Direct Shear Experiments Research Articles

Rapid Mineral Precipitation During Shear Fracturing of Carbonate‐Rich Shales

AbstractTarget subsurface reservoirs for emerging low‐carbon energy technologies and geologic carbon sequestration typically have low permeability and thus rely heavily on fluid transport through natural and induced fracture networks. Sustainable development of these systems requires deeper understanding of how geochemically mediated deformation impacts fracture microstructure and permeability evolution, particularly with respect to geochemical reactions between pore fluids and the host rock. In this work, a series of triaxial direct shear experiments was designed to evaluate how fractures generated at subsurface conditions respond to penetration of reactive fluids with a focus on the role of mineral precipitation. Calcite‐rich shale cores were directly sheared under 3.5 MPa confining pressure using BaCl2‐rich solutions as a working fluid. Experiments were conducted within an X‐ray computed tomography (xCT) scanner to capture 4‐D evolution of fracture geometry and precipitate growth. Three shear tests evidenced nonuniform precipitation of barium carbonates (BaCO3) along through‐going fractures, where the extent of precipitation increased with increasing calcite content. Precipitates were strongly localized within fracture networks due to mineral, geochemical, and structural heterogeneities and generally concentrated in smaller apertures where rock:water ratios were highest. The combination of elevated fluid saturation and reactive surface area created in freshly activated fractures drove near‐immediate mineral precipitation that led to an 80% permeability reduction and significant flow obstruction in the most reactive core. While most previous studies have focused on mixing‐induced precipitation, this work demonstrates that fluid–rock interactions can trigger precipitation‐induced permeability alterations that can initiate or mitigate risks associated with subsurface energy systems.

Influence of Water-rock Chemical Erosion on the Shear Strength of Coastal Buildings

Lai, L., 2020. Influence of water-rock chemical erosion on the shear strength of coastal buildings. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 262–266. Coconut Creek (Florida), ISSN 0749-0208.This paper formulates the chemical solution preparation scheme of water-rock and the scheme of preparation and shearing of specimens, and carries out the experiment of shear strength of building under different water-rock chemical solution erosion conditions, and carries out the direct shear experiment, and analyzes the influence law of water-rock chemical action on the shear strength of building, quantificationally expresses the evolution process of the shear strength parameters of building with the chemical damage of water-rock through the introduction of chemical water-rock damage degree, and uses the method of chemical kinetics to simulate the evolution process of damage degree buildings with time in the given chemical water-rock conditions, then predicts the evolution law of shear strength parameters. The degradation of shear strength of building is analyzed and verified. Experimental results show that the degradation of shear strength of building in alkaline environments not as good as that of an acid environment, and the internal friction angle and cohesion of the building decreases with the increase of water-rock chemical damage degree.

Effect of contact surface area on frictional behaviour of dry and saturated rock joints

Frictional behaviour of joints/faults is of great importance in many geo-engineering applications across different scales. It is known that the joint surface roughness is one of the critical parameters controlling the frictional behaviour of rock joints. While nominal normal and shear stresses acting across the joint asperities are calculated based on overall shearing area, the actual normal and shear stresses can be much greater due to smaller actual contact area. The actual contact area on the other hand is known to be related to the joint surface roughness. In order to investigate the effect of contact area on the shear behaviour of rock joints, an extensive set of direct shear experiments at different normal stresses on dry and saturated shale, limestone and sandstone tensile joints were undertaken. To compare the frictional properties of natural and artificial samples, synthetic samples were also fabricated, and their frictional properties were evaluated. Results of this study revealed that the predominant parameter controlling the frictional strength is the actual contact area. While the common models in predicting the shear strength of rock joints were unable to fit the experimental data based on nominal contact area, the results based on actual contact area fit the measured values accurately. In addition, it was observed that when the actual contact area is considered, Mohr-Coulomb criterion is sufficient to fit the experimental data irrespective of the rock type, joint surface roughness or whether dry or saturated. This highlights that the Mohr-Coulomb criterion can appropriately predict the frictional strength if the actual contact area can be estimated. More importantly, the results showed that the surface roughness cannot dictate the shear behaviour of rock joints without considering the actual contact area.

Physical Modeling of Stone Columns in Unsaturated Soil Deposits

Abstract This article focuses on evaluating the load–settlement response of circular footings on unsaturated soil layers improved with stone columns using 1-gravity (1-g) physical modeling experiments. The initial (preloading) conditions in the soil layers were varied by compaction to the same dry density but different initial degrees of saturation. An effective stress analysis calibrated using direct shear experiments was found to satisfactorily predict the measured bearing capacities of unimproved soil layers, considering a change in failure mode for soil specimens at certain initial degrees of saturation. As the bearing capacity of the unsaturated soil layers increased, the amount of improvement gained by incorporating stone columns decreased. Bulging deformations of the stone column exhibited a close relationship with the bearing capacity, with smaller amounts of bulging in soil layers with low initial degrees of saturation that also have high bearing capacity. The stress concentration ratio increases with increasing initial degree of saturation, indicating that stone columns carry a greater fraction of the applied footing stress for soil layers closer to saturated conditions.

Direct Shear Experimental Study on the Mobilized Dilation Behavior of Granite in Alxa Candidate Area for High-Level Radioactive Waste Disposal

Dilation behavior is of great importance for reasonable modeling of the stability of the host rock of the repository for high-level radioactive waste disposal. It is a suitable method for carrying out direct shear experiments to analyze the dilation behavior of rock with well understood physical meanings. Based on a series of direct shear experiments on granite samples from the Alxa candidate area under different normal stresses, the shear stress‒shear strain and shear stress‒normal strain relations have been studied in detail. Five typical stages have been divided associated with the fracturing process and deformation behaviors of the granite samples during the experimental process, and the method to determine the typical stress thresholds has been proposed. It has also been found that the increasing normal stress may reduce the maximum dilation angle, and when the normal stress is relatively lower, the negative dilation angle may occur during the post-peak stage. According to the data collected from the direct shear tests, an empirical model of the mobilized dilation angle dependent on normal stress and plastic shear strain is proposed. This mobilized dilation angle has clear physical meanings and can be used in plastic constitutive models of the host rock of the repository, and this analysis can also be put forward to other types of geomechanical problems, including the deformation behaviors related to landslide, earthquake, and so on.

An investigation of plant-induced suction and its implications for slope stability

Both above- and below-ground biomass of plants provide additional strength to soil through different mechanisms, which help to increase the stability of a vegetated slope. Nonetheless, shallow landslides on steep slopes covered with vegetation still occur, often being triggered above the groundwater table, due to loss of suction subsequent to rainfall. Therefore, it is essential to know to what extent vegetation enhances slope stability, and to quantify the contribution of vegetation to the shear strength of soil to determine factors of safety. Results of large-scale direct shear experiments on root-permeated soils and slope geometry from a landslide database were synthesised through an infinite slope analysis under partially saturated conditions to find critical combinations of slope angle and suction stress. Monte Carlo simulations yielded a clear separation of stable and unstable zones, which can be used to define the susceptibility of a slope to near surface failure. This method, based on the simulations, has the potential to be used as a regional early warning system.

An experimental and numerical investigation on the hydromechanical behaviour of carbonate fault zones upon reactivation: the impact of carbonate mud sealing layers and overall research outcomes

Abstract To contribute to the understanding of the impacts of fault reactivation induced by reservoir exploitation, we describe the final series of laboratory experiments, numerical simulations and microstructural analysis conducted during the ‘Fault Reactivation in Carbonates’ research project. In the project, the structure and hydromechanical properties of carbonate-hosted fault zones were investigated. For the analyses here reported, faults were artificially generated by direct shearing composite blocks made of layers of reservoir analogue rocks (outcrop travertine or synthetic grainstone) intercalated with one layer of a sealing analogue rock (synthetic carbonate mudstone). Post-direct shearing, cylindrical plugs containing the fault zone and parts of intact rock were cored out from the blocks and tested in a triaxial test rig, simulating fault reactivation. Varied stress paths and pore-pressure conditions representative of fluid depletion and injection were considered. In parallel, two-dimensional mechanical models representative of the direct shear experiments were developed using smoothed particle hydrodynamics (SPH). We observed a continuous reduction in fault transmissibility during direct shearing, followed by a permeability reduction of 50–80% with increasing mean effective stress in the subsequent fault reactivation tests. Experimental fault zone geometries produced during direct shear were broadly reproduced by the two-dimensional modelling approach. We also detected that the inclusion of the carbonate mud sealing rock into the fault zone caused greater compaction of the fault materials when compared to experiments conducted without carbonate mud layers. We conclude that with fault displacement, increasing incorporation of carbonate mud sealing material into the fault zone and the concomitant development of gouge results in the continuous reduction of fault transmissibility/permeability. This occurs in the two very different limestone host-rock types and for all the stress configurations investigated. Discussions on these results and also on the outcomes of the research project as a whole are presented in the paper.

Experimental Study on Shear Behavior of Expansive Soil Modified by Gravel Sand

Using the strain-controlled direct shear equipment, some direct shear experiments were carried out on the soil samples of Nanyang expansive soil with different gravel ratios under the specific water content, and the shear strength, cohesion and internal friction angle of the expansive soil samples were studied. The experimental results show that the shear strength of the expansive soil samples increases with the increase of vertical pressure. As the proportion of gravel sand increases, the shear strength of expansive soil samples increases to varying degrees. The shear strength of expansive soil increases significantly when the proportion of gravel sand is in the range of 10%~40%, and the shear strength of remolded soil is the highest when the gravel sand content is 10% under the lower vertical pressure. Under the higher vertical pressure, the shear strength of the remolded soil is the largest when the gravel content is 20%, and the shear strength of expansive soil is close to that of pure expansive soil when the proportion of gravel sand is 50%. The cohesive force of the sample increases first and then decreases sharply with the increase of the proportion of gravel sand, and is maximum when the content of gravel sand is 10%. The internal friction angle increases first and then decreases slowly with the increase of the proportion of gravel sand, and is maximum when the gravel sand content is 40%. The results show that gravel sand can effectively enhance the shear strength index of expansive soil, which can be used as a good engineering application material for improving expansive soil.

Quantitative Characterization of the Aqua-Dispersing Nano-Binder Effects on the Slip Resistance of Borrowed Soil of a Rock Slope

Soil loss can be reduced by enhancing the slip resistance of soil. Therefore, the slip resistance of borrowed soil of rock slopes is key to ecological restoration. An effective way to enhance the soil slip resistance is to improve the permeability and shear strength of the soil by using polymeric materials. Most current projects have confirmed the amount and effect of additions via repeated tests because of the lack of a quantitative characterization model. This leads to high implementation costs and inhibition of the method’s popularization and application. Therefore, based on different dosages and application processes of aqua-dispersing nano-binder (ADNB), the present study aims to examine the change rules of the particles, permeability, and shear strength after soil improvement through particle analysis, permeability, and direct shear experiments. It was found that ADNB could increase the average particle diameter, permeability coefficient, and shear strength of soil, and significantly enhanced the slip resistance of rock slope soil. On this basis, this paper establishes quantitative characterization models of the slip resistance of rock slopes, reveals the clay improvement mechanism, and proposes the optimal addition range. The study also verified the reliability of the models through an engineering application. It is proved that the appropriate amount of ADNB based on the model can enhance the slip resistance of borrowed soil as well as the ecological restoration effect of rock slopes. The model in question solves the problem of repeated tests during implementation due to the lack of a quantitative characterization model and provides theoretical guidance for the ecological restoration of rock slopes.

Experimental research on a novel optic fiber sensor based on OTDR for landslide monitoring

In this paper a novel optic fiber sensor (OFS) based on optical time domain reflectometer (OTDR) with simple construction and innovative structure was developed for landslide monitoring. The instrument had a maximum measurement distance and an effective initial measurement displacement and a low economic cost of 36 mm, 0.98 mm and $ 0.45/m. A direct-shear experiment with an OFS was conducted to investigate its effectiveness in monitoring the damage from shear sliding on rock slopes. A detailed field-monitoring test of the proposed OFS on a shallow artificial slope was conducted and the progressive deformation behavior of the slope was accurately captured by the OFS. In addition, it is possible to connect these OFSs in series to explore the underground properties of deep-seated landslides. This capability supports the promising application of the OFS in civil engineering for real-time monitoring of the stability of slopes, foundation pits and dams.

Temperature and Gas/Brine Content Affect Seismogenic Potential of Simulated Fault Gouges Derived From Groningen Gas Field Caprock

AbstractWe investigated the rate‐and‐state frictional properties of simulated anhydrite‐carbonate fault gouge derived from the basal Zechstein caprock overlying the seismogenic Groningen gas reservoir in the NE Netherlands. Direct shear experiments were performed at in situ conditions of 50–150 °C and 40‐MPa effective normal stress, using sliding velocities of 0.1–10 μm/s. Reservoir pore fluid compositions were simulated using 4.4 Molar NaCl brine, as well as methane, air, and brine/gas mixtures. Brine‐saturated samples showed friction coefficients (μ) of 0.60–0.69, with little dependence on temperature, along with velocity strengthening at 50–100 °C, transitioning to velocity weakening at 120 °C and above. By contrast, gas filled, evacuated and partially brine‐saturated samples showed μ values of 0.72 ± 0.02 plus strongly velocity‐weakening behavior accompanied by stick slip at 100 °C (the only temperature investigated for gas‐bearing and dry samples). A microphysical model for gouge friction, assuming competition between dilatant granular flow and thermally activated compaction creep, captures the main trends seen in our brine‐saturated samples but offers only a qualitative explanation for our gas‐bearing and dry samples. Since the reservoir temperature is ~100 °C, our results imply high potential for seismogenic slip nucleation on faults that cross cut and juxtapose the basal Zechstein anhydrite‐carbonate caprock against the Groningen reservoir sandstone, specifically in the gas‐filled upper portion of the reservoir system.

Assessment of the Shear Strength of Pile-to-Soil Interfaces Based on Pile Surface Topography Using Laser Scanning.

This article presents in situ research on the side surface of continuous flight auger (CFA) foundation piles using a three-dimensional (3D) laser scanner (Leica ScanStation C10) in order to evaluate the morphology assessment of pile concrete surfaces in various geotechnical layers. Terrestrial laser scanning describes the 3D geometry of the construction with high spatial resolution and accuracy. A total of six areas were selected from the acquired point cloud for which a two-step approach for removing the form was applied. In the first step, the reference surface was fitted using the least squares method, and then, cylindrical projection of the surface was performed. In the second step, an operator of removal of the multi-plane form was applied. For each sample, height parameters (Sq, Ssk, Sku, Sp, Sv, Sz, Sa) and functional volume parameters (Vmp, Vmc, Vvc, Vvv) according to the standard ISO 25178-2:2012 were determined. Significant differences in the values of surface height and functional volume parameters were observed for each geotechnical layer where piles were formed. Because the piles remain embedded in the ground, in situ tests of the side surface of piles are rarely performed and taken into account in the assessment of pile bearing capacity. The study of surface topography is a crucial stage in the assessment of the shear strength at the interface between a concrete pile and the soil layer. The obtained concrete morphology assessments are applicable during the determination of the skin friction factor in the analytical or numerical estimation of pile shaft resistance. The proposed procedure of morphology evaluation may improve the fidelity of the assumed friction factor between the concrete and soil and increase the reliability of direct shear experiments.

Crystallographic preferred orientations of ice deformed in direct-shear experiments at low temperatures

Abstract. Synthetic polycrystalline ice was sheared at temperatures of −5, −20 and −30 ∘C, to different shear strains, up to γ=2.6, equivalent to a maximum stretch of 2.94 (final line length is 2.94 times the original length). Cryo-electron backscatter diffraction (EBSD) analysis shows that basal intracrystalline slip planes become preferentially oriented parallel to the shear plane in all experiments, with a primary cluster of crystal c axes (the c axis is perpendicular to the basal plane) perpendicular to the shear plane. In all except the two highest-strain experiments at −30 ∘C, a secondary cluster of c axes is observed, at an angle to the primary cluster. With increasing strain, the primary c-axis cluster strengthens. With increasing temperature, both clusters strengthen. In the −5 ∘C experiments, the angle between the two clusters reduces with strain. The c-axis clusters are elongated perpendicular to the shear direction. This elongation increases with increasing shear strain and with decreasing temperature. Highly curved grain boundaries are more prevalent in samples sheared at higher temperatures. At each temperature, the proportion of curved boundaries decreases with increasing shear strain. Subgrains are observed in all samples. Microstructural interpretations and comparisons of the data from experimentally sheared samples with numerical models suggest that the observed crystallographic orientation patterns result from a balance of the rates of lattice rotation (during dislocation creep) and growth of grains by strain-induced grain boundary migration (GBM). GBM is faster at higher temperatures and becomes less important as shear strain increases. These observations and interpretations provide a hypothesis to be tested in further experiments and using numerical models, with the ultimate goal of aiding the interpretation of crystallographic preferred orientations in naturally deformed ice.

The Role of Mineral Composition on the Frictional and Stability Properties of Powdered Reservoir Rocks

AbstractThe growing hazard of induced seismicity driven by the boom in unconventional resources exploitation is strongly linked to fault activation. We perform laboratory measurements on simulated fault gouges comprising powdered reservoir rocks from major oil and gas production sites in China, to probe the control of mineral composition on fault friction and stability responses during reservoir stimulation. Double direct shear experiments were conducted on gouges with phyllosilicate content ranging from 0 to 30 wt.% and grain sizes <150 μm, at constant normal stresses of 10–40 MPa and conditions of room temperature and water saturation. The velocity step and slide‐hold‐slide sequences were employed to evaluate frictional stability and static healing, respectively. Results indicate that the mineralogy of the gouges exhibit a strong control on the frictional strength, stability, and healing. Phyllosilicate‐rich samples show lower frictional strength μ and higher values of (a − b), promoting stable sliding. For the gouges studied, the frictional strength decreases monotonically with increasing phyllosilicate content, and a transition from velocity weakening to velocity strengthening behavior is evident at 15 wt.% phyllosilicates. Intermediate healing rates are common in gouges with higher content of phyllosilicates, with high healing rates predominantly in phyllosilicate‐poor gouges. As an indispensable component in reservoir rocks, the carbonates are shown to affect both the frictional stability and healing response. These findings can have important implications for understanding the effects of mineralogy on fault behavior and induced seismic potential in geoengineering activities, particularly in reservoirs in China.

Rate Effects on Shear Deformation of Rough Limestone Discontinuities

The relationship between slip velocity, resistance to shear, and geometrical evolution of initially rough limestone surfaces is investigated in servo-controlled direct shear experiments. We focus on shear stress evolution throughout the deformation process, as function of the shear rates applied. Direct shear tests are performed on tensional fractures positioned in a perfectly mating configuration, using a hydraulic, closed-loop, servo-controlled system. The discontinuities are sheared to prescribed displacement target of 10 mm under a constant normal stress of 5 MPa while shear stress and dilation are recorded under controlled shear rates between 0.1 and 50 µ/s. Surface geometry is measured using laser profilometer before and after tests, and roughness evolution is statistically quantified. We find that, before peak shear stress is attained, the shear stiffness increases with slip velocity, manifesting asperity strain hardening during this largely elastic deformation stage. Peak shear stress and stress drop, however, are both inversely proportional to velocity, indicating velocity weakening coupled with rate-dependent healing of the interfaces. The final roughness of the sheared surfaces increases moderately with increasing slip velocity indicating enhanced fragmentation and brittle deformation with increasing shear rate.

Experimental Study on Shear Failure Mechanism and the Identification of Strength Characteristics of the Soil‐Rock Mixture

Soil‐rock mixture (SRM) is a special geological material that has the unique properties of rock or soil. Studies on the strength characteristics of the SRM have very important theoretical significance and practical value. In this study, the gravel proportions, gravel sizes, gravel shapes, and repetitive results of shear experiments are considered in laboratory experiments and for the identification of strength parameters. To the gravel shapes, from the angle of the composition materials of SRM, the experimental samples include samples with breccia gravels and with subrounded gravels. And, in this study, the laboratory model experiment is used to research the strength characteristics of SRM. In addition, the shear failure mechanism is used to establish the relationship between the microfailure mechanism and the macrostrength parameters identification. Taking gravel proportion, gravel size, gravel shape, and repetitive shear process as the influencing factors of the SRM, the laboratory models have been remolded, and laboratory direct shear experiments have been carried out. The shear deformation laws of the SRM are researched on the basis of the analysis of the curves of shear stress and the horizontal displacement.

Scalable En Echelon Shear‐Fracture Aperture‐Roughness Mechanism: Theory, Validation, and Implications

AbstractShear fractures can facilitate fluid conductivity through rock. Aperture and roughness are controlling characteristics for a fracture's fluid conductivity. Inspired by en echelon fractures, we develop a shear “fracturelet” model that predicts anisotropic aperture with respect to the direction of shearing, rougher (nonplanar) rather than smoother (planar) fractures, and the bounds of this roughness for a coalesced fracture. This tendency for rougher fracture creation is validated by in situ X‐ray images and fluid conductivity measurements from triaxial direct shear experiments on anhydrite and shale. These experiments were conducted at confining stresses from 4 to 30 MPa and shear displacement magnitudes from 0 to 2 mm on initially intact rock specimens. Hydraulic, dilatational, and local fracture apertures were measured in the experiments. Apertures exhibited strong anisotropy with more conductive flow paths forming perpendicular to the direction of shearing. Local and dilatational aperture were found to be positively correlated with increasing shear displacement but hydraulic aperture was found to vary significantly, always having values smaller than the other aperture measures at factors ranging from 0.6 to 0.0. An implication of these results is that shear fractures have a mechanism for simultaneously exhibiting very low fluid conductivity and high fluid storage volume.

Physical and numerical modelling of the inherent variability of shear strength in soil mechanics

In this study the spatial variability of soils is substantiated physically and numerically by using random field theory. Heterogeneous samples are fabricated by combining nine homogeneous soil clusters that are assumed to be elements of an adopted random field. Homogeneous soils are prepared by mixing different percentages of kaolin and bentonite at water contents equivalent to their respective liquid limits. Comprehensive characteristic laboratory tests were carried out before embarking on direct shear experiments to deduce the basic correlations and properties of nine homogeneous soil clusters that serve to reconstitute the heterogeneous samples. The tests consist of Atterberg limits, and Oedometric and unconfined compression tests. The undrained shear strength of nine soil clusters were measured by the unconfined compression test data, and then correlations were made between the water content and the strength and stiffness of soil samples with different consistency limits. The direct shear strength of heterogeneous samples of different stochastic properties was then evaluated by physical and numerical modelling using FISH code programming in finite difference software of FLAC3D. The results of the experimental and stochastic numerical analyses were then compared. The deviation of numerical simulations from direct shear load-displacement profiles taken from different sources were discussed, potential sources of error was introduced and elaborated. This study was primarily to explain the mathematical and physical procedures of sample preparation in stochastic soil mechanics. It can be extended to different problems and applications in geotechnical engineering discipline to take in to account the variability of strength and deformation parameters.

An experimental study on stick-slip of gelatin hydrogels using fracture mechanics

Stick-slip (SS) is a mechanical instability which occurs on sliding interfaces in low velocity regimes in form of jerky motions. In this article, SS behaviour of gelatine hydrogels sliding on a smooth glass substrate is investigated in direct shear experiment using fracture mechanics. It is observed that average value of shear stress amplitudes and corresponding energy release rate (ERR) decrease with increase in pulling velocity upto the critical velocity. Time of relaxation is also found to be sliding rate dependent in the experiment. The video analysis of interfacial crack propagation reveals that crack length increases to a critical value and becomes unstable at the last phase of ‘stick’ period, but the same crack begins to restore to its original length just after ‘slip’ phase. The hysteresis present during the stick-slip motion diminishes at the critical velocity. Finally, this present study establishes that stick-slip is basically governed by the competition between rate dependent crack growth and crack healing along the sliding interface.

Direct shear testing of GCLs at elevated temperature and in a non-standard solution

The objective of this study was to develop, verify, and assess a direct shear apparatus to measure the internal shear strength of geosynthetic clay liners (GCLs) when exposed to elevated temperature and non-standard hydration solutions. Shear boxes were developed to facilitate testing 300 mm square and 150 mm square specimens. Preliminary experiments documented the effectiveness of a GCL gripping system and a two-stage hydration procedure. Direct shear experiments were performed to evaluate the new apparatus with respect to the following variables: (i) specimen size; (ii) GCL peel strength; (iii) GCL heat treatment; (iv) test temperature; and (v) hydration solution. Direct shear results were verified via comparison to literature, which also supported the use of 150 mm square GCL specimens to measure internal shear behavior and shear strength. Shear tests on heat-treated and non-heat-treated GCLs at 20°C and 80°C yielded a 40% reduction in internal peak strength with increase in temperature. Hydration in a high-alkaline synthetic mining solution indicated a loss in peak strength with increasing hydration times from 15 to 60 days. A systematic reduction in peak and large-displacement shear strength with increasing normal stress was observed for 60 days’ hydration in the alkaline solution relative to hydration for 4 days in de-ionized water.