argillaceous rock from the Colombian Andes. A wetting path was applied to the rocks using the vapor transfer technique. Uniaxial compression tests were conducted on specimens at different levels of suction. Also, microstructural observations are carried out using a Mercury Intrusion Porosimeter Apparatus. Results from the laboratory tests indicate that anisotropic deformations took place during the wetting path. Also, total suction contributed to a considerable and non-linear reduction in the peak compressive stress, elastic modulus and stress thresholds of the tested samples. Microstructural analysis indicated the influence of suction on the dominant pores.

Plastic deformation of unsaturated soils under cyclic thermo-mechanical loads is important to the serviceability of many earth structures, such as the high-speed railway embankment and energy pile. In this keynote, experimental and theoretical studies of cyclic thermo-mechanical behaviour of unsaturated fine-grained soils are presented. Particular attentions are given to cyclic shear behaviour of unsaturated soil at various suctions and temperatures, as well as soil volume changes under heating and cooling at different suctions.

For typical shales (void ratio 0.15 to 0.42), modal pore throat sizes range from a few nm to a few tens of nm. Unstressed shales, even when fully saturated, have negative pore water pressure (capillary tension). However, the total suction is often greater than this, especially for highly-compacted shales with extremely small pore size. The additional suction is due to effects associated with clay surfaces. Osmotic pressures can be directly measured, and they can easily be several MPa, a combination of solute suction and clay-related effects. The small pore sizes in shales also result in extremely low values of permeability and of consolidation coefficient. All these characteristics directly impact testing protocols. The first step in any test should be to apply sufficient confining stress to raise the pore pressure up to a positive, measured value. Undrained consolidation, combined with undrained triaxial compression and with small sample sizes (and drainage screens when necessary), results in acceptable test durations. A range of effective consolidation stress values is attained by first equilibrating shale samples in varying amounts of suction, to vary the water content. Non-aqueous fluids are required when sampling, to avoid swelling, and are often necessary for pore lines if osmotic pressures are to be avoided.

The paper presents a constitutive model for argillaceous rocks, developed within the framework of elastoplasticity, that includes a number of features that are relevant for a satisfactory description of their hydromechanical behaviour: anisotropy of strength and stiffness, nonlinear behaviour and occurrence of plastic strains prior to peak strength, significant softening after peak, time-dependent creep deformations and permeability increase due to damage. Both saturated and unsaturated conditions are envisaged. The constitutive model is then applied to the simulation of triaxial and creep tests on Callovo-Oxfordian (COx) claystone and to the analysis of the excavation of a drift in the Meuse/Haute-Marne Underground Research Laboratory. The pattern of observed pore water pressures and convergences during excavation are generally satisfactorily reproduced. The effect of incorporating creep is also demonstrated.

The osmotic, hydraulic and self-healing efficiency of bentonite based barriers (e.g. geosynthetic clay liners) for containment of polluting solutes are governed by both the chemico-physical intrinsic parameters of the bentonite, i.e. the solid density (ρ _sk ), the total specific surface (S), the fixed negative electric surface charge (σ), the Stern fraction (f _Stern ), and by the chemico-mechanical state parameters able to quantify the solid skeleton density and fabric, i.e. the total (e) and nano (e _n ) void ratio, the average number of platelets per tactoid (N _l,AV ), and the effective electric fixed-charge concentration ($$ \bar{c}_{sk,0} $$c¯sk,0). In turn, looking at saturated active clays only, the state parameters seem to be controlled by the effective stress history (SH), ionic valence (ν _i ) and related exposure sequence of salt concentrations in the pore solution (c _s ). A theoretical framework, able to describe chemical, hydraulic and mechanical behaviors of bentonites in the case of one-dimensional strain and flow fields, has been set up. In particular, the relationships, linking the aforementioned state and intrinsic parameters of a given bentonite with its hydraulic conductivity (k), effective diffusion coefficient ($$ D_{s}^{*} $$Ds∗), osmotic coefficient (ω) and swelling pressure (u _sw ) under different stress-histories and solute concentration sequences, are presented. The validity of the proposed theoretical hydro-chemico-mechanical framework has been tested by comparison of its predictions with some of the available experimental results on bentonites (i.e. hydraulic conductivity tests, swelling pressure tests and osmotic efficiency tests).

The paper outlines the multiscale mathematical formulation of clay-rich shales as a swelling capillary porous medium with a resolution as fine as the nanoscale. The starting point is the description of the physicochemical interactions between elementary crystalline units–the so-called clay sheets or platelets. By way of homogenization, the clay platelet physics is upscaled to represent a system of randomly dispersed shale particles at the microscale with the void spaces partially saturated with a liquid, i.e. water. The end result is the constitutive description of clay shales enriched with microstructural details down to the clay platelet level that can readily describe swelling or shrinkage in terms of physicochemical loading.

In CO₂ capture and storage (CCS) projects, measuring permeability of supercritical CO₂ in porous rock is a key important factor in predicting the migration and evaluating the long-term stability of injected CO₂. In this paper, a new device for measuring CO₂ permeability coefficient in porous rock at reservoir conditions is described, in which the pressure and temperature of injected CO₂ are usually at supercritical state. In the measurement, the supercritical CO₂ flowing through rock samples is depressurized to gaseous phase whose flow rate could be easily measured with normal gas meters. An average flow rate over a period of time is used to calculate the permeability of CO₂ in rock sample under supercritical state. An application example using a siltstone for testing proves that the device can measure the permeability of supercritical CO₂ in porous rock effectively.

Here, we focus on the hydromechanical behavior of self-standing clay films with a thickness of a few dozen microns. We measure their elastic and creep properties and how those properties depend on the interlayer cation and on the relative humidity (or water content). Those experimental results are compared with the elastic and creep behavior of nanometric clay particles, which we characterize by molecular simulations. Significant qualitative differences between the behavior of the clay films and that of the clay particles are observed, which suggests that the hydromechanical behavior of the clay films is significantly impacted by their mesostructure (i.e., by how the clay particles or tactoids are arranged in space). Upscaling the hydromechanical behavior of the clay films from that of the clay particles may be challenging.

The effects of coupled thermo-mechanical processes under consideration of micro-fracturing of the solid geomaterial on mechanical and thermal properties of geomaterials are investigated and subsequently simulated using advance Lattice Element Method (LEM). As a result of that extension, the alteration of effective parameter due to structural changes become numerically understandable. Hence, the simulation of the coupled processes on the meso-scale helps to develop and validate reliable identification method for real cases. The obtained results make it obvious that LEM has a large potential for fracture problems in geomaterials.

The paper addresses the issue of how physical-chemical aspects affect the mechanical behaviour of clays and how they could be integrated in the mechanical modelling. A significant influence of the high-plastic-clay fraction on the behaviour of clayey materials was experimentally highlighted, such as the compression index variation, or the strong and very marked decrease of friction angle. Microstructural investigation provides further understanding allowing to relate the observed phenomena to an activated mechanisms acting at the level of clay particles. A new approach using the Chang and Hicher micromechanical model was proposed, where physical-chemical effects, acting between clays clusters, are incorporated. Local mechanisms are introduced through repulsive and attractive forces obtained from the derivation of energy potentials.

Highly active clays, such as sodium bentonites, are known to behave as solute restricting, semipermeable membranes under specific physical and chemical conditions. This behavior has important implications for practical applications, including a reduction in solute mass flux due to diffusion. After description of a method for simultaneously measuring membrane behavior and solute diffusion of active clays, the results of experimental studies using the method to test bentonite-based barriers are presented. The effects of effective stress and/or void ratio (dry density) of the specimen, and deviations from anticipated coupled membrane and diffusion behavior resulting from extending the evaluation to the limit where membrane behavior becomes nil are illustrated. The presentation should be of interest to those evaluating the use of bentonite-based barriers for waste containment applications, such as dense bentonite-buffers for high level radioactive waste disposal.

Peats are soils containing a significant component of organic matter. Biochemical degradation of this fraction generates gases such as CO₂, H₂S and CH₄, which tend to saturate the pore water eventually resulting in exsolution and expansion. The effects of these gases on the hydro-mechanical behaviour of peats are under investigation at Delft University of Technology. The results of a series of triaxial tests are discussed, in which gas was exsolved under controlled conditions by flushing natural samples with carbonated water, and undrained isotropic unloading and shear were performed. A significant reduction in the effective stress acting on the soil skeleton was observed during undrained unloading due to gas exsolution. However, different stages were observed in time, which appear to be ruled by the very high compressibility of peat. The mechanical response upon shearing is dominated as well by the ratio between the compressibility of the fluid and the soil skeleton. Although the ultimate strength does not differ much between the samples tested, the mobilised shear strength for a given axial strain does, which has to be accounted for cautiously in the choice for an operative shear strength.

The FEBEX large-scale test simulating an underground repository of nuclear waste excavated in granite was dismantled after 18 years of operation. The engineered barrier between the heaters and the host rock consisted of compacted blocks of FEBEX bentonite which were retrieved during dismantling. The water content, dry density and suction of some of these blocks were measured at the laboratory and retention curves (WRC) were inferred from these values. The WRC of samples trimmed from the blocks was also determined using the vapour transfer technique. The results obtained with both methodologies were compared with the WRC obtained previously for the reference untreated FEBEX bentonite. The results are comparable, which indicates, on the one hand, that both methodologies give consistent results and, on the other, that the water retention capacity was not altered by 18 years operation under barrier conditions.

Some fault rupture propagation tests were conducted in which wet soil were used to model a cohesion soil in real condition. Results of these tests show that by increase of water content, the shear band of fault rupture propagate in a narrower zone so the distorted area above the ground was decreased. Ductile soil has the ability to deform more before a developed shear band reaches to the surface. This behavior can be interpreted by strength parameters of the wet soil. For this purpose, some series of direct shear tests were conducted, studying the effect of water content on some strength parameter of granular soil. All of these tests were carried out in both low and high vertical stress. The results indicated that increase of water content to a certain value lead to increase in internal friction angle and beyond this limit it decreases. The response of cohesion was vice versa. Also the internal friction angle was increased as the vertical stress decreased. The results were discussed in both high and low vertical stress.

In this study, specimen preparation techniques for testing fully and partially saturated specimens were investigated. Various techniques were evaluated for obtaining loose to dense fully saturated sand specimens in a flexible membrane of DSS-C device, with minimal distortion to the specimen. Moist undercompaction technique in stages and then saturation with back pressure seem to be the best technique for obtaining loose to dense fully saturated uniform sand specimens. Gas/air entrapped partially saturated sand specimens were prepared by using a chemical powder: sodium percarbonate. The powder was mixed with water and predetermined amount of dry sand was rained into this solution. The chemical powder gets into reaction with water and generates oxygen gases which get entrapped in the sand voids. The techniques developed for obtaining uniform, repetitive sand specimens practically were discussed herein. The specimens prepared were tested under undrained conditions in Dynamic Simple Shear with Confining Pressure (DSS-C) testing device.

Compacted bentonite which is one component of structure framework of barrier system. The hydro-thermal-mechanical properties of compacted bentonite have been investigated in the past decades in the area such as geoenvironmental engineering (Olivella et al. Olivella et al. 1996). This study conducted creep test for bentonite using a modified relative humidity circulation system. Various vertical stresses were applied on basis of unconfined compressive strength that only relative humidity of 98% as hydration effort influenced to creep behaviour in deformation. Thus, either expansion or shrinkage in vertical direction was measured, and all specimens approached to be completely destruction due to apply the combined effort of mechanical effect and hydration effect.

Various kinds of laboratory experiments were carried out on the Callovo-Oxfordian and Opalinus claystones to investigate response of clay rock to moisture change, including swelling and shrinking with variations of the environmental humidity, stress-bearing capability of bound porewater or buildup of swelling pressure, influence of water content on the mechanical stiffness and strength, and moisture-enhanced sealing of fractures. Significant responses of the claystones were observed.

The paper presents some experimental results collected on samples recovered from an experimental embankment obtained by compacting a lime-treated clay. Samples were collected soon after the in situ compaction and they were cured in controlled environmental conditions for at least 18 months. Mercury intrusion porosimetry tests (MIP) were carried out on freeze-dried specimens to characterize the microstructure of the material. In order to assess the durability of the improved material, laboratory tests focused on the effects of cyclic variations of the degree of saturation on the water retention properties and the volumetric behaviour of the stabilized clay. Collected results show that the lime-treated clay undergoes an almost irreversible volumetric behaviour; this irreversible contraction is associated to severe drying processes .

The aim of this research is to analyze, by experimental approach, the impact of suction in the tensile strength and cracking phenomenon of unsaturated clayey soils samples. Through a bending tests, and a clay submitted to different level of suction (361 MPa, 110 MPa and 38 MPa), the approach consisted first in estimating the tensile strength which controls the initiation of tensile cracks. Then, using digital image correlation method, the propagation of cracks was precisely followed through the local strains development around the crack and close the crack tip.

Experimental determination of water permeability in unsaturated conditions is a critical issue. Among the existing experimental techniques, the instantaneous profile method is frequently used. When applied to bentonite-based materials, the method often shows that the water permeability–suction function significantly differs depending on the distance from the wetting face. Such behaviour has been interpreted as a consequence of structural changes in the sample which directly affect the water flow properties. In order to better understand the involved processes, a hydromechanical simulation of an infiltration test is performed. While structural changes are shown to affect the hydraulic properties, the computed water permeability–suction evolution is strongly affected by the interpretation of the raw experimental data.

To study thermo-hydro-mechanical behaviour of unsaturated soil, some apparatuses are developed and reported in the literature. Most of the existing apparatuses, however, cannot apply cooling and control temperature lower than room temperature. Moreover, an accurate measurement of thermal volume changes is still challenging, particularly for unsaturated soil. In this study, a triaxial apparatus with double cell total volume change measuring system is modified to fulfil temperature control in a wide temperature range (both higher and lower than room temperature). Temperature is regulated by circulating water with a controlled temperature in a spiral copper tube installed between the inner and outer cells. Detailed calibrations are carried out to determine the response of heating/cooling system and double cell to heating and cooling, such as the thermal equilibrium time and the volume change of inner cell. By using the new apparatus, a series of test is carried out to investigate the volume changes of normally consolidated intact and recompacted loess at different suctions over a wide thermal cycle ranging from 5 °C to 53 °C. It is found that contractive volumetric strain increases as temperature increases. During the cooling process, soil volume keeps contracting until the temperature decreases to 5 °C. An irreversible contraction at a much higher rate is observed from 13 °C to 5 °C. The observed plastic strain during cooling cannot be captured by existing thermo-mechanical models.

To characterize the influence of temperature and relative humidity on the mechanical behavior of geomaterials, an experimental device was designed based on a conventional oedometer testing device. The aim of this work is to provide fundamental information about Thermo-Hydro-Mechanical coupling of unsaturated porous geomaterials such as sand or clay. Several methods were tested and compared to impose relative humidity and temperature. Two systems of control of relative humidity were developed: one using salt solutions to impose constant relative humidity with accuracy in the range of ±4% and the other one using the variation of the saturated vapor pressure of water with temperature to impose relative humidities and potentially make them vary over time. A special attention was paid to thermal insulation of the entire system to reduce temperature variations. A Proportional–Integral–Derivative controller (PID controller) permits to control temperature of samples between 20 and 60 °C for a week with accuracy in the range of ±0.5 °C. This system makes it possible to test 3 samples in parallel, at the same temperature but at potentially different relative humidities. For each sample the vertical displacement, the temperature close to the sample, the relative humidity of the air injected into the sample and the lateral pressure (since zero lateral strain boundary conditions are imposed) are measured. Finally the experimental device was tested on Hostun sand at two temperatures and three relative humidities.

Current experimental techniques used to understand the shear banding process in sands provide little insight into the internal micro-structure evolution. To this end, Acoustic Emission (AE), as a non-destructive testing technique, was proposed in this paper with great interest in characterizing the internal micro-structure response leading to the evolution of shear bands formed in laboratory triaxial compression. Silica sand was used to conduct consolidated-drained triaxial compression tests at a constant axial strain rate under an effective confining pressure of 100 kPa. AE events were collected and analyzed. Insights regarding relations of the deviatoric stress, source rates and dissipated energy rates of AE events with the increasing global axial strain are offered. The result indicated that with the increase of relative densities, the evolution envelope of AE source rates transits from a steep shape to a flat shape, and total amount of AE source events decreases gradually. According to the evolution of AE energy rate, shear banding process can be divided into four stages in terms of O-A, A-B, B-C and C-D, corresponding to the strain hardening regime, incipient strain softening regime, highest rate of strain softening regime and residual stress regime. From which point A could be considered as an omen of the initiation of strain localization, point B as the initiation of visible shear band and point C as the completion of shear banding. AE technologies can be provided as an alternative means to clarify and indicate the initiation and evolution of shear banding in sand.

A series of undrained cyclic triaxial liquefaction tests were conducted to observe directly and indirectly the local deformations of sand specimen through a transparent membrane. Black-colored silica sand, mixed in white-colored silica sand, was used for the direct evaluation. Black-colored dots, used for observing the local deformations indirectly, were pasted on membrane with a constant interval of 5 mm. Digital photographs were taken in front of the triaxial cell to evaluate the deformations of sand particles patterns and dots by Particle image Velocimetry method. The results indicated that image analysis is an efficient method for direct and indirect measurements of local deformation. Comparison of direct and indirect evaluations revealed that relative movement between sand particles pattern and an adjacent dot on the membrane had a leap when excess pore water pressure ratio reached unity.

Naturally-occurring hydrates are promising energy resource with abundant reserves that easily surpass the other resources all combined. Much work has been done in predicting the properties of gas hydrates. This is especially true for laboratory formed pure gas hydrates. But many physical properties of hydrates in reservoir are not completely understood yet. In effort to research the hydrate behavior in the subsurface conditions, a new laboratory technique is developed to effectively measure the formation and dissociation of the hydrates in pore space. This newly-developed method has other advantages, when compared with the conventional setup. Details of the experimental design and measurement procedure will be discussed. This chapter will also present the findings of the conducted experimental studies and test results on methane hydrate to illustrate the new technique’s usefulness. It was found that the solution in the pore space retains a memory-effect when metane hydrate is melt at moderate temperatures. This can be eliminated only when the system is heated to sufficiently high temperature.

Soil Water Characteristic Curve (SWCC) is the core of unsaturated soil mechanics. The accuracy of modeling unsaturated soil seepage and consolidation behavior are highly governed by SWCC. The conventional testing methods only measured two state variables under the equilibrium condition. Those investigations were usually conducted on a Representative Elementary Volume (REV) scale less than the full length of suction/moisture profile of sandy soil. To further investigate both point-wise and the global SWCC measurement under transient flow condition, a dynamic SWCC testing platform is set up in The University of Queensland. This experimental platform integrates Spatial Time Domain Reflectometry (Spatial TDR) technique, tensiometers and outflow logging by electrical bench scales, so that the consistent logging of moisture profile, suction profile and in/outflow can be achieved. In this study, the experimental platform is briefly presented with some preliminary outcomes and the potential problems of setup are discussed.

Predicting pore water pressures due to wave induced pressure fluctuations on quasi saturated porous beds is important e.g. for the design of bank protections in channels. Therefore, the influence of fluid compressibility and the coefficient of permeability both depending on the degree of saturation were analyzed by means of laboratory testing with the alternating flow apparatus of the Federal Waterways Engineering and Research Institute. A methodology for the determination of the saturation dependent parameters based on high precision pressure and discharge measurements is presented. As shown in the present paper, test results and finite element simulations based on the determined parameters coincide very well.

The simple shear test has been employed regularly during the last few decades in soil characterisation studies for earthquakes, liquefaction, offshore construction and other applications. A variety of simple shear devices are available and the main difference between them is the method of applying the horizontal confining stress. According to the ASTM (D6528-07) standard for direct simple shear testing of cohesive soils, circular specimens are generally confined by a wire reinforced membrane or stacked rings. In this paper a comparison of results from monotonic undrained simple shear tests with wire reinforced and flexible membranes is reported. A simple shear apparatus is utilized in which lateral confinement of the specimens can be achieved, either by flexible membrane and confining pressure applied to the specimen through compressed air to maintain K₀ condition during compression, or by conventional wire reinforced membrane without confining pressure. It is shown that the membrane confinement method influences the measured undrained strength. For the normally consolidated specimens reported herein the shear strength using a flexible membrane is higher than for the wire reinforced membrane when the confining stress is adjusted to keep the vertical stress constant and is similar when the confining stress is kept constant throughout the shearing stage.

Shales and stiff clays when deformed under direct shear stress conditions develop narrow shear zones within which different sets of discontinuities or cracks are present. These cracks form in the shear zone in a progressive manner. In the first stage of deformation, a small set of unconnected cracks called Riedel shears form. In the second stage of deformation, a series of cracks called Thrust shears form. They connect with the Riedel shears. In the third stage of deformation, The Riedel and Thrust discontinuities interact, forming an undulating and rough failure surface. In this study, the fractal dimension concept from fractal theory is used to evaluate the progressive degree of cracking in the shear zone that causes the failure of shales and stiff clays forming part of natural slopes and earth dams. It was established that the intensity of cracking in the samples was reflected in the fractal dimension values. High levels of cracking were associated with high values of the fractal dimension.

There has been a renewed emphasis in the U.S. on the use of fully softened shear strength for slope stability analysis of cuts in stiff clays and the stability of compacted clay embankments. A detailed investigation on the proper use and measurement of the fully softened shear strength has been recently undertaken, examining different test apparatuses and procedures. A summary of the recent development and guidelines on the use of this concept in slope stability analysis and the proper way to measure the fully softened shear strength in the laboratory is presented.

Hydromechanical properties of shales are complex due to the involved material structure, with the solid matrix being mainly formed by swelling clays and porosity dominated by nanometer scale tortuous voids with large aspect ratios. Intrinsic permeability of restructured Opalinus Clay (Swiss shale) brought to shallow geological storage conditions was measured with in situ brine. Under constant temperature, vertical stress, and downstream fluid pressure, steady-state flow experiments show a significant trend of permeability decrease with increasing differential (upstream minus downstream) fluid pressure, thus contradicting the conventional Darcy’s description. To interpret these experimental measurements, brine permeability is derived using a one-step self-consistent homogenization scheme based on the knowledge of material’s pore structure. While mechanical and thermal effects cannot explain the permeability decrease, the trend is reproduced with the correct order of magnitude by considering a chemical effect: a pore size reduction in the sample due to water adsorption at mineral surface.

Construction of tunnels, slopes, deep basements, foundations and retaining walls are common in shale. Quality of shale can be assessed based on the Q index system. This paper presents the analysis of charts developed for Q index of shale from three different parameters such as block size (RQD/J_n), inter-block shear strength (J_r/J_a) and active stress (J_w/SRF). For the range of values chosen for RQD/J_n, J_r/J_a and J_w/SRF a Q index was estimated and charts were established between Q index and RQD/J_n for various values of J_r/J_a and J_w/SRF. The charts presented in the paper for Q index can be used to classify the shale. Shale should have minimum J_r/J_a, J_w/SRF and RQD/J_n as 0.5, 0.2 and 10 respectively to be classified as good to very good category.

Gas transport properties in argillaceous rocks are becoming an important issue within different contexts of energy-related geomechanics (disposal of radioactive waste, production of shale gas, CO₂ sequestration). The present investigation aims at describing the pathways generated on a deep Cenozoic clay during gas injection using different microstructural techniques. Mercury intrusion porosimetry results have allowed detecting fissures after gas injection tests that have not been observed on intact samples. The opening of these pressure-dependent fissures plays a major role on gas permeability. A complementary insight into the connectivity of these fissures has been quantified by micro-computed tomography.

Grouted-in vibrating wire pressure transducers (VWPs) can be used to measure the in situ constrained (1-D) compressibility (m_v) of deep claystone aquitards through measurement of barometric loading efficiency. Measurements of in situ m_v for a Cretaceous shale in southern Saskatchewan, Canada, were undertaken using data collected from 27 VWPs installed in multiple boreholes at two sites over depths of 10 to 325 m below ground. The measured m_v profiles at both sites produced similar trends of decreasing m_v with increasing depth. 1-D consolidation testing was used to measure pre-consolidation pressure (Pc’), compression index (C_c), and the swelling index (C_r) on nine core samples collected from Site 1. These tests yielded C_c values ranging from 0.1–0.5 ($$ \bar{x} $$x¯ = 0.29 ± 0.12), and C_r from 0.03–0.07 ($$ \bar{x} $$x¯ = 0.05 ± 0.02). Laboratory measurements of C_c and C_r were used to estimate variations in in situ m_v with depth. A theoretical relationship between in situ void ratio (e) and effective stress (σ′) was determined using the laboratory determined Pc’ values, compression indices (C_c, C_r), and measurements of in situ e. Varying the values of Pc’, C_c, or e exerted minor influences on these profiles relative to C_r. The resulting theoretical patterns of in situ m_v with depth (or σ′), exhibited a similar pattern to the laboratory and field observations, however to replicate the in situ profiles the C_r values had to be an order of magnitude lower than the laboratory values. The good agreement between the theoretical and measured m_v profiles with depth highlight the potential to combine in situ measurements of m_v with laboratory consolidation test results to characterize the mechanical properties of deep claystone aquitards and potentially improve upon our understanding of how the stress history of these formation has resulted in their present day geomechanical properties.

To obtain the influence of surface roughness of the fracture on hydraulic characteristics of rock mass. Firstly, the roughness of three kinds of fracture surfaces were determined using TR600 roughness profiler. The vertical stress and horizontal stress and water head pressure of samples are simulated by normal load, shear load, and seepage water pressure respectively. The coupled shear-seepage tests were carried out using a JAW-600 shear–seepage coupled test system. The changes in hydraulic opening and permeability with shear displacement were obtained under different initial normal stress and normal stiffness conditions. This study shows that the hydraulic opening and permeability of fracture rock mass can be divided into three stages of reduction, accelerated rise and stability with the increase of the shear displacement. Larger initial normal stress correspond to greater the hydraulic opening and transmission rate of the rock mass. Greater roughness of the fracture surfaces correspond to greater hydraulic opening and transmission rate, and worse stability. The water infiltration is proportional to three times the degree of hydraulic opening. A small change in the degree of hydraulic opening can cause great changes in the transmission rate. Under the same stress condition, the degree of the fracture surface roughness will produce important influence on the volume of gushing water.

An experimental investigation to analyse the anisotropic volumetric response of shaly and sandy facies of Opalinus Clay upon suction variations is presented. Obtained results demonstrate the different behaviour of the tested facies to a wetting-drying cycle. The shaly facies exhibits higher water retention capacity and stronger volumetric response than the sandy facies. Anisotropic response is experienced by both facies with the strain perpendicular to bedding higher than in the parallel direction. The sandy facies exhibits a more pronounced anisotropic behaviour in particular during the drying phase. A detailed analysis of the response in the two directions with respect to the bedding orientation proves that the different anisotropic behaviour between the two facies is mainly caused by a different response parallel to bedding rather than perpendicular.

One of the main concerns related to tunnel excavations, drilling operations and wellbore stability in shales is the generation of excess pore water pressure due to changes in mechanical stress; therefore the consolidation of shales is a fundamental process that must be considered. This paper presents a comprehensive methodology for analysing the compression and consolidation behaviour of shales. An apparatus to perform high-pressure oedometric tests is presented and an analytical method is introduced to analyse the shale consolidation behaviour, which allows information to be gathered on the coefficient of consolidation, stiffness, poroelastic properties, and permeability of the tested material as a function of the applied stress conditions. Results obtained on Opalinus Clay shale using the developed methodology are presented and discussed.

Specific equipment and procedures developed for geomechanical testing of hydrocarbon caprocks were adopted to conduct truly undrained triaxial tests with Opalinus Clay. The amount of pore pressure development during consolidation, and the resulting effective stress, is managed by equilibrating the samples in vacuum desiccators of different relative humidities (vapor equilibration technique) prior to assembling into the test apparatus. We present test results of five Opalinus Clay samples covering a laboratory mean effective consolidation stress range from 5 MPa to 50 MPa. A drained consolidation test was first conducted to determine the appropriate strain rate for consolidated-undrained (CU) triaxial testing. The Skempton ‘B’ parameter was quantified prior to the deformation tests and found to be stress dependent. A distinct stress dependency of elastic moduli is also observed, but normalized with the undrained shear strength there is only a relatively small variation. Within the explored stress range the different stress paths to peak indicate a transition from over consolidated to rather normally consolidated state. However, failure is in all cases dilatant, i.e. associated with a drop in pore pressure and strain-softening (more so at low effective stress). Caprock shales of similar porosity to the Opalinus share many similarities in overall behavior, but also exhibit some slight differences.

First experimental results on Opalinus Clay from shallow depth (<30 m depth) are presented and compared to results on cores from Mont Terri Underground Rock Laboratory (~300 m depth). Samples were tested in one dimensional condition using an advanced experimental technique. The samples from the two sites show similar properties in terms of geotechnical characterization and one dimensional compressibility/swelling indexes, despite the different source depths.

Using a combined approach of ion-beam milling and electron microscopy, we observe, describe and quantify the microstructure of naturally and synthetically deformed Opalinus Clay (OPA) and deduce its microstructural evolution and underlying deformation mechanisms. The investigated samples derive from the so-called Main Fault, a 10 m offset fold-bend thrust fault crossing the Mont Terri Rock Laboratory in the Swiss Jura Mountains. The samples are slightly overconsolidated, experienced a burial depth of 1350 m and a maximum temperature of 55 °C. Most impact on strain is attributed to frictional sliding and rigid body rotation. However, trans-granular fracturing, dissolution-precipitation of calcite, clay particle neoformation and grain deformation by intracrystalline plasticity have a significant contribution to the fabric evolution. The long-term in-situ deformation behavior of OPA is inferred to be more viscous than measured at laboratory conditions.

Monitoring the behavior of the rock mass combined with a sound knowledge of rock mechanical properties is of great importance. Already in the very beginning of the Mont Terri rock laboratory in 1996, rock samples were extracted for the assessment of elastic parameters. The transverse isotropic Opalinus Clay exhibits a strong dependence on its orientation with respect to the applied load. Furthermore, rock strength is dependent on water content/suction and microcracking due to desiccation and/or mechanical load. Swelling potential, very low hydraulic conductivity and non-linear temporal behavior are further important properties of Opalinus Clay, which require well-defined sampling procedures, sample conditioning and test setup in the lab. Monitoring deformation and pore water pressure of the temporal behavior of galleries and niches prior, during and after excavation is standard procedure in the Mont Terri rock laboratory. Besides classical survey of deformation as well seismic, and geoelectric methods are applied in order to assess sedimentary heterogeneity, tectonic discontinuities and the extent of the excavation damaged zone (EDZ). Swisstopo has furthermore developed a technique based on resin impregnation, which allows for characterizing geometry, properties and temporal behavior of the EDZ. These datasets allow for establishing and continuously improving conceptual models of the rock mechanical situation and behavior in the Opalinus Clay. Conceptual models and long-term datasets are important for the calibration and validation of HM coupled models. In the framework of the Mont Terri Project, several numerical models have been developed and tested for the specific use in consolidated shales, which are important instruments for future predictions.

A short series of cyclic triaxial tests has been carried out on chalk samples from the English Channel. The specimens are installed with a minimum of handling. The state of specimens after the cyclic testing ranged between no apparent changes to the structure (no degradation) and total loss of structure (degradation); whereas pronounced failure planes were observed in some of the specimens. The current article summarizes details of geological description and handling of the tested specimens in respect to the failure mode of the chalk.

Some deep recognition boreholes were executed in relation to the construction of the Gotthard Base Tunnel, in the locality of Bodio. They reached about 200 m under the surface. From about 100 m, laminated lacustrine sediments were encountered, in the form of fine sands, argilleous silts and sandy clays. Two probes were submitted to an oedometric creep test lasting about eight months, under constant load. The purpose was to assess the parameters needed to for see tertiary settlement. The paper presents the results and discuss the method.

This article presented the effect of mixing patterns for the bio-cement in two different soils (Kaolin clay and Bangkok sand). Bio-cementation process initiated the crystal forms of calcium carbonate (CaCO₃) to bind the soil particles. Soil samples were mixed with the solution containing 250 mM of CO(NH₂)₂, 250 mM of Ca2+ (by CaCl₂) and 20% (v/v) of urease. In simulated deep mixing of Kaolin clay, the research found that blades used in solution mixing affected the strength development of Kaolin clay after bio-cementation. The V _s of 210 m s−1 was found in comb blade mixer. In simulated deep mixing of Bangkok sand, the research found that solution injecting method affected the V _s of Bangkok sand. The period drop and the period injecting methods provided better V _s results than the one time pouring and the continuous injecting methods.

In this study, we focus on organic matter structure and its interaction with the pore space of shales during hydrocarbon (HC) generation. Rock samples collected from Domanic horizon of South-Tatar arch were heated in the pyrolyzer to temperatures closely corresponding to different catagenesis stages. X-ray microtomography method was used to monitor changes in the morphology of the pore space and organic matter structure within studied shale rocks. By routine measurements we made sure that all samples had similar composition of organic and mineral phases. All samples in the collection were grouped according to initial structure and amount of organics. They were processed separately to: (1) study the influence of organic matter content on the changing morphology of the rock under thermal effects; (2) study the effect of initial structure on the primary migration processes for samples with similar organic matter content. After heating the morphology of altered rocks was characterized by formation of new pores and channels connecting primary voids. However, it was noted that the samples with a relatively low content of the organic matter had less changes in pore space morphology, in contrast to rocks with a high organic content. Second part of the study also revealed significant differences in resulting pore structures depending on initial structure of the unaltered rocks and connectivity of original organics.

Two laboratory experiments are used to provide preliminary evidence towards upscaling the microbially induced calcite precipitation (MICP) for soils. Fine sand is packed and subsequently subjected to MICP treatment in 11-liter cylindrical tanks. Different conditions are tested regarding the bacteria injection source as well as their propagation in the porous medium. The goal is to associate the adopted application strategy with the final shape, size and total mass of precipitated calcite in the yielded bio-cemented soil volume. Observations carried out at the micro-scale reveal the presence of calcified biofilms as a distinct “habit” of CaCO₃ precipitate. Finally, a series of drained triaxial shear tests is carried out on fine and medium MICP-treated sands. Results reveal a more pronounced enhancement in terms of strength for the medium sand, despite the same CaCO₃ content.

Intergranular strain parameters of the extended Hypoplastic model are determined from laboratory experiments. Simple static triaxial setup coupled with controlled stress path test method is employed to determine the parameters. Parameters are determined for 4 different naturally existing sands acquired from field. The dependence of the intergranular strain parameters, on density and stress state of the sand is studied and recommendations are made for the selection of mean values in the relevant range of stresses and densities. The variation in the magnitude of intergranular strain parameters is studied in accordance with the varying grain assembly properties. The strain range within which the incremental stiffness remains constant after strain reversal is studied in conjunction with the grain properties and the validity of the assumption that the governing parameter is a material independent constant is commented upon.

The use of energy piles remains a topic of discussion in Belgian practice. The main concern is the lack of knowledge and documented experience with regard to the energy performance. Another issue is the potential impact of temperature changes and temperature cycles on the pile response (bearing capacity, settlements), in particular for piles with a relative small diameter. To address this challenge, an extensive full-scale test campaign on several types of energy piles has been set up. The energy piles are instrumented over their entire length with Fibre Bragg Grating (FBG) optical sensors and thermocouples. The tests aim at thermally characterizing the piles, as well as determining the combined thermo-mechanical behaviour. This paper presents an overview of the first, preliminary results of the test campaign and assesses the potential of a fully coupled analysis with the Finite Element Method (FEM) in Plaxis 2D software, showing a good agreement with the measurements.

The analysis of foundations in fine grained, mostly soft soils leads to the question if either an undrained or drained analysis is appropriate to describe the ultimate limit state of a certain structure. Triaxial tests show that the undrained shear strength s_u is no proper soil parameter but strongly dependent on the initial conditions (OCR-value) and on the developing stress path. The analytical modelling of raft foundations in drained and undrained conditions shows the difficulties of using s_u based on triaxial tests. More triaxial tests and the study of different geotechnical structures are needed to give advice whether the short term behaviour (undrained) or the long term behaviour (drained) is supposed to be most critical.

This paper expands on the impact of the thermally induced deformation of the soil on the serviceability mechanical performance (i.e., deformation-related) of energy pile groups. The work is based on the results of a full-scale in-situ test that was performed on a group of energy piles at the Swiss Tech Convention Centre, Lausanne, Switzerland, and on a series of 3-D thermo-mechanical finite element analyses that were carried out to predict the considered experiment. This study proves that the serviceability mechanical performance of energy pile groups crucially depends on the relative thermally induced deformation of the soil to that of the energy piles. The relative thermally induced deformation of the soil to that of the energy piles is governed by (i) the thermal field characterising the energy pile group and (ii) the relative thermal expansion coefficient of soil to pile. Considering these aspects in the analysis and design of energy pile groups is key because they profoundly characterise the deformation of such foundations.

The failure of pile foundations in recent strong earthquakes showed that the current analysis and design method of pile foundation need improvement. In strong earthquakes, the mechanical behavior of the pile foundations, the surrounding soil and the structure are completely nonlinear. Considerations of their nonlinearities are important in improving the analysis method. In recent years, the nonlinearity of the soil and the piles have become inevitable in the analysis of pile foundations. However, the nonlinearity of the structure is simplified. In this paper, a section of an elevated bridge supported by a 3 × 3 group-pile foundation in model scale is considered. 1g shaking table test and three-dimensional nonlinear dynamic finite element method (FEM) are conducted to investigate the seismic behavior of the mentioned model. In the numerical analysis, a FEM program called DBLEAVES is used. In the numerical modeling, the soil, the piles and the structure are modeled by nonlinear constitutive equations. The purpose of this study is to confirm the accuracy of the mentioned nonlinear analysis method by the 1g shaking table test. The recorded data of the shaking table test are reproduced qualitatively and quantitatively by the numerical test. This implies that the discussed numerical method is a comprehensive tool. Applicability of the method is to study the seismic behavior of piles with a high accuracy. Study of reinforcing of existing piles with the ground improvement is its other applicability.

The soil-structural interface is involved in many geotechnical engineering problems. Previous investigations have mainly relied on the macro-scale observations from laboratory experiment. However, soil is a type of granular material, and recent research works reveal that the macroscopic responses of granular materials originate from the evolution of the microstructures. Therefore, to understand well the shearing behavior of soil adjacent to an artificial interface, it is necessary to explore this problem on the particle scale. In this study, an interface shear test is modeled using the three-dimensional discrete element method. Five shear boxes of distinct size are modeled to verify the scale effect. According to the comparison between the interface shear tests in terms of computation time, shear strength measured on the interface and volumetric strain of the specimen, an interface shear box containing 14,000 particles is sufficient for this study. Then, a series of three-dimensional interface shear tests with distinct normalized roughness “$$ \varvec{R}_{\varvec{n}} $$Rn” is modeled. The results show that (1) two failure modes exist in an interface shear test, elastic-perfectly plastic for a smooth surface and stress softening observed for a rough surface, and (2) the shear strength of the soil-structural interface increases with the increasing of the roughness of the interface. The displacement field is obtained by interpolating the movement of each particle. The field of $$ \varvec{u}_{\varvec{x}} $$ux (displacement in the direction of shearing) indicates that a narrow zone of intense shearing deformation, called the shear band, emerges from the contact interface and expands during the shearing process. A discontinuous feature is characterized after the shear band appears.

This paper presents a hydromechanical coupled framework for unsaturated soils, which differentiates the capillarity and adsorption. The proposed framework is a thermodynamic-based modelling framework and it was derived from energy-conjugate variables. Independent hydromechanical models are considered for each mechanism, including independent measures of effective stresses and water retention curves. Hydromechanical coupling at each mechanism is efficiently achieved by liking the effective stress formulation with the water retention model. Finally, the models are linked through a structure parameter to obtain the global response. The framework will pave the way for developing constitutive models for unsaturated soils, in particular the expansive soils.

Caprock stability is crucial to be assessed in the carbon dioxide underground storage. The CO₂ injection not only generates pressurization but also induces thermal stresses because of the difference in injected CO₂ and in situ temperature. Exhibiting the combination of both effects, the caprock stability needs to be investigated. Numerical simulations are carried out to investigate the effects of thermo-mechanical coupled properties on the caprock stability. The results indicate that for a given geometrical configuration and a given temperature difference between injected CO₂ and reservoir, the deviatoric stress may increase or decrease, depending on the combination of thermal-hydro-mechanical properties: thermal expansion coefficient, stiffness and Poisson’s ratio. Therefore the effects of material properties on the caprock stability should be addressed in a combined way for CO₂ injection problems.

In the framework of the research for the thermo-hydro-mechanical effects of the clay host rock of a deep repository a numerical model to analyse the rock behaviour in response to heating was developed. The paper shows the developement of a 3D Thermal-Hydraulic-Mechanical (THM) simulator by coupling parametric modeling and implicit FE simulation environment in ANSYS. To calibrate the numerical model to the in-situ experimental results, powerfull methods of sensitivity analysis using optimized stochastic sampling strategies and optimization algorithms where used.

Geomaterials exhibit complicated rate-dependent behaviors. A deeper understanding and interpretation of these rate-dependent behaviors possess important theoretical and practical values. From the development of the rate-dependent constitutive model point of view, the paper deeply investigates the rate-dependent physical mechanisms and the mathematical models of soils. Based on the classical elasto-plastic models, traditional elasto-viscoplastic models apply viscous dissipations, similar to what has been used in fluid mechanics, for the description of rate-dependent dissipations. Traditional thermodynamic models are rate-independent. Starting from the theory of non-equilibrium thermodynamics and by introducing the matrix of migration coefficients, a thermodynamic rate-dependent model for soil is established, which is able to represent both the viscous dissipations and the non-viscous dissipations concerning rate dependency. The paper explores to which degree non-viscous dissipation, which is not familiar for the researchers of rock and soil mechanics, is able to describe the rate-dependent behaviors of soil, and carries out the constitutive modeling of one-dimensional compressional and three-dimensional shear behaviors of soil. The simulations are compared with those using elasto-viscoplastic models and the experimental results.

This paper presents a thermo-viscoplastic subloading soil model with a mobile centre of homothety. The model is formulated to describe the influence of non-isothermal conditions on the stress-strain-time behaviour of soils and is restricted to isotropic stress and strain conditions. Numerical simulations of three isotropic drained heating tests at constant isotropic effective stress for different overconsolidation ratios were performed. The model was able to accurately reproduce the experimental results.

Global warming is an important environmental issue today, and the increasing concentration of carbon dioxide (CO₂) in the atmosphere is considered to be the main causative factor. CO₂ sequestration in geological formations is regarded as one of the most promising approaches for reducing the emission of anthropogenic CO₂. Predicting the migration and evaluating the long-term stability of CO₂ injected into geological formations is a major concern in CO₂ sequestration projects. A numerical method is an effective and economical approach to achieve this goal. In this paper, a numerical method based on the three-phase (rock-water-CO₂) theory and energy conservation law was established. To verify this numerical method, the Norwegian Sleipner CO₂ geological sequestration project in Utsira formation was simulated to obtain the distribution of CO₂ after a certain time of continuous injection.

In nature, weakly cemented granular materials are encountered in the form of soft rocks such as limestone, sandstone, mudstone, shale, etc. The mechanical behaviour of these materials is quite different from the purely frictional granular materials. The presence of cementation between the grains causes a significant variation in mechanical response under complex boundary conditions. In order to understand the manifestation of this interparticle cohesion at the ensemble level, we have used a hollow cylinder torsional testing apparatus which is capable of independently controlling the magnitude and the direction of the three principal stresses. From this experimental programme, the small strain response, peak strength and post peak behaviour with changing intermediate principle stress ratio (b) and initial mean effective stress (I₁) is studied. In addition to the analysis of stress strain behaviour at different b and I₁, stress-dilatancy characteristics of these cohesive frictional material are also discussed. This experimental study is followed by calibration and validation of a single hardening constitutive model which considers cementation as additional confinement. Observations from validation exercises suggest that this consideration works well for stress-strain response whereas it fails to predict the volumetric behaviour.

The undrained behavior of partially saturated sands under repetitive loading has been investigated by many researchers experimentally. The research to date demonstrate that partially saturated sands do not liquefy however, significant amount of excess pore water pressures may develop especially in loose sand specimens with degrees of saturation above 80%. Cyclic Simple Shear Liquefaction Box (CSSLB) was developed by Eseller-Bayat et al. to perform cyclic simple shear tests on large specimens using a shaking table. In this study, based on the experimental results, the behavior of loose to dense partially saturated sand specimens to cyclic simple shear strains under drained and undrained conditions was modelled numerically in finite difference software program FLAC3D (Itasca Consulting Group). The CSSLB was first modelled and the sand specimen was numerically tested under drained conditions. Then Finn and Byrne liquefaction models were used to simulate the liquefaction behavior of sands. Sand specimens with different degrees of saturation (40–83%) were tested under several cyclic simple shear strain amplitudes and the excess pore water pressures were numerically obtained. Finally, excess pore water pressure ratio (r_u) values were compared both in numerical and experimental tests. The results were also compared with the excess pore water pressures generated in fully saturated sand specimens.

The study explores the feasibility of fracturing clays during fast electromagnetic (EM) heating. An oedometer setup customized with an EM wave source is simulated numerically at the continuum level. Solid matrix equilibrium, fluid flow and heat transfer equations are solved together with the laws of electromagnetism, i.e. Maxwell’s equations, using the COMSOL multiphysics code. Numerical simulations involve EM excitation in a fluid-saturated isotropic, homogeneous, linear thermo-poro-elastic and lossy (EM wave loses power as it propagates in a lossy material.) medium at both 50 MHz and 2.45 GHz frequencies. It is found that the thermal expansion contrast between the fluid and solid phase results into a local buildup of fluid pressure that cannot dissipate due to the very low permeability of the medium, hence promoting fracturing.

The results of laboratory tests for examining the mechanical properties of a geomaterial always vary because of the changes in the microscopic structure. Same results for the tests cannot be obtained even on using the same particle aggregation owing to the influence of particle arrangement. In this study, we attempt to replicate the particle arrangement using 3D printing and examine the feasibility of evaluation of this influence experimentally. A 3D printed specimen is made from numerically simulated packing samples using discrete element method (DEM). This printed specimen shows good reproducibility as verified from the comparison of the cross-section of the specimen obtained using X-ray CT scanning. To validate the mechanical parameters, the value of the permeability coefficient of the 3D printed specimen obtained from laboratory testing is compared with that obtained from numerical simulations using DEM coupled with simplified maker and cell method and permeability-estimating equation. The permeability coefficients obtained using each method are found to be qualitatively consistent. Conclusively, we demonstrate the reproducibility of numerical simulation through experimental validation focusing on permeability.