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An experimental technique is presented to quantify the lateral restraint provided by a geogrid embedded in granular soil at the particle level. Repeated load triaxial tests were done on transparent sand specimens stiffened with geosynthetic inclusions simulating geogrids. Images of laser illuminated planes through the specimens were segmented into particles using a deep learning-based segmentation algorithm. The particle outlines were characterized in terms of Fourier shape descriptors and tracked across sequentially captured images. The accuracy of the particle displacement measurements was validated against Digital Image Correlation (DIC) measurements. In addition, the method's resolution and repeatability is presented. Based on the measured particle displacements and rotations, a state boundary line between probable and improbable particle motions was identified for each test. The size of the zone of probable motions was used to quantify the lateral restraint provided by the inclusions. Overall, the tests results revealed that the geosynthetic inclusions restricted both particle displacements and rotations. However, the particle displacements were found to be restrained more significantly than the rotations. Finally, a unique relationship was found between the magnitude of the permanent strains of the specimens and the area of the zone of probable motions. • Triaxial specimens of transparent sand (fused quartz) with geosynthetic inclusions were tested under repeated loading. • A deep neural network originally developed to segment biological tissue was trained to segment particles of fused quartz. • Lateral restraint provided by the inclusions was quantified in terms of reduced particle rotation and displacement. [ABSTRACT FROM AUTHOR]

Geotechnical stability analyses of mine waste rock (WR) piles require the critical friction angle (ϕ cr) of the coarse blasted rock. However, due to the presence of oversized rock clasts, shear strength can only be characterized on small samples prepared using grading scaling techniques, such as scalping. Thus, considering a testing device able to handle samples of characteristic size D, the material should be scaled down to a maximum particle size d max given by the minimum sample aspect ratio α = D/d max. However, a practical concern about how far the size scale can be reduced while keeping representative results remains a matter of debate in the geotechnical community. International standards do not agree on the minimum recommended α, and its effects on the mechanical behavior remain poorly understood. This paper aims to investigate the grading effects and sample size effects on ϕ cr of WR materials using the scalping technique, to provide insights on the minimum recommended α. Triaxial tests were conducted on loose and dense samples of diameters D = 150 and 300 mm. Samples were scalped from field material having d max = 75 mm, to allow a range of α from 4 to 30. Additionally, one of the world largest in-situ direct shear boxes (120 × 120 × 38 cm3) was developed to test the same WR material. The results show that scalping is an appropriate technique to assess the critical shear strength of WR. The minimum α for ϕ cr assessment in triaxial testing is not sensitive to grading nor sample size, but it is affected by sample density. The aspect ratio was found to be α ≥ 12 and α ≥ 16 for loose and dense samples, respectively. This finding advocates that α values recommended by worldwide standards, such as ASTM D7181-20 , might be too low and should be revisited after comprehensive testing. [ABSTRACT FROM AUTHOR]

The state parameter is a fundamental component of critical state compatible, bounding surface plasticity models. Several definitions for the state parameter exist, expressing how far the current state is from critical. However, there is rarely the opportunity to directly assess the chosen state parameter definition within a given model. Here, we use a plasticity model of the SANISAND family to simulate partially drained paths in triaxial space, by imposing a constant ratio of volumetric to axial strain. We show that for such paths, the locus of post-phase transformation and post-peak stress ratio points where instability is triggered represents a constant state parameter curve in e − p ′ space. We then present results from a series of partially drained triaxial compression experiments on Hostun sand, where volumetric and axial strains are coupled. We demonstrate that the chosen constitutive model does not recreate the experimentally observed paths well and more specifically is unable to match the experimentally obtained locus of constant state instability points. Consequently, we propose an updated state parameter definition through which the selected bounding surface plasticity model is shown to not only capture the desired locus of instability points, but to also achieve an overall improved performance in simulating partially drained paths. [ABSTRACT FROM AUTHOR]

Moraine soils are widely distributed in southeast Tibet of China, which poses a serious threat to local railway construction. The mechanical behavior of moraine soil containing ice in cold regions is difficult to capture under the joint action of stress conditions and temperature. To study the strength characteristics of ice-rich moraine soil, a total of 112 groups of thermal-mechanical triaxial tests under different ice forms, ice contents, and temperature conditions are carried out. The test results show that the mechanical properties of moraine soil with crushed ice and block ice are different, showing the characteristics of strain hardening and strain softening, respectively. Overall, the peak strength of moraine soil with block ice is greater than that of crushed ice. The cohesion and internal friction angle of moraine soil decreases with the temperature rise. With the increase of ice content, the peak strength of moraine soil with block ice increases, while that of crushed ice shows the opposite trend. In addition, the increase of ice content increases the cohesion of moraine soil with block ice, but there is a threshold value of ice content (25%) for moraine soil with crushed ice, which leads to the maximum cohesion at this time. Based on the test results, a unified function is proposed to describe the quantitative relationship between the strength parameters of ice-rich moraine soil, temperature, and ice content. Finally, to explore the nonlinear strength behavior of moraine soil, a binary-medium model is introduced to describe its stress-strain relationship, and the evolution of the main parameters in the model is analyzed. Comparing theoretical and experimental results demonstrates that the established model is of satisfactory applicability to simulate the mechanical behavior of moraine soil with different ice forms. • The stress-strain curve of soil containing crushed ice shows strain-hardening characteristics. • The stress-strain curve of soil containing block ice shows strain-softening characteristics. • There is an ice content threshold of moraine soil with crushed ice which makes the most effective cohesion. • The strength parameter model of moraine soil considering temperature and ice content is described. • The stress-strain relationship of ice-rich moraine soil is modeled utilizing a binary medium model. [ABSTRACT FROM AUTHOR]

The Indian National Gas Hydrate Program Expedition 02 (NGHP-02) was the most comprehensive gas hydrate drilling expedition carried out to date. From 74 pressure cores that were successfully recovered from various site locations offshore India, 19 subsamples were obtained for further detailed geotechnical analysis using the PCATS Triaxial apparatus to investigate the geomechanical behaviour of the cores, including small-strain stiffness, G max , undrained shear strength, S u , as well as direct-flow measurements of permeability, k. Samples tested included fine-grained muds with no appreciable hydrate through to coarse-grained sands that contained extensive hydrate within the pore space. Although the subsampling and transfer of pressure cores into the PCATS Triaxial under zero effective confining stress, σ′ , remains challenging, 12 samples were successfully transferred and underwent detailed geotechnical characterization. During consolidation, after reapplication of in-situ σ′ , all samples experienced axial and radial consolidation, with variations between samples seemingly dependent on the degree of sample disturbance, stress relaxation between coring and testing, hydrate saturation, S h and grain size, D 50. The largest values of G max were obtained from samples with high S h in the pore space, with values much higher than that expected for similar sediments without hydrate under comparable σ'. Samples with high permeability had both high S h and larger grain size, suggesting that in coarse-grained sediments sufficient pathways are available for reasonable fluid flow even with high hydrate saturations. Triaxial testing and unconfined compression tests highlighted the influence of hydrate on increasing sediment strength, although the size of the sediment grains appeared to have an important control on the magnitude of the strength increase and the overall stress-strain behaviour. The results from the detailed geotechnical characterization of the recovered pressure cores will be invaluable in helping India identify potential locations for future gas production wells, and assess the long-term performance of such reservoirs. • Testing of pressure core samples using PCATS Triaxial apparatus was undertaken. • Induced strains during consolidation are influenced by test and sample conditions. • Permeability of hydrate-bearing sediments are greater than fine-grained sediments. • Hydrate cement increases strength/stiffness of coarse-grained sediments. • Increases in strength/stiffness influenced by grain size and clay content. [ABSTRACT FROM AUTHOR]

The liquefaction of cargoes of metallic ores during maritime transportation is believed to have caused a number of ships to capsize during the past 30 years. To minimise the risk of liquefaction, shipping standards specify a transportable moisture limit (TML), which is the maximum moisture content for ore cargoes to be loaded onto a ship. However, the mechanics leading to the liquefaction of these cargoes is not well understood. This study uses an unsaturated soil mechanics perspective to understand the cyclic liquefaction behaviour of partially saturated materials, similar in grading to iron ore fines, a metallic ore that is known to liquefy during shipping transportation. Iron ore fines are transported at relatively low densities and have variable gradings containing a wide range of particle sizes and fines contents. Therefore, the effects of the degree of saturation and the fines content on the cyclic liquefaction behaviour of well-graded materials have been investigated by performing unsaturated, compression-only cyclic triaxial tests on samples prepared with four different gradings containing particle sizes from 9.5 mm to 2 μm with fines (<75 μm) contents of 18%, 28%, 40% and 60%. The trends in the data are discussed and used to develop a simple method that can conservatively estimate the number of cycles that samples with different degrees of saturation and fines contents are able to resist. The use of this method to describe the liquefaction behaviour of cargoes containing iron ore fines, in practice, is discussed. [ABSTRACT FROM AUTHOR]

Drained triaxial tests have been performed to explore the effect of particle loss on shearing behaviour and critical states in granular mixtures. The mixtures comprise Leighton Buzzard sand (d 50 = 0.8 mm), to which was added 15% by mass of salt particles of different nominal sizes: 0.063 mm, 0.25 mm and 0.5 mm. Shearing behaviours before and after particle loss (by dissolution) were compared. A good fit is observed between the test data and a stress-dilatancy relationship for the post-dissolution tests, highlighting the ability of the stress-dilatancy analysis as a means to interpret the effects of particle loss on shearing. It was noted that critical state strength parameter M is determined by the post-dissolution grading regardless of size of removed particle. However, the duration of contractant volumetric strain increased with the larger removed particles (0.25 mm & 0.5 mm) even when initial specific volumes were virtually identical. It is suggested that a loose volumetric state is reached if the sand particle network is initially disrupted by the amount and/or size of salt particles, which following dissolution results in structural or fabric phenomena that are not reflected in scalar volumetric measures such as specific volume. [ABSTRACT FROM AUTHOR]

Abstract Volumetric strain and permeability are strictly interconnected properties and important controlling parameters for deformation patterns in rock masses. Under reservoir conditions, stresses may be highly inhomogeneous and anisotropic, leading to porosity changes and consequently affecting fluid flow. Therefore, it turns out be a challenging issue in rock mechanics to evaluate volume change based on traditional soil mechanics background, originally intended for soft materials under low and mostly isotropic pressures. In this respect, triaxial compression tests were carried out to describe the interplay between physical properties, volume change and permeability of two hard sandstones by quantifying porefluid volume change with fully water saturated rock specimens (14 cm length and 7 cm radius). The investigated sedimentary rocks are (1) the greyish Trendelburg beds, a silica cemented subarkose Bunter Sandstone of Triassic age (porosity of ca. 12%), and (2) the red-brownish Rotliegend Sandstone (Bebertal), a carbonate and silica cemented sandstone of Permian age, clearly less porous (ca. 6% of effective porosity) and less permeable (3.5 × 10−10 m/s) than the Bunter Sandstone. Both materials present a pronounced brittle behaviour influenced by coring direction: permeability, volumetric strain and fracture pattern are direction-dependent. Effective porosity and pore pressure level affect the fracturing development, which therefore influences the permeability after stress fall. For the Bunter Sandstone, increasing porefluid pressure leads to an earlier microcraking stage, which shortens the forestage of compaction and induces a more pronounced dilatant behaviour with decreased compressive strength. For the Rotliegend, the increase of pore fluid pressure enhances compaction. Altogether, this examination is valuable to understand the combined effects of pore pressure change and pore space quality on the mechanical behaviour of rock masses. Highlights • Bunter Sandstone and Rotliegend have an inverse anisotropic behaviour and therefore they accomodate strain differently. • Strength, permeability and volume change are direction dependent and highly affected by the pore pressure level. • Fracture pattern have their development strongly associated with effectice porosity and effective pressure. [ABSTRACT FROM AUTHOR]

• Monitored the formation and dissociation of CO 2 hydrates in fine-sand under triaxial condition. • Formation and dissociation processes were divided into three phases based on V P & V S. • Proposed micro-scale models applicable to CO 2 hydrate formation and dissociation based on wave velocities. • Induction time and water to hydrate conversion ratio depend on the initial water content. • Wave velocities of CH 4 & CO 2 hydrate-bearing sands were found to be comparable. Natural gas hydrates in oceanic and permafrost sediments are considered a future energy resource. Injecting CO 2 into these sediments is believed to aid in the safe and efficient extraction of CH 4 and the geological sequestration of carbon. The effectiveness of this strategy can be ensured by assessing the formation of CO 2 hydrates in the pores of these sediments at in-situ stress conditions by investigating their elastic properties (viz., compressional and shear wave velocities, elastic modulus, and shear modulus) by resorting to a suitable method such as seismic and/or acoustic logging. However, conducting such a study in field is seldom opted due to the requirement of expensive paraphernalia such as seismic equipment, pressure coring tools, logging while drilling tools, modular dynamic tester, etc. With this in view, efforts were made to synthesize CO 2 hydrates in partially saturated fine-sands under triaxial conditions by employing the excess gas method. Ultrasonic wave transducers were employed in tandem to measure the compressional (V P) and shear wave (V S) velocities of the sample, both under formation and dissociation processes. These results have been utilized to propose hydrate formation and dissociation models which encompass three phases each. The rapid reduction in V P and V S observed during thermal dissociation indicates loss of cementation between the sand grains and hydrates, and commencement of hydrate dissociation at their interface. It has been demonstrated that the induction time for CO 2 hydrate formation increases, and the water-to-hydrate conversion ratio decreases, with an increase in the initial volumetric water content. Further, the results generated in the present study were compared vis-à-vis those available in the literature for CH 4 and CO 2 hydrate-bearing sediments (HBS) and it has been found that both V P and V S of the CO 2 - and CH 4 - HBS are comparable for a given hydrate saturation. [ABSTRACT FROM AUTHOR]

This article introduces a novel testing system for investigating rock hydromechanical behavior with neutron and X-ray imaging techniques. The system comprises four subsystems: an axial compression system, a confining pressure system, a fluid flow system, and a triaxial cell. In order to enable imaging with both modalities to track fluid flow and deformation in situ and 3D, the cell was designed to have sufficient transparency to X-rays and neutrons. The system's capabilities are demonstrated by showing neutron and X-ray tomography data of Idaho Gray sandstone samples during in situ coupled flow-triaxial tests. Quasi-single-phase flow analysis was enabled by directly visualizing the fluid front advance via neutron tomography by exchanging light water (H 2 O) and heavy water (D 2 O). Digital Volume Correlation (DVC) could be performed on both the X-ray and neutron tomography images to quantify the mechanical strain field evolution in the samples, which can be compared to the evolution of the fluid flow fields. In addition, the cell and a sample of Idaho Gray sandstone were imaged in a laboratory X-ray tomography machine, generating 3D reconstructed volumes with grain-scale resolution. Improvements are proposed for future experiments to obtain grain-scale resolution images during coupled flow-triaxial tests using neutron and X-ray beams simultaneously and conducting in situ experiments on laboratory tomographs. • A novel system for conducting flow and triaxial tests with neutron and X-ray imaging. • The triaxial cell is sufficiently transparent to both X-rays and neutrons. • 3D flow path tracking can be achieved using high-speed neutron tomography. • Digital volume correlation can be performed from X-rays and neutron images. [ABSTRACT FROM AUTHOR]

The paper studies the effect of soil strength and stiffness degradation on the undrained cyclic performance of offshore foundations in low-plasticity cohesive soil using 3D finite element modelling. Cyclic triaxial tests on reconstituted kaolin are conducted at the ETH Zurich laboratory, providing insights into key parameters affecting the degradation process. A simplified soil constitutive model accounting for cyclic degradation is developed and encoded in Abaqus via a user subroutine. The model is calibrated against experimental results and validated with published centrifuge model tests of monopiles under cyclic lateral loading. It is subsequently used to evaluate the performance of suction caisson foundations with different aspect ratios (L/D = 0.5 and 2) under short-term cyclic and seismic loading. Due to its ductile resistance mechanism, the L/D = 0.5 caisson exhibits superior performance under vertical cyclic loading in fast-degrading soil. Under inclined cyclic loading, the slower degradation rate of the L/D = 2 caisson governs response, reversing the trend. Under seismic shaking, the degradation-induced resistance imbalance amplifies the irrecoverable settlements produced by kinematic shearing at the caisson sidewalls. For the fast-degrading soil examined, degradation is shown to increase settlements by up to 50%. • Cyclic strength and stiffness degradation of cohesive soils is explored via undrained triaxial tests on kaolin samples. • A simplified total stress-based constitutive model is developed for capturing such effects. • The model is validated against published centrifuge tests of a monopile subjected to lateral cyclic loading. • The undrained behaviour of caissons under cyclic and seismic loading is assessed, considering cyclic degradation effects. • For the case of fast-degrading soil examined, soil degradation may increase foundation settlements by up to 50%. [ABSTRACT FROM AUTHOR]

Oil wells subjected to cyclic steam injection often present cement sheath failures due to thermal loads. Currently, different Portland based systems are employed to mitigate this risk. We developed and tested geopolymer systems that meet oil well cementing requirements and achieve high mechanical performance. A metakaolin-potassium-based formulation, containing microsilica, mineral fiber and retarder, was adjusted to the desired rheology and thickening time. Triaxial compression and indirect tensile tests were performed and the properties were used in finite element simulations of the life of a model onshore well. Compared to the Portland based system currently in use, the geopolymer has superior uniaxial compressive strength and lower stiffness. Tensile strength increased significantly with the addition of mineral fibers and became higher than that of the Portland system. Numerical simulations show that the geopolymer can withstand steam injection temperatures comparable to a conventional system, remaining in the elastic region. These results show that geopolymers are a viable alternative well cementing system for this application, with significant improvement in mechanical performance. [ABSTRACT FROM AUTHOR]

Abstract Gas sorption in coal, in particular CO 2 , is known to cause swelling and internal structural change or damage. For instance, CO 2 adsorption on coal causes swelling which in turn can close the fractures or cleats whereas its desorption can open pre-existing fractures as well as create new fractures, thus enhancing gas flow paths in coal seams. Gas release and the associated induced damage are relevant for several applications such as coal burst in coal mining operations and for geological sequestration of CO 2 into coal seams to reduce both the carbon footprint and increase the efficiency of energy extraction. However, the sorption-induced internal structural damage in coal, in particular the link between the micro-scale damage phenomena to macro-scale processes in confined and unconfined stress states, is not yet fully understood. We carried out a series of experiments to study the potential sorption-induced damage in both confined and unconfined stress states at micro (μm) and macro (cm) scale. 3D images, obtained by X-ray microcomputed tomography (micro-CT) technique, showed that in the unconfined stress state, adsorption of CO 2 closes some pre-existing fractures while new fractures form in the specimen of coal. This is well supported by high pressure CO 2 adsorption analyses that were independently conducted. After CO 2 release, the overall fracture intensity, defined as the area of fractures per volume of rock mass, was considerably increased by opening both pre-existing fractures together with new fractures. In the confined condition, the gas adsorption was significantly lower confirming the closure of the initial fractures by swelling without creation of induced fractures. However, the gas release under shearing stress was significantly higher showing that the shearing stress causes further damage assisted by gas release and its induced micro-structural damage. Highlights • Multi-scale sorption-induced damage in coal in confined and unconfined stress states was studied. • Micro-CT was able to map the changes in the fracture system due to CO 2 sorption. • Gas release is unlikely to cause coal failure but it can induce micro-fractures. • Gas release can cause coal failure if the sample is under shearing stress. • Macroscopic tensile failure is dominant when sample is failed by gas release. [ABSTRACT FROM AUTHOR]

Studies of structural anisotropy of rocks contribute to the understanding of their mechanical behaviour variation in a broad spectrum of geological settings. In this work we characterise the lithological variation and the mechanical behaviour of the Trendelburg beds, a fine-grained subarkose from the Bunter Sandstone, with moderate effective porosity (10%) and low permeability (0.5 mD). Traditional triaxial compression tests were carried out with varying confining- and pore pressures in water saturated specimens (7 cm diameter ×14 cm length). Ultrasonic velocity, permeability, deformability, compressive strength, mechanical work and fracture pattern were determined in two directions of anisotropy (0° and 90°) with respect to bedding. Changing the angle of anisotropy leads to different reactions to the influence of lithological heterogeneities, which reaches a maximum when arranged parallel to σ 1 . The Trendelburg beds has significant anisotropy effects, which tend to increase with effective pressure. The effects of a structural anisotropy due to bedding are associated with an anisotropy of physical properties, stress state, pore pressure and likely pore space distribution. Mechanical properties are direction dependent, however, the influence of lithological factors may diverge between both directions of anisotropy as pressure increases. [ABSTRACT FROM AUTHOR]

CO 2 geological storage in saline aquifers results in acidification of resident brine. Chemical reactions between acidified brine and rock minerals lead to dissolution and precipitation of minerals at various time scales. Mineral dissolution and precipitation are often neglected in assessing the mechanical integrity of target storage formations, yet, changes in rock strength and deformational behavior can impact trapping mechanisms. This study shows the impact of exposure to CO 2 -charged brine on shear strength and stiffness of various outcrop rocks evaluated through triaxial testing. The tested rocks were exposed to CO 2 -charged brine over geological time at a naturally occurring near-surface seepage along the Little Grand Wash Fault and Salt Wash Grabens, which include the Crystal Geyser site near the town of Green River, Utah. Prior work suggests that this site provides a near-surface structural analog for possible fault-controlled CO 2 leakage over time scales that exceed expected injection time scales (10–100 years). Results show mechanical alteration in various aspects: (1) CO 2 -charged brine alteration at near-surface conditions results in mineral dissolution/precipitation and reduction of shear strength and brittleness of Entrada sandstone and Summerville siltstone samples, and (2) carbonate precipitation in fractured Mancos shale leads to matrix stiffening and fracture mineralization resulting in overall stiffer and likely tighter shale. Additional discrete element simulations coupled with a bonded-particle-model confirm the role of cement bond size alteration as one of the main controls for rock chemo-mechanical alteration in sandstones. The chemo-mechanical alteration path that mimics cement dissolution (under stressed subsurface conditions) results in vertical compaction and lateral stress relaxation. Overall, results show that rock exposure to CO 2 -charged brine can impart distinct petrophysical and geomechanical changes according to rock lithology and location with respect to major CO 2 conduits. While mineral dissolution in the storage rock may result in undesired reservoir strains and changes of stresses, mineral precipitation downstream from a leakage path can help seal potentially induced fractures. [ABSTRACT FROM AUTHOR]

Acoustic emission (AE) monitoring offers the potential to sense particle-scale interactions that lead to macro-scale responses of granular materials. However, there remains a gap in fundamental understanding of how particle-scale mechanisms and properties influence AE generation, which limits the application of AE monitoring and interpretation of AE measurements. Addressing this gap in knowledge was the principal focus of this study. A benchmarking study was conducted first whereby a programme of seven 3D DEM simulations of drained triaxial tests with energy tracking were performed and compared with experimental measurements, which ensured the adopted simulation approach captured realistic behaviour. Dissipated plastic energy was influenced in the same way as measured AE by imposed confining pressure, displacement rate, and load-unload-reload compression and shearing. A parametric analysis was subsequently conducted using a programme of 25 3D DEM simulations. The findings show that sliding and rolling friction were the dominant mechanisms in plastic energy dissipation and hence particle-scale properties that influence sliding and rolling friction have the most significant influence on AE generation (e.g., particle shape, surface roughness, hardness). Relationships have been established to quantify how changes in particle-scale properties in DEM (sliding and rolling friction coefficients, normal and shear stiffness and their ratio, and local damping coefficient) lead to changes in dissipated plastic energy (R2 of 0.99); however, it should be noted that the relations in the sensitivity analysis cannot be interpreted as representative of specific granular materials. This new knowledge enables improved interpretation of AE and underpins the development of theoretical and numerical approaches to model and predict AE behaviour in particulate materials. [ABSTRACT FROM AUTHOR]

The presence of veins in the rock matrix influences the behaviour of a rock. This results in a behaviour that is distinct from intact rock without any veins and structure and from jointed rock. In this context, intact veined rock is a rock containing mineral veins and no joints. This paper investigates the influence of vein mineralogy, thickness, and orientation on the strength of intact veined andesite from the El Teniente mine. This investigation has focused on a comprehensive vein characterization and analyses of the results of triaxial compression experiments. In a comprehensive laboratory investigation, it was demonstrated that the peak strength of intact veined rock specimens was strongly influenced by the orientations of individual veins, whereby vein mineralogy and thickness had a lesser role. A further analysis of the mineral composition of the intact veined rock specimens following a series of triaxial tests revealed a more dominant influence of the mineral composition of the veins. It was shown that rock specimens dominated by veins containing < 30% of hard minerals, i.e. minerals with Moh's hardness > 4, attained significantly lower peak strengths than specimens where the average content of hard minerals exceeded 60% of the vein infill. Analyses of the mobilised shear strength of individual veins demonstrated a strong correlation between the presence of hard minerals and the strength of individual veins in the rock specimens. This relationship was stronger when the vein volumetric content of hard minerals was < 40–50%. For veins, in which the hard mineral content exceeded the 40–50% threshold, the estimated strength values were influenced by mineral content to a lesser degree. [ABSTRACT FROM AUTHOR]

One major objective of rock mechanics testing in a laboratory is to characterize strength and deformational behaviours under in-situ stress state. It is a well-known fact that in-situ rock masses are under a stress state with three principal stresses i.e. triaxial stress state. Knowledge of the mechanical behavior of rocks under triaxial stress conditions comes mainly from the Karman-type triaxial tests (conventional triaxial test) where a cylindrical specimen is axially loaded keeping the lateral load constant. However, the stress path that is specific to these tests is certainly not unique in practical situation. Loading direction in-situ rock masses subjected to engineering or tectonic processes can be arbitrary orientation where principal stress axes can have arbitrary orientation with respect to vertical and horizontal direction. It is difficult to carry out these types of tests because of the difficulties in controlling the stress in those arbitrary orientations and also due to the fact that no detailed guideline on the methodologies of these tests is available in literature. Therefore, with due need, starting from the background of the triaxial testing, this paper will discuss the technical aspects of developed experimental methodologies for these tests in our servo controlled rock mechanics system from MTS corporation and are verified with the case study of deformational behavior in selected special stress regimes. This article will also address the difficulties that one can face during the testing along with their possible solutions. [ABSTRACT FROM AUTHOR]

An experimental study was carried out on grout samples prepared from both Stratabinder and BU100 cementitious products. Samples were prepared with various water to grout ratios and tested for uniaxial compressive and shear strength. Triaxial tests were performed on cylindrical samples to determine values for internal friction angle, cohesion and tensile strength. It was found that the water to cement ratio affects the uniaxial compressive and shear strength of grout. The triaxial test indicated that both internal friction angle and cohesion of Stratabinder do not differ significantly from BU100. [ABSTRACT FROM AUTHOR]

In material sciences, it is a well-known fact that linear or linearized theory based on Hooke's law does not offer a satisfactory description of solids in special regimes, which include e.g. too high strains under large uniaxial stresses. Therefore, in general, the response to biaxial or triaxial loading cannot be obtained as superposition of uniaxial load responses. Striking paper book example of material demonstrating such behavior is rubber subjected to uniaxial or isotropic compression. Despite this fact, linear mechanical moduli, being secant or differential, determined through standard rock-mechanics tests, mostly from the uniaxial compression, are still widely used for description of deformational behavior of rocks. Without doubt, an appropriate interpretation of these effective quasi-elastic or stiffness moduli can give useful information about mechanical properties of the rocks, especially in comparative sense. However, for more reliable constitutive modeling of any solid materials, paricularly rocks, an experimental investigation of deformational responses to uniaxial, biaxial and triaxial loading or unloading regimes is very useful. This contribution presents the results of an experimental case study on homogeneous sandstone exposed to isotropic triaxial, and equi-biaxial or uniaxial loading regimes. The measured deformational response of this rock is compared with behavior of elastic solid materials. Finally, benefit of the experimental testing for constitutive modeling based on phenomenological description is briefly discussed. [ABSTRACT FROM AUTHOR]

The article presents the stress-strain and volume change behavior, shear strength and stiffness parameters of landfilled municipal solid waste (MSW) collected from two dump sites located in Delhi, India. Over 30 drained triaxial compression (TXC) tests were conducted on reconstituted large-scale specimens of 150 mm diameter to study the influence of fiber content, age, density and confining pressure on the shear strength of MSW. In addition, a few TXC tests were also conducted on 70 mm diameter specimen to examine the effect of specimen size on the mobilized shear strength. It is observed that the fibrous materials such as textiles and plastics, and their percentage by weight have a significant effect on the stress-strain-volume change behavior, shear strength and stiffness of solid waste. The stress-strain-volume change behavior of MSW at Delhi is qualitatively in agreement with the behavior reported for MSW from different countries. Results of large-scale direct shear tests conducted on MSW with an identical composition used for TXC tests revealed the cross-anisotropic behavior as reported by previous researchers. Effective shear strength parameters of solid waste evaluated from this study is best characterized by ϕ ′ = 39° and c ′ = 0 kPa for the limiting strain-based failure criteria of K 0 = 0.3 + 5% axial strain and are in the range of the data reported for MSW from different countries. Data presented in this article is useful for the stress-deformation and stability analysis of the dump sites during their operation as well as closure plans. [ABSTRACT FROM AUTHOR]

Triaxial strength tests were performed on artificially jointed standard-size granite specimens with two sub-vertical and three sub-horizontal (2+3) joints. A fully servo-controlled press was used to load the samples and attain residual strength. These tests add to an existing test database for intact and artificially jointed (1+2 joints) samples of the same rock. The main parameters from each test were used to analyse tendencies, in particular, changes in elasticity and peak and residual strength in line with increased jointing. Peak strength and apparent Young's modulus decreased from the intact to the jointed specimens, reflecting a tendency analogous to that deriving from the use of classification systems to characterize rock masses. Mohr-Coulomb and Hoek-Brown failure criteria were fit to peak and residual strength tests results. In the case of the Hoek-Brown criterion, it seems that an equivalent of the geological strength index (GSI), which reflects structure at the specimen scale, could be of use in estimating the decrease in strength of increasingly jointed samples. Residual strength envelopes were practically the same for the intact and jointed specimens. Our results for granite would suggest that the so-called rock mechanics scale effect at large (i.e. the relevant change in material properties from rock samples at lab scale to that of the rock mass including the occurrence of natural discontinuities at the engineering work scale), is more due to structure (number, orientation, spacing and features of occurring joints), than to sample scale effect (i.e. the variation in properties when testing different size, from a few mm to some decimetres, unjointed intact rock samples at the laboratory). [ABSTRACT FROM AUTHOR]

• Stress- and temperature-dependent mechanical behavior of CRM was evaluated. • CRM were prepared with 100% RAP and bitumen emulsion using an SGC. • The effect of adding 1% of Portland cement or hydrated lime was assessed. • Active filler additions affected the compactability and mechanical properties of CRM. • Master curves based on a new proposed stress–temperature formulation were plotted. The asphalt paving sector is currently embracing and enhancing cold mixture technologies to reduce carbon emissions and decarbonize its operations. Cold recycled mixtures (CRM) have proven to be a promising alternative in road construction and rehabilitation, becoming a primary research focus. This study evaluates the effects on compactability and volumetric properties of adding 1% Portland cement or 1% hydrated lime to CRM made with 100% reclaimed asphalt pavement and bitumen emulsion. Dynamic triaxial tests with different confinement pressures (ranging from 20 to 200 kPa) and temperatures (5, 15, 25, and 35 °C) were used to determine the influence of these active filler additions on the resilient modulus (Mr) of the mixtures. All the mixtures exhibited increased Mr values at lower temperatures and reduced stress–dependency. Cement addition had the most favorable effect on compactability, reducing the necessary compaction energy, while mixtures with hydrated lime had more substantial increases in Mr and reduced stress dependencies. Finally, the results were analyzed by plotting master curves using a proposed novel approach called Stress–Temperature Superposition Principle (STSP), which allows for simpler and more straightforward analyses. [ABSTRACT FROM AUTHOR]

A comprehensive experimental program was instigated to test intact veined andesite from the El Teniente Mine (Chile) under triaxial compression with confining pressure ranging from 2 to 60 MPa. The experimental program established that the veins acted as weak mechanical components in the specimens, promoting rock fracture under stress. The experimental results indicated that the stress thresholds identifying the onset of dilatancy and the transition from the stable to unstable fracturing were higher in intact veined rock than in intact rock. The observed behaviour illustrated a significant departure from intact rock where the onset of Acoustic Emissions (AE) correlates with the onset of specimen dilatancy. The experimental results from intact veined rock indicated that the onset of AE correlated with the crack damage stress. The departure from the behaviour of intact rock is attributed to the presence of veins. [ABSTRACT FROM AUTHOR]

In rock masses containing multiple joint sets the joints will intersect. The geometric nature of such joint intersections can influence the hydro-mechanical response of jointed rock. An undrained experimental study was performed on fully-saturated, doubly-jointed sandstone specimens. Testing considered a variety of joint orientations and interconnected joint configurations, and various values of confining and initial pore-water pressure. Test results revealed three failure mechanisms: (1) shearing (through intact material); (2) crushing failure in the vicinity of the joint junction point (with associated sliding on joints), and (3) sliding along a preferred joint plane. Using these results, a ‘failure mode matrix’ was developed to assist in prediction of failure mechanisms for different joint angles and intersection configurations, at different confining pressures. Greater peak induced pore-water pressures were observed for symmetric interconnected joint configurations than skew-symmetric configurations, for all confining pressures, though the difference was significantly more pronounced at higher confining pressures. Greatest peak strength values were obtained for joint geometries in which both interconnected joints were at a low angle. When both joint angles were greater than the friction angle of the joints, peak strength did not show a noteworthy difference between symmetric and skew-symmetric joint configurations for relatively low confining pressures. However, at higher confining pressures, skew-symmetric joint configurations showed considerably higher peak strength than symmetric joint configurations. These behaviors were attributed to differences in failure mechanism (between symmetric and skew-symmetric specimens) and the influence of failure mechanism and confining pressure on end friction and specimen response. [ABSTRACT FROM AUTHOR]

Well cement is the most common barrier material in wells for geothermal and hydrocarbon production. As cements are exposed to hydrostatic loads and periods of deviatoric loading in wellbore environments, it is important to understand the mechanical behavior of cement under relevant conditions. We study effects of porosity, saturation, confining pressure and drainage conditions on the mechanical behavior of class G well cement using two basic formulations, one which produces a highly porous cement paste. The high-porosity cement exhibited lower strength and reduced elastic moduli compared to the stabilized formulation. The elastic moduli for both formulations were reduced with increasing confining pressure, and the most pronounced effect of confinement was increased ductility and pronounced strain hardening behavior of the two cements, likely due to compaction. Under saturated and undrained conditions, the stabilized cement exhibited an increase in stiffness and essentially brittle failure even at 20 MPa confining pressure. The porous cement showed less sensitivity to drainage conditions. We attribute this observation to possible generation of internal micro-cracks and dislocations instead of a macroscopic failure plane. The results contribute to increased understanding of the mechanical response of conventional and porous cements under relevant confining pressures and different saturation and drainage conditions. [Display omitted] • Triaxial testing of oil well cement at different drainage and saturation conditions. • Mechanical strength and elastic moduli decrease with increasing cement paste porosity. • Confining pressures promote cement paste ductility and results in strain hardening. • Saturated and undrained test conditions result in brittle failure under confinement. [ABSTRACT FROM AUTHOR]

Membrane penetration effect can be observed on the cylindrical specimen in the consolidated drained triaxial test when the rubber membrane is deflected into the space among soil particles over its side. It occurs gradually upon the commencement of consolidation stage in which the soil sample experiences the elevation of cell pressure. The volume of the specimen and stress-strain interpretation in consolidation and shearing stages respectively are affected by considering the membrane penetration effect. This study presents a consolidated drained triaxial testing on granitic residual soil grade V under effective confining pressure of 50, 100, 150, 200, 300, 350 and 400 kPa. The testing of effective confining pressure consisted of real specimens and dummy specimens. For real specimens, compacted granitic residual soil in cylindrical with initial diameter of 50 mm and 100 mm height was used. As for dummy specimens, a solid rigid cylindrical core was enclosed with a thin layer of soil material. Dummy specimens were used to observe the behaviour of membrane penetration in triaxial testing. The graphical method was applied in finding the effect of considering membrane penetration in volume changes of the soil sample accurately. The compliancy of the soil material used towards the graphical method has also been determined. The proportion of membrane penetration from this study ranged between 25% and 58%. The obtained angle of friction at failure, φ' of specimen with and without membrane correction was 27° and 25° respectively, which is insignificantly different. Further investigation is required by considering other factors including specific soil particles size, larger size of soil sample, and variety of thickness and materials of the rubber membrane. • Membrane penetration effect is observed in consolidated drained triaxial test. • Granitic residual soil grade V, real specimen, dummy specimen. • Graphical method is utilized to determine the actual volume change and effective stress at failure. • Confining effective stress increases, volume change increases. • Stress-strain curves, non-linear shear strength envelope, FoS of a slope stability. [ABSTRACT FROM AUTHOR]

Recent research efforts at the University of Illinois have aimed at studying geogrid applications in railroad track structures, specifically focusing on ballast and subballast reinforcement. Ballast, typically comprising large sized aggregate particles with uniform gradation, is an essential layer in the railroad track substructure to facilitate load distribution and drainage. The primary mechanism of load transfer within the ballast layer involves inter-particle contact between ballast particles. Similarly, the effectiveness of ballast reinforcement with geogrids is primarily governed by the geogrid-aggregate interlock. Such interaction and the effectiveness thereof can change significantly as the level of grain size and shape degradation or fouling increases in the ballast layer with accumulation of train traffic. Although several studies in the past have investigated the effects of geogrid reinforcement on ballast shear strength and permanent deformation behavior, the effectiveness of geogrid reinforcement at different levels of ballast degradation needs to be further understood. In this study, monotonic triaxial shear strength tests were conducted on both new and degraded ballast materials with and without geogrid reinforcement. Two geogrid types, with square- and triangular-shaped apertures, were used in the laboratory to calibrate an aggregate imaging-based Discrete Element Method (DEM) modeling approach, which is capable of creating actual ballast aggregate particles as three-dimensional polyhedron blocks having the same particle size distributions and imaging quantified average shapes and angularities. The DEM model was observed to adequately capture the shear strength behavior of geogrid-reinforced triaxial ballast specimens prepared using both new and degraded ballast samples. [ABSTRACT FROM AUTHOR]

Recent publications have emphasised the importance of addressing cyclic behaviour when designing piled foundations. Laboratory tests may be conducted to provide site-specific cyclic soil characteristics, but questions arise concerning: (i) how to take into account the pile installation process and (ii) how to apply the results to assess pile capacity and deformation responses under cyclic loads. This paper describes an investigation into the cyclic behaviour of Dunkerque and NE34 Fontainebleau sands, performed to support and to help analyse field-scale and model pile cyclic loading tests on the same soils. Forty computer-controlled, locally instrumented, cyclic and static triaxial tests were performed, following testing schemes that were developed to reflect the conditions adjacent to the pile shafts. Assessments were made of how the cyclic variations in stress imposed during installation and the period allowing for the two types of sand to creep following such ‘installation’ effects, affect the response to subsequent cycling. Constant-volume cyclic tests, involving up to 4500 cycles, were imposed from alternative sets of initial conditions that revealed the relationships among the cyclic deviatoric amplitude, the changes in mean effective stress and the number of cycles as well as the permanent strain accumulation and the cyclic stiffness characteristics. Monotonic compression and extension tests are also presented for both sands to help frame their strength, stiffness and critical state properties. A synthesis with previously obtained cyclic pile test trends confirms the practical applicability of the results obtained. [ABSTRACT FROM AUTHOR]

Understanding the physical nature and mechanical behaviour of hydrate-bearing sediments is of fundamental importance in assessing the resource potential of methane gas hydrates. Advances in pressure coring techniques and associated processing equipment have enabled intact samples to be recovered under in-situ pressures. However, testing of these samples under the in-situ stress conditions has not been possible. To help address this issue, the PCATS Triaxial apparatus was developed to enable the physical properties of such samples to be measured. The apparatus was deployed for the first time during the JOGMEC funded site investigation of the Eastern Nankai Trough in the summer of 2012, prior to a planned hydrate production test in 2013. A number of pressurized core were recovered and sub-samples successfully tested in PCATS Triaxial to determine a range of geomechanical properties, including small strain stiffness (from resonance testing), stress–strain properties (triaxial shear tests) and permeability. Samples tested included fine-grained soils with no appreciable hydrate, sands with hydrate saturation greater than 20%, and one sample that had a combination of both materials. Testing showed an increase in stiffness and undrained shear strength with increasing grain size, hydrate saturation and applied effective stress. Permeability was significantly reduced for hydrate-bearing sands compared to clayey samples with no hydrate present. [ABSTRACT FROM AUTHOR]

Sand–tyre chip (STCh) mixtures can be used in many geotechnical applications as alternative backfill material. The reuse of scrap tyres in STCh mixtures can effectively address growing environmental concerns and, at the same time, provide solutions to geotechnical problems associated with low soil shear strength and high dilatancy. In this paper, the shear strength and dilatancy behaviour of STCh mixtures have been investigated. A series of monotonic triaxial tests has been carried out on sand mixed with various proportions of tyre chips. It has been found that tyre chips significantly influence the shear strength and the dilatancy behaviour of STCh mixtures. The effects of confinement and relative density on the shear strength, dilatancy and initial tangent modulus of the STCh mixtures have also been investigated. Moreover, a dilatancy model for STCh mixtures has been proposed and validated with the experimental results. [ABSTRACT FROM AUTHOR]

Two model soils, one made from sand with plastic fines and one made from sand with non-plastic fines, were tested in a triaxial apparatus to examine the basic mechanics of transitional soils, that is, soils whose initial density has a very strong influence on the behaviour. The sand with non-plastic fines defined parallel critical state lines and state boundary surfaces at the medium strain levels that can be reached in triaxial testing, the locations of which depended on the initial specific volume of the soil when it was created. Overconsolidating the soil, or shearing it a second time at an increased stress level, was not found to change this behaviour. There was also no significant effect of the initial specific volume on the soil stiffness, which was predominantly a function of the current stress level for normally consolidated states. A variety of different sample preparation methods was used, but the methods did not have any clear influence on the critical state volumes. Although the testing for the sand with non-plastic fines was more limited in extent, the patterns of behaviour seemed to be similar. The relationships between the initial specific volume and the critical state line intercept, Γ , gave gradients of 0.52 and 0.97 for the sands with plastic and non-plastic fines, respectively, showing significant differences in volume remaining after isotropic compression and monotonic compressive shearing. [ABSTRACT FROM AUTHOR]

Abstract: We investigated the complete stress–strain response of artificially jointed granitic rock specimens with two joint sets. A series of confined compressive tests on these jointed rock specimens were performed in a fully servo-controlled press. Elastic, strength and post-failure parameters were calculated and analysed, with special attention paid to the dilation angle. The results were compared to those for corresponding intact specimens investigated in a previous study. Although the peak strength of the jointed cores was much lower than that for the intact cores, the residual strength envelopes were practically equal. The apparent elastic and drop moduli tended to be lower for jointed specimens than that for intact specimens. At low confining stress levels, the jointed rock tended to dilate less than the intact rock, but at higher confinement dilation behaviour converged. Another finding with respect to post-yield behaviour was that the dilation angle of the jointed specimens could adequately be represented by an existing plastic-shear–strain and confinement-dependent model developed for intact specimens. [Copyright &y& Elsevier]

Moraine soils are widely distributed in the Qinghai-Tibet Plateau. With increased global warming, moraine soil-related geohazards have become increasingly common, posing a serious threat to the infrastructure (e.g., the Sichuan–Tibet Railway) and inhabitants of this region. This paper aims to investigate the mechanical behavior of ice-rich moraine soil in the Tianmo Valley by conducting a series of triaxial constant strain rate (CSR) and coupled thermomechanical (CTM) tests on artificially moraine soil containing different ice forms (crushed ice and block ice). The results show that in general, compared to moraine soil containing crushed ice, moraine soil with block ice has a higher peak strength, a similar internal frictional angle and a considerably larger cohesive strength. The stress–strain curve of the soil containing crushed ice shows a strain-hardening form, while that of the soil containing block ice is represented by a strain-softening model similar to that of dense soil. In the CTM tests, it is revealed that the rising temperature could cause a sharp increase in strain and lead to sample failure, even when the axial load is far below the material's peak strength. A comparison between samples with different ice forms reveal that the soil containing crushed ice is more sensitive to temperature change. The tests demonstrate that the ice form has a significant influence on the mechanical behavior of moraine soil, and the temperature rise can result in a dramatic decrease in soil strength. Therefore, efforts should be made to detect the occurrence form of buried ice and the changes in the environmental temperature as well as the stress state of moraine soil slopes in situ. • The strength of the moraine soil containing block ice is higher than that of the soil containing crushed ice. • The stress-strain curve of the soil containing crushed ice shows a strain-hardening form. • The stress-strain curve of the soil containing block ice is represented by a strain-softening model. • The mechanical behaviors of samples containing crushed ice are more sensitive to temperature rise than that of block ice. • A stress threshold value exists when the temperature rise acts as a dominant role affecting the deformation of moraine soil. [ABSTRACT FROM AUTHOR]

• Numerical simulations of cold in-place recycling sections were conducted. • The evolution of the CIR material stiffness with curing was analysed. • The influence of the CIR-base thickness and the effect of the wearing-course were studied. • Response in sections with flexible subbases was more sensitive to variation in the parameters. • Response in sections with semi-rigid subbases was more stable and had a higher rutting resistance. Cold in-place recycling (CIR) technologies are becoming one of the main bets in the road sector to promote the reduction of greenhouse gas emissions. This technique also contributes to the circular economy, reusing 100% of the RAP from worn roads. In this research, numerical simulations of CIR sections are presented. The nonlinear behaviour of the CIR-base material is modelled using three predictive models based on triaxial test results. Variations in the performance depending on the type of subbase, the curing of the CIR-base material and its thickness, and the effect of the wearing-course were analysed. The response of the sections with unbound granular subbase proved to be very sensitive to variations in the parameters studied, and the increase in CIR base thickness was beneficial, while the opposite occurred with a cement-treated subbase. [ABSTRACT FROM AUTHOR]

Abstract: Any failure criterion can be represented as a surface in principal stress space σ 1, σ 2, σ 3 (with no order implied), and the shape of the surface depends on the functional form of the criterion. For isotropic rock that exhibits a pressure dependence on strength, the simplest failure criterion is a linear function, and the failure surface is a hexagonal pyramid with a common vertex V o on the tension side of the hydrostatic axis, where V o =(theoretical) uniform triaxial tensile strength. An example of a pyramidal failure surface is the popular Mohr–Coulomb criterion, which is independent of the intermediate principal stress σ II (σ I ≥σ II ≥σ III ) and contains two material parameters, such as V o and the internal friction angle ϕ. The Paul–Mohr–Coulomb failure criterion Aσ I +Bσ II +Cσ III =1 is linear with three principal stresses, and it is formulated with three identifiable material constants, where A=(1−sinϕ c )/(2V o sinϕ c ), B=(sinϕ c −sinϕ e )/(2V o sinϕ c sinϕ e ), C=−(1+sinϕ e )/(2V o sinϕ e ) and ϕ c , ϕ e are internal friction angles for compression (σ II =σ III ) and extension (σ I =σ II ). The convex nature of the failure surface at constant mean stress can be approximated by additional planes with appropriate material parameters. To demonstrate the utility of the linear failure criterion, a series of conventional triaxial compression and extension experiments were performed on an isotropic rock. The results were processed using the developed data fitting techniques, and the material parameters for the six-sided pyramidal failure surface were determined. A multi-axial experiment was also performed to evaluate the convexity of the failure surface, and a twelve-sided pyramid was constructed and the appropriate equations were derived. [Copyright &y& Elsevier]

Abstract: Carbon dioxide (CO2) sequestration in deep, unmineable coal seams may provide an immediate and economically feasible solution for mitigation of anthropogenic CO2 emissions. Coal contains natural cleats, which largely control fluid movement in coal seams. This study uses experimental and numerical methods to investigate the variables (i.e. injection pressure, injection depth, and coal temperature) that will influence cleat performance in the CO2 sequestration process for black coal. The steady-state downstream pressure (and pressure differential from the injection value) that develops during undrained triaxial tests performed on fractured black coal, without deviatoric load, is taken as a measure of cleat performance. The results of tests show that increasing injection pressure has a significant detrimental effect on cleat performance, in accord with greater adsorption-induced swelling (and cleat closure) with higher injection pressure. Testing also shows that increasing injection depth has a moderate (negative) effect on cleat performance and that temperature has a minor (positive) effect on cleat performance. The experimental results were used to validate a numerical model produced using the COMET 3 numerical simulator, which was used to further explore coupling between the variables that influence cleat performance in coal-seam CO2 sequestration. The numerical results show that the influence of injection pressure on cleat performance is dependent on injection depth, whereby a given increase in injection pressure will have more detrimental effects on cleat performance at greater depths than it would at more shallow depths. The influence of injection depth on cleat performance is similarly dependent on the absolute depth range considered. Coupling was observed in the influence of temperature on cleat performance and both injection pressure and depth. Modeling shows that increasing temperature has a negative influence on cleat performance for low injection pressures (i.e. 6MPa), but a positive influence on cleat performance for higher injection pressures (i.e. 12MPa). Modeling shows that temperature will have an influence on the manner by which depth effects cleat performance if injection pressures are high (i.e. 12MPa), but no such influence if injection pressures are low (i.e. 6MPa). [Copyright &y& Elsevier]

Abstract: A comprehensive laboratory investigation was performed on municipal solid waste (MSW) from a landfill located in northern California using a large-scale triaxial (TX) apparatus. An improved, standardized waste specimen preparation method was developed and used to prepare 27 large-scale TX specimens (d =300mm, h =600–630mm). The effects of waste composition, confining stress, unit weight, loading rate, and stress path on the drained stress–strain response of MSW were investigated. Waste composition has a significant effect on its stress–strain response. The commonly observed upward curvature of the stress–strain response of specimens composed of larger-sized waste materials results from the fibrous constituents (primarily paper, plastic and wood) reinforcing the waste matrix. This effect is greatest when the MSW specimen is sheared across the long axis of the fibrous particles. Due to this significant strain hardening effect and waste’s in situ stress state, a limiting strain failure criterion of 5% axial strain from the K o field consolidation state is judged to be most appropriate. Results from this test program and data from the literature indicate that the TX compression secant friction angle of MSW varies from 34° to 44°, with 39° as a best estimate, at a confining stress of one atmosphere (assuming c =0). The friction angle decreases as confining stress increases. The friction angles measured in this testing program are representative of failure surfaces that are oriented at an angle to the predominant orientation of the long axis of the fibrous waste particles. These friction angles are higher than those obtained in direct shear tests where shearing typically occurs parallel to the orientation of the fibrous waste particles. [Copyright &y& Elsevier]

Abstract: This study investigates an application of genetic programming (GP) for modeling of coarse rotund sand–mica mixtures. An empirical model equation is developed by means of GP technique. The experimental database used for GP modeling is based on a laboratory study of the properties of saturated coarse rotund sand and mica mixtures with various mix ratios under a 100kPa effective stresses, because of its unusual behavior. In the tests, deviatoric stress, and pore pressure generation, and strain have been measured in a 100mm diameter conventional triaxial testing apparatus. The input variables in the developed GP models are the mica content, and strain, and the outputs are deviatoric stress, pore water pressure generation. The performance of accuracies of proposed GP based equations is observed to be quite satisfactory. [Copyright &y& Elsevier]

Abstract: In the 1920s and 1930s, Terzaghi and coworkers realized that the failure of various porous geomaterials under internal pore pressure is given through evaluating the failure function for the same materials at zero pressure, with ‘total stress plus pore pressure’ instead of ‘total stress alone’ as argument. As to check, probably for the first time, the relevance of this (‘Terzaghi''s’) failure criterion for trabecular bone, a series of poromechanical and ultrasonic tests was conducted on bovine and human trabecular bone samples. Evaluation of respective experimental results within the theoretical framework of microporomechanics showed that (i) Terzaghi''s effective stress indeed governs trabecular bone failure, (ii) deviatoric stress states at the level of the solid bone matrix (also called tissue level) are primary candidates for initiating bone failure, and (iii) the high heterogeneity of these deviatoric tissue stresses, which increases with increasing intertrabecular porosity, governs the overall failure of trabecular bone. Result (i) lets us use the widely documented experimental results for strength values of bone samples without pore pressure, as to predict failure of the same bone samples under internal pore pressure. Result (ii) suggests a favorable mode for strength modeling of solid bone matrix. Finally, result (iii) underlines the suitability of microfinite element simulations for trabecular bone microstructures. [Copyright &y& Elsevier]

Abstract: Coal permeability is sensitive to the effective stress and is therefore coupled to the geomechanical behaviour of the seam during gas migration. As coal shrinks with gas desorption and swells with adsorption, understanding this coupling to geomechanical behaviour is central to interpreting coal permeability. Existing coal permeability models, such as those proposed by Shi and Durucan (2004) and Palmer and Mansoori (1996), simplify the geomechanical processes by assuming uni-axial strain and constant vertical stress. However it is difficult to replicate these conditions in laboratory tri-axial permeability testing and during laboratory core flooding tests for enhanced coal bed methane. Often laboratory tests involve a hydrostatic stress state where the pressure in the confining fluid within the tri-axial cell is uniformly applied to the sample exterior. In this experimental arrangement the sample is allowed to undergo tri-axial strain. This paper presents two new analytical permeability model representations, derived from the general linear poroelastic constitutive law, that include the effects of tri-axial strain and stress for coal undergoing gas adsorption induced swelling. A novel approach is presented to the representation of the effect of coal sorption strain on cleat porosity and thus permeability. This involves distinguishing between the sorption strain of the coal matrix, the pores (or cleats) and the bulk coal. The developed model representations are applied to the results from a series of laboratory tests and it is shown that the models can predict the laboratory permeability data. As part of this characterisation the various sorption strains are identified and it is shown that the pore strain is significantly larger than (approximately 50 times) the bulk sorption strain. The models also provide further insight into how coal permeability varies with coal shrinkage and swelling and how the permeability rebound pressure depends upon the effective stress applied. [Copyright &y& Elsevier]

Abstract: The paper describes the development of a constitutive model for a poorly graded sand, which was used in geotechnical experiments on buried pipes (reported elsewhere). The sand was tested extensively in the laboratory to determine the state parameter constants. Triaxial tests on the sand included conventional drained triaxial compression tests, as well as more specialized shearing tests at constant mean effective stress and others at constant volume. Single element simulation of the triaxial tests was performed to validate the proposed constitutive model. The adopted model allowed non-linear elastic behaviour prior to yielding. After yielding of the sand, the state parameter-based model for the sand permitted non-associated plastic flow. Dilation and frictional strength were both dependent on the current value of the state parameter. The combination of laboratory testing and single element modelling resulted in the selection of a single set of material constants for the soil, which adequately described the full range of triaxial tests. Subsequently the model was applied to the problem of a plate loading test on the sand and the model predictions were compared with the test data. [Copyright &y& Elsevier]

• Monotonic and RLPD triaxial tests of CIR mixtures were conducted. • Shear parameters and Mohr–Coulomb envelopes were obtained. • Higher BC provided higher cohesion but was detrimental to internal friction. • Permanent strains under different SR were assessed, and two M−E prediction models were fitted. • Mixtures with 2.50% BC exhibited the best response to permanent deformation. The cold in-place recycling (CIR) technique is a road rehabilitation method suitable for a circular economy. Triaxial tests have proven to be suitable for studying the mechanical behaviour of cold mixtures. In this study, CIR specimens were prepared using a gyratory compactor with different proportions of bitumen emulsion. The Mohr–Coulomb diagrams and shear parameters were obtained by conducting monotonic triaxial tests at different confining pressures. The binder content provided adequate cohesion; however, an excess of binder reduced internal friction. Subsequently, repeated load permanent deformation triaxial tests were performed. Accordingly, critical stress ratios between 20% and 30% were obtained, and the mix with a 2.50% binder exhibited the best response. Lastly, two permanent deformation prediction models were fitted to the measured results. For high deformations, the Paute model underestimated the deformations, whereas the Huurman model demonstrated the best fit. [ABSTRACT FROM AUTHOR]

• The nonlinear elastic behaviour of CIR mixtures with 100% RAP was studied. • Triaxial tests were conducted assuming CIR behaviour similar to granular material. • The influence of different proportions of bitumen emulsion and water was assessed. • The evolution of the resilient moduli was observed for different curing times. • The resilient moduli obtained were adjusted using prediction numerical models. This study evaluates the short-term resilient behaviour of cold in-place recycled (CIR) asphalt mixtures with bitumen emulsion when they are more similar to non-cohesive granular materials than to conventional hot mixtures. Gyratory compacted CIR specimens were manufactured using different proportions of bituminous emulsion and water, and dynamic triaxial tests were conducted at different curing times. The resilient moduli were obtained experimentally and fitted using three numerical models. Characteristic nonlinear elastic behaviour and an increase in stiffness with curing time were observed. Water loss during curing and stiffness increase were found to be related. Mixtures with 2.50% residual bitumen and 2.75% added water showed the best short-term stiffness evolution. [ABSTRACT FROM AUTHOR]

Abstract: The experimental investigation of clayey rocks which are typical for cap rock formations above CO2 storage reservoirs is described and results are given. One series of claystone samples is gathered from the injection well of the Ketzin pilot test site in Germany. Another series comes from the same Keuper formation in a near-surface outcrop in the Southwest of Germany. The near-surface condition allowed drilling of samples with a standard size of 100 mm diameter and 200 mm height as well as large samples with a diameter of 550 mm and a height of 1200 mm. A special triaxial testing procedure is applied both on standard and large size samples which allows determination of the strength, stiffness and viscosity behavior of the rock in one path. A multi-stage technique (stepwise variation of the confining pressure) gives the strength behavior of each single sample. Stepwise varied deformation rates, over two orders of magnitude, lead to steps in the stress–strain-curve from which the viscosity index can be calculated. The viscosity index can be directly used in the Norton’s constitutive relations for a viscoplastic simulation. The comparison of results from standard and large samples shows that for clayey rocks a scale effect is negligible. Strength, stiffness and viscosity are not dependent from the sample size. The observed failure modes of the samples show that the transition from cataclastic to non-cataclastic behaviour occurs in the range of the applied levels of pressure and deformation rates even at room temperature. This transition limit is very important for the judgment of the sealing capacity of a cap rock. If numerical simulations result in deformation rates which, for the given pressure and temperature conditions in a cap rock formation during the CO2 injection, stay sub-critical for cataclastic behaviour, the formation of new cracks can be excluded. [Copyright &y& Elsevier]

Abstract: The poromechanical behaviour of hardened cement paste under isotropic loading is studied on the basis of an experimental testing program of drained, undrained and unjacketed compression tests. The macroscopic behaviour of the material is described in the framework of the mechanics of porous media. The poroelastic parameters of the material are determined and the effect of stress and pore pressure on them is evaluated. Appropriate effective stress laws which control the evolution of total volume, pore volume, solid volume, porosity and drained bulk modulus are discussed. A phenomenon of degradation of elastic properties is observed in the test results. The microscopic observations showed that this degradation is caused by the microcracking of the material under isotropic loading. The good compatibility and the consistency of the obtained poromechanical parameters demonstrate that the behaviour of the hardened cement paste can be indeed described within the framework of the theory of porous media. [Copyright &y& Elsevier]

Abstract: This paper deals with the evaluation of the critical-state parameters with respect to the matric suction for saturated and unsaturated undisturbed residual clayey soils from Turkey. In order to conduct the unsaturated triaxial compression testing procedures a conventional triaxial compression apparatus was redesigned. The data for critical-state conditions from these tests are presented with respect to matric suction, based on the critical-state parameters of M, q o, Γ, λ, which is commonly proposed by many authors. The critical state of the unsaturated samples is compared with that of the saturated samples. This experimental study has demonstrated that matric suction has no influence on parameters of M and λ. The parameters of M and λ are approximately 0.85 and 0.074 respectively for saturated and unsaturated conditions. The relationships between matric suction (u a − u w) and the intercepts q o and Γ have been observed as nonlinear, and thus they can be defined as a function of matric suction (u a − u w). Furthermore, a method is developed to predict the intercepts q 0 according to matric suction for unsaturated clayey soils. [Copyright &y& Elsevier]

Abstract: The structure of constitutive laws for powder compaction is examined in a companion paper, where a suitable framework for incremental and deformation theory models is developed. In this paper the response of a commercial steel powder is investigated using triaxial compaction experiments. The tests were designed to critically evaluate the underlying assumptions of the two modelling approaches and to calibrate and validate the models. It is demonstrated that for loading histories that are characteristic of industrial die compaction processes, a deformation plasticity model is capable of describing the material response, while the incremental plasticity response is very sensitive to the detailed form of the model in the early stages of compaction, where the material has a very low yield stress and undergoes large deformations without being able to sustain a sizeable load. [Copyright &y& Elsevier]