Triaxial Compression Tests Research Articles

A new layered rock constitutive model incorporating confining pressure and bedding angle weights based on orthogonal matrix analysis

Under the combined influences of confining pressure, lithology and bedding, the deformation and failure characteristics of layered rocks become more complex posing a significant challenge in safe underground excavations. In this study, two groups of specimens were prepared with various bedding angles including 0°, 10°, 20°, 30°, and 45°. X-ray diffraction and scanning electron microscopy were employed to analyze the mineral composition and internal microstructures of the specimens. Triaxial compression tests were conducted on the specimens subject to different confining pressures. Then, orthogonal matrix analysis was utilized to determine the specific influence weights of confining pressure, lithology, and bedding on peak differential stress, elastic modulus and peak strain of the rock samples. Finally, by implementing the proposed weight matrix, a statistical damage constitutive model for the layered rock under the coupled effects of confining pressure, lithology, and bedding was developed. It was found that the peak stress difference, cohesion, and internal friction angle decrease as the bedding angle increases, while the elastic modulus increases with the bedding angle. Additionally, the two groups of specimens exhibited distinct failure patterns under the influence of bedding angle and confining pressure. The averaged influence weights of confining pressure, lithology, and bedding on the rock deformation and failure characteristics are 40.99%, 23.87%, and 35.14%, respectively. The analytical simulation results were validated with the experimental results in this study and literature confirming the capability of the proposed constitutive model for capturing the stress–strain behavior of layered rock with various bedding angles subject to different confining pressures.

Study of Rock Damage Constitutive Model Considering Temperature Effect Based on Weibull Distribution

The deformation and damage process of rocks is accompanied by crack extension and penetration. The rock strength criterion, as a macroscopic characterization of the rock strength microelement, is the basis for establishing the damage constitutive modeling of rock. Aiming at the problem of the Hoek–Brown (H–B) strength criterion having a large strength prediction value under high confining pressure, the H–B strength criterion is corrected by considering the influence of the initial cracks on the development of the rock strength, and its applicability is verified. Based on the damage theory, assuming that the rock strength microelement obeys the Weibull distribution and considering the influence of residual strength, the damage correction coefficient is introduced, and a thermal damage statistical constitutive model that can reflect the whole process of the development of initial cracks inside the rock is established. The degree of penetration up to the damage is established, and the method of determining the parameters of the model is given. The theoretical curves of the established model are compared and analyzed with the curves of a conventional triaxial compression test of rock samples, and the study shows that the statistical constitutive model of the thermal damage of rock, established based on the modified H–B strength criterion, can better simulate the stress–strain relationship of rock under a conventional triaxial test. It also verifies the reasonableness and applicability of the model, which is expected to provide a basis for the exploitation of deep resources and the safety assessment of underground engineering.

Undrained shear strength of artificially flocculated clays

The flocculation and sedimentation of suspended clay particles in marine environments are an important process to form natural marine clays. In this study, the process of flocculation and sedimentation is simulated using appropriate flocculating agents in the laboratory. A series of laboratory sedimentation tests are carried out on various floc samples prepared with different concentrations of flocculating agents, and the size and density of flocs are measured and are found to serve as good parameters to characterise the development of flocculation. Multiple series of undrained triaxial compression tests under normal consolidation, as well as under over-consolidation, are then carried out on a slightly flocculated sample and a largely flocculated sample, as well as an ordinary remoulded sample. The values of the undrained shear strength of the flocculated samples are compared with those of the ordinary remoulded sample. The largely flocculated sample exhibits a higher undrained shear strength under normal consolidation as well as under over-consolidation. A comparison with past studies on natural marine clays also indicates that the highly variable values of the undrained shear strength of marine clays can be attributed to the different levels of development of flocculated structures in clays.

Assessing the zeolite’s effectiveness on the mechanical performance of cement-stabilised sand through triaxial test and microstructure analysis

ABSTRACT In this research, a series of consolidated drained compression triaxial tests were carried out to investigate the mechanical performance of Babolsar sand. Next, the parameters such as cement zeolite substitution percentage, cement content, and curing time were determined. The results revealed significant mechanical strength of specimens by replacing zeolite up to 35% compared with cemented sand mixtures for all ranges of cement percentages. Also, the samples showed more ductile behaviour due to increasing strains at failure. Increasing the zeolite substitution by up to 35% generated shearing band failure. However, exceeding the 35% substitution, the barrelling shear mode was predominant. All samples were compressed initially until reaching a point of instant zero dilation and then dilated until the end of the tests. The dilation tendency was higher in zeolite percentages up to 35%. Moreover, increasing the cement content enhanced the dilation of specimens due to hardening and interlocking cement and sand with zeolite. Finally, microstructure analysis results showed the Si/Ca ratio as a key index to approximate triaxial shear strength development.

Effect of freeze‒thaw cycles on root-Soil composite mechanical properties and slope stability.

Natural disasters such as landslides often occur on soil slopes in seasonally frozen areas that undergo freeze‒thaw cycling. Ecological slope protection is an effective way to prevent such disasters. To explore the change in the mechanical properties of soil under the influence of both root reinforcement and freeze‒thaw cycles and its influence on slope stability, the Baijiabao landslide in the Three Gorges Reservoir area was taken as an example. The mechanical properties of soil under different confining pressures, vegetation coverages (VCs) and numbers of freeze‒thaw cycles were studied via mechanical tests, such as triaxial compression tests, wave velocity tests and FLAC3D simulations. The results show that the shear strength of a root-soil composite increases with increasing confining pressure and VC and decreases with increasing number of freeze‒thaw cycles. Bermuda grass roots and confining pressure jointly improve the durability of soil under freeze‒thaw conditions. However, with an increase in the number of freeze‒thaw cycles, the resistance of root reinforcement to freeze‒thaw action gradually decreases. The observed effect of freeze‒thaw cycles on soil degradation was divided into three stages: a significant decrease in strength, a slight decrease in strength and strength stability. Freeze‒thaw cycles and VC mainly affect the cohesion of the soil and have little effect on the internal friction angle. Compared with that of a bare soil slope, the safety factor of a slope covered with plants is larger, the maximum displacement of a landslide is smaller, and it is less affected by freezing and thawing. These findings can provide a reference for research on ecological slope protection technology.

Study on failure characteristics and evaluation index of aquifer shale based on energy evolution

AbstractThe presence of abundant clay components and microporous structure in shale results in its high hydrophilicity, making a water-rich environment inevitable in petroleum exploration projects. Therefore, it is crucial to consider the influence of bedding structure, moisture content, confining pressure, and their combined effects on the geomechanical properties of shale. This article aims to investigate the mechanical properties of deep shale under varying water content conditions, elucidate the failure mode and failure mechanism of shale in actual engineering scenarios, and explores the interplay between stress, structure, moisture content, and other factors on its mechanical properties. The evaluation of wellbore stability and fracture propagation effects is proposed based on laboratory experiments using triaxial stress and strain data, along with the application of energy evolution theory. The experimental procedures encompass an analysis of shale's microscopic components and structure, as well as anisotropic shale triaxial compression tests conducted under different moisture contents and confining pressures. The results demonstrate that shale exhibits dense pores in its microstructure and displays pronounced anisotropic characteristics in its macrostructure. The presence of water within these pores, combined with the in situ stress within the formation, significantly influences the mechanical properties of shale. This anisotropy decreases with increasing moisture content, but the mechanical performance still decreases. Under triaxial compression conditions, the increase in confining pressure to some extent enhances the anisotropy of shale's deformation characteristics, which is related to the failure modes of shale. However, the detrimental effect of moisture content on shale's mechanical properties still persists. In order to quantify the impact of these factors, this study utilizes the elastic modulus as an indicator of the coupling effect. It combines the triaxial strain curve obtained from laboratory tests and proposes an evaluation index for shale mechanical properties based on the energy evolution theory. This index is suitable for assessing wellbore stability (the stability index called SIr) and crack expansion (the brittleness index called BIr). The calculation results reveal that, during the wellbore drilling process, excavating parallel to the direction of shale bedding while maintaining low moisture content and high confining pressure yields a higher SIr value, indicating better wellbore stability. On the other hand, during reservoir fracturing, fracturing perpendicular to the shale bedding direction and maintaining low confining pressure and moisture content result in a smaller BIr value. This approach is more beneficial for the expansion of shale fracture network in engineering.

Evaluating the anisotropic thermal–mechanical coupling strength of slate: A discrete element method approach

This study utilizes the discrete element method (DEM) to explore the thermal–mechanical coupling behavior of slate. Initially, a series of triaxial compression tests are conducted on slate specimens with varying orientation angles, temperatures, and confining pressures. Subsequently, the DEM is employed to replicate the mechanical and failure characteristics of the slate under identical laboratory conditions. The DEM constitutive models incorporate the parallel bond model for the rock matrix and the smooth-joint model for foliation. A thermal-degradation function for the parallel bond model is proposed, reasonably capturing the temperature's effect on the strength and failure characteristics of the slate. The simulation results exhibit a series of U-shaped curves in peak strength at different orientation angles, mirroring experimental findings. The study's findings indicate that orientation angles significantly influence the mechanical behavior and failure characteristics of slate. Elevated temperatures have minimal effects on strength anisotropy. Furthermore, applying high confining pressure is observed to mitigate the impact of high temperatures on bond weakening. This proposed method provides a useful tool for tunnel designs and geothermal energy development.

Experimental investigation of the intensity and conditions for strainburst triggering in intact hard rocks

Strainburst occurs during deep hard rock excavations. A series of single-free-face true triaxial compression (TTC) tests were conducted on seven hard rock types to investigate strainburst intensity and triggering conditions. Five of the seven rock types failed by strainburst, and two failed by spalling. The differences between the strainburst intensities are attributed to the mineral composition and structure. High-hardness minerals and mosaic-type microstructures facilitate stress conduction, which promotes rock microcrack initiation and propagation, causing severe rock fragmentation and fragment ejection, as observed for the Jinzhou granite. However, if the proportions of high- and low-hardness rock minerals are high and the microstructure is stable, such as in the triangular microstructure of the Chifeng basalt, then microcrack initiation and propagation are inhibited, decreasing the likelihood of rock fragmentation. The triggering conditions for strainburst were further analysed by considering the source of kinetic energy. For intact rock specimens, the kinetic energy for rock fragment ejection mainly originates from the intrinsic potential energy (Wbk) released by the specimen itself; thus, strainburst triggering requires that the applied maximum principal stress exceeds the bearing capacity and that the rock Wbk exceeds zero. However, a high Wbk is needed to trigger strainburst for structural rock specimens. Based on these findings, the possibility of triggering strainburst during excavation is greater when the Wbk (>9.1 kJ/m3), rock quality designation (>75%), and in situ stress of the rock are all high.

Investigation on failure mechanism and time-delay fracturing behavior of hard-rock tunnel under extremely high geostress state

To investigate the failure mechanism and time-delay fracturing behavior of hard-rock tunnel under extremely high geostress state, a series of conventional triaxial compression tests and time-delay loading–unloading mechanical tests on rocks were conducted. Combining testing results and on-site data of deep-buried tunnels, the generation mechanism and failure process of time-delay rockbursts were analyzed. The results indicate that with the increase of confining pressure, the failure mode of hard-rock exhibits a transition from axial splitting type and shear-slip type to special squeezing-expansion type. The progressive damage of rocks under extremely high stress state mainly manifests as aging lateral and volumetric expansion. Under time-delay compression tests, the long-term strength of hard-rock is lower than the threshold stress of σeb. In extremely high stress state, the fracture and dissipated energy evolution of rocks has significant time-dependent characteristics. Under extremely high stress conditions, rocks can store a large amount of releasable energy internally and gradually generate numerous cracks without the need for external work, ultimately leading to sudden unstable failure. Affected by the superposition of time-delay pressure and loading–unloading effects, the hard-rock also exhibits the same time-dependent fracturing behavior as soft-rock under high stress state, but its deformation mechanism is dominated by lateral expansion and crack volumetric dilatancy. When the stress level is far from reaching the ultimate strength required for rockmass failure, it is sufficient to cause the propagation of internal microcracks. Under the action of time-delay loads, the spalling and peeling failure of surrounding rock is significantly reduced, but the scale of unstable blocks is larger. Based on the failure process and microseismic monitoring date of rockburst in hard-rock tunnels, the entire evolution process of time-delay rockbursts can be divided into the following six stages: before excavation, rockmss excavation, aging cracking, squeezing-expansion, instability and spalling, time-delay rockburst. Time-delay rockburst has an obvious acoustic emission calm period before its occurrence, making its incubation process has strong suddenness and randomness.

Compression Mechanical Properties and Constitutive Model for Soft–Hard Interlayered Rock Mass

The properties of soft–hard interbedded rock masses are significantly impacted by the strength of rock layers and the characteristics of interface surfaces. This study investigates the mechanical properties of soft–hard interlayered rock masses by preparing rock-like specimens with different interface angles. Uniaxial and triaxial compression tests were conducted to examine the compression mechanical characteristics of the specimens. Experimental results demonstrated that in the uniaxial compression tests, the peak strength of the two-layer rock-like specimen exhibits an initial decrease followed by an increase as the interface angle increases. Similarly, the peak strength of the three-layer rock-like specimen also follows a “U-shaped” pattern. The failure of both specimens shifts from tensile failure to shear failure. In the triaxial tests, the strength of the two-layer rock-like specimen initially increases and subsequently decreases as the interface angle increases. In contrast, the intensity of the three-layer rock-like specimen exhibits a decreasing trend, transitioning from shear dilation or tensile failure to shear failure. By utilizing the damage constitutive model to compute the compressive strength of the composite specimen, it was observed that the deviation from the experimental value did not exceed 2.5%, and the overall shape of the curves was in good agreement. Consequently, it is affirmed that the damage constitutive model developed in this study can accurately capture the pre-peak phase of the stress–strain relationship in soft–hard interlayered rock-like specimens, thus providing a valid representation of their mechanical behavior.

A damage constitutive model of layered slate under the action of triaxial compression and water environment erosion

Based on the macroscopic structure control theory, The slate with a significant bedding plane is a composite rock mass composed of rock blocks containing microscopic defects, joint surface closure elements, and shear deformation elements. Considering the coupling damage effect of water erosion and triaxial compressive load on bedding structure plane, the transversely isotropic damage constitutive model of slate under triaxial compressive load is derived with the dip angle of bedding and confining pressure as the variable. Firstly, based on the statistical theory of continuous damage mechanics and the maximum tensile strain criterion, the transversely isotropic deformation constitutive model of rock block with micro-defects is given; Secondly, based on the phenomenological theory of closed deformation and shear-slip deformation mechanism of layered structural plane under the coupling action of water erosion and triaxial compression load, the calculation formula of axial deformation of layered structural plane under the coupling action is given; Finally, to verify the accuracy of the established constitutive model, triaxial compression tests are carried out to study the influence of dip angle and confining pressure on the macroscopic mechanical properties and mechanism of slate. The results show that: the established triaxial compression damage constitutive model of bedding slate can accurately describe the stress–strain relationship of bedding slate after water environment erosion. With the increase of bedding dip angle, the strength and deformation capacity of the bedding slate first decreases and then increases, showing a U-shaped distribution as a whole. There are three main types of failure: tension shear composite failure, shear slip failure, and splitting tension failure.

Modified Mohr–Coulomb criterion for nonlinear strength characteristics of rocks

AbstractThe classical Mohr–Coulomb criterion has an extremely wide range of applications in the field of engineering geology, while it cannot accurately characterize rock failure under high confining stresses. Therefore, a nonlinear modification is implemented based on the Mohr–Coulomb criterion for achieving wider usage in all stress environments. The cohesion and internal friction angle are firstly calculated by regression based on the triaxial experiments of diorite and other typical rocks in different confining stress ranges, and the variation modes of cohesion and internal friction angle are then formalized and integrated into the classical Mohr–Coulomb criterion. Using the experimental results of 435 triaxial compression tests in existing publications, the predictions of the modified criterion are compared with those of the previous nonlinear criteria, which show that the predicted values of the modified criterion are in good agreement with the measured strength. Compared with the previous nonlinear criteria, the modified criterion has a wider application range and always maintains a higher accuracy.

Anisotropic Mechanical Behaviors of Shale Rock and Their Relation to Hydraulic Fracturing in a Shale Reservoir: A Review

Shale gas is an important supplement to the supply of natural gas resources and plays an important role on the world’s energy stage. The efficient implementation of hydraulic fracturing is the key issue in the exploration and exploitation of shale gas. The existence of bedding structure results in a distinct anisotropy of shale rock formation. The anisotropic behaviors of shale rock have important impacts on wellbore stability, hydraulic fracture propagation, and the formation of complex fracture networks. This paper briefly reviews previous work on the anisotropic mechanical properties of shale rock and their relation to hydraulic fracturing in shale reservoirs. In this paper, the research status of work addressing the lithological characteristics of shale rock is summarized first, particularly work considering the mineral constituent, which determines its physical and mechanical behavior in essence. Then the anisotropic physical and mechanical properties of shale specimens, including ultrasonic anisotropy, mechanical behavior under uniaxial and triaxial compression tests, and tensile property under the Brazilian test, are summarized, and the state of the literature on fracture toughness anisotropy is discussed. The concerns of anisotropic mechanical behavior under laboratory tests are emphasized in this paper, particularly the evaluation of shale brittleness based on mechanical characteristics, which is discussed in detail. Finally, further concerns such as the effects of bedding plane on hydraulic fracturing failure strength, crack propagation, and failure pattern are also drawn out. This review study will provide a better understanding of current research findings on the anisotropic mechanical properties of shale rock, which can provide insight into the shale anisotropy related to the fracture propagation of hydraulic fracturing in shale reservoirs.

A new multiscale Cosserat model for size effect simulation in granular media

The Cosserat continuum, which contains a length scale factor l reported to link with the material’s grain size, is seemingly reasonable to describe the size effect of granular media. However, the physical meaning of l is still unclear, and its potential in describing the size effect is seldom studied. In this work, the performance of the conventional Cosserat model in capturing the size effect of granular media is firstly assessed. A new multiscale Cosserat constitutive model with an additional higher-order internal length scale vector Dq is then proposed based on the conventional Cosserat theory. The advantage of the proposed multiscale Cosserat model in size effect simulation for granular media is discussed through numerical simulations of centrifuge slope experiments and triaxial compression tests. The analysis results denote that the multiscale Cosserat continua are more sensitive to size effect than the conventional Cosserat continua, and the length scale vector Dq is like a symmetry-breaking factor that can trigger asymmetric deformation in structures; the influences of length scale factor l and Dq are seemingly opposite on stress concentration. This subtly improved constitutive model has the potential to simulate the size effect of a wider variety of materials.

Influence of orientation of the intermediate principal stress on fracture reactivation in granite

Fracture (fault) reactivation can lead to dynamic geological hazards including earthquakes, rock collapses, landslides, and rock bursts. True triaxial compression tests were conducted to analyze the fracture reactivation process under two different orientations of σ2, i.e. σ2 parallel to the fracture plane (Scheme 2) and σ2 cutting through the fracture plane (Scheme 3), under varying σ3 from 10 MPa to 40 MPa. The peak or fracture reactivation strength, deformation, failure mode, and post-peak mechanical behavior of intact (Scheme 1) and pre-fractured (Schemes 2 and 3) specimens were also compared. Results show that for intact specimens, the stress remains nearly constant in the residual sliding stage with no stick-slip, and the newly formed fracture surface only propagates along the σ2 direction when σ3 ranges from 10 MPa to 30 MPa, while it extends along both σ2 and σ3 directions when σ3 increases to 40 MPa; for the pre-fractured specimens, the fractures are usually reactivated under all the σ3 levels in Scheme 2, but fracture reactivation only occurs when σ3 is greater than 25 MPa in Scheme 3, below which new faulting traversing the original macro fracture occurs. In all the test schemes, both ε2 and ε3 experience an accumulative process of elongation, after which an abrupt change occurs at the point of the final failure; the degree of this change is dependent on the orientation of the new faulting or the slip direction of the original fracture, and it is generally more than 10 times larger in the slip direction of the original fracture than in the non-slip direction. Besides, the differential stress (peak stress) required for reactivation and the post-peak stress drop increase with increasing σ3. Post-peak stress drop and residual strength in Scheme 3 are generally greater than those in Scheme 2 at the same σ3 value. Our study clearly shows that intermediate principal stress orientation not only affects the fracture reactivation strength but also influences the slip deformation and failure modes. These new findings facilitate the mitigation of dynamic geological hazards associated with fracture and fault slip.

Determination of the characteristic values of the undrained shear strength of organic soils according to Eurocode 7

The paper presents the methods of determining the characteristic value on the basis of the standards: PN-B-03020:1981, PN-EN 1997-1:2008, prEN 1997-1:2022-09 and Schneider formula. Determination of the characteristic value of the undrained shear strength τfu was carried out using statistical method on the basis of the prEN 1997-1:2022-09 standard and Schneider formula. The statistical calculations were based on the results of field vane tests carried out in organic subsoil of test embankment in Antoniny test site before loading and after the 2nd embankment stage. In order to determine the undrained shear strength τfu of organic soils from field vane tests, the measured values of shear strength τf v were corrected using the average values of correction factors μ = μ(lab) determined on the basis of triaxial compression, simple shear and triaxial extension tests. The analysis of the calculation results shows that with relatively numerous data sets, large values of the coefficient of variation Vx result in significantly lower characteristic values of τfu obtained according to prEN 1997-1:2022-09, compared to the values obtained according to the Schneider formula. In the case of few data sets, for which high values of the coefficient kn are obtained, with high values of the coefficient of variation Vx , the comparison of the values according to prEN 1997-1:2022-09 with the values obtained according to the Schneider formula shows the greatest differences.

Coupled DEM-FDM numerical model for EPB shield tunnelling simulation with foam conditioning

It is widely recognized that an appropriate application of soil conditioning significantly improves the tunnelling performance of earth pressure balance (EPB) shield tunnel boring machines (TBMs). This study presents a numerical approach that simulates an EPB shield tunnelling, considering the injection of foam into the soil. To enhance computational efficiency and reduce the number of particles simulated in the ground formation, the numerical study coupled the discrete element method (DEM) and the finite difference method (FDM). The properties of in-situ soil and foam-conditioned soil were incorporated into the DEM domain by determining the contact parameters of particles using two calibration models: the triaxial compression test and the pressurized vane shear test models. With the established model, tunnelling analyses were conducted under specific TBM operation conditions, such as the penetration rate, and rotational speed of the cutter head and the screw conveyor. The effect of foam injection on EPB shield TBM operation was evaluated by adjusting the contact parameters of DEM elements. The numerical simulations revealed that foam conditioning reduced the torque, thrust force, and forces on cutting tools. These simulations provided valuable insights into the applicability of the numerical approach in assessing foam conditioning for EPB shield TBMs.

Experimental investigation on the influence of NaCl concentration on triaxial mechanical behaviors of a low-clay shale

During a hydraulic fracturing operation, a large quantity of salinity fracturing fluids will be retained in the shale reservoir, which may cause the problem of wellbore instability. Previous studies mainly focused on the change mechanism of clay-rich shale after brine immersion. However, the influence of NaCl concentration on mechanical behaviors of a low-clay shale is still unclear. Therefore, in this investigation, conventional triaxial compression tests were carried out on low-clay shale specimens saturated with NaCl solutions at different concentrations (0%, 5%, 15%, 25%) under different confining pressures (0 MPa, 15 MPa, 30 MPa, 45 MPa), and their mechanical properties were analyzed systematically. Optical microscopy, polarized light microscopy and scanning electron microscopy were used to understand the microscopic changes in shale samples after saturation in NaCl solutions. The brittleness indices of saturated shale specimens were evaluated based on rock strength properties and the energy evolution law. According to the experimental results, compared with the water-saturated specimens, the peak strength, elastic modulus, cohesion and internal friction angle of specimens saturated with 5%NaCl solution all showed a downward trend. However, for the specimens saturated with the NaCl solution at concentrations of 15% and 25%, higher NaCl concentration showed a greater rebound effect on the mechanical parameters compared with the 5%NaCl saturation. The microscopic observation showed that the brine could invade the interior of shale specimens from micropores, microcracks and bedding fractures, which were mainly parallel to the bedding direction. And the bedding fractures and some of micropores seemed to be filled by deposited NaCl crystals and mineral particles. The brittleness index of the brine-saturated shale specimen decreased at the low-concentration (5%) and increased at the high-concentration (15% and 25%). This investigation could provide a theoretical basis and mechanical parameters for analyzing the stability of shale wellbore affected by the salinity fracturing fluids.

Numerical study on the shear strength of granular materials under the low confining pressure

The relationship between stress and strength in granular soil differs under low confining pressure as compared to high confining pressure, which is of great importance for the geotechnical engineering. Numerical simulations using discrete element method (DEM) are conducted to model consolidated-drained triaxial compression tests, with the confining pressure σc ranging from 1 to1000 kPa. From the analysis of the shear strength of granular soils under various confining pressures and void ratios, it is found that the peak stress ratio ηp under a low σc is significantly lower than that under the high σc, particularly for samples with the same initial void ratio. As the confining pressure increases, ηp for samples with the same initial void ratio increases up to 29 % under the low σc while stabilizing under the high σc. Conversely, ηp for samples with the same void ratio after the consolidation can increase up to 13.5 % under the low σc while declining under the high σc. In addition, the critical confining pressure distinguishing the low and high confining pressures is about 200 kPa for dense samples with initial void ratio of 0.605 used in the study, and it increases as the sample void ratio increases. From the microscopic analysis, the variation of ηp under the low σc can be attributed to the enhanced anisotropy of normal contact force and contact normal under the low confining pressure. Finally, a modified Mohr-Coulomb failure criterion is proposed, which can determine the strength of granular soils under the low confining pressure with different void ratios.

Experimental and model investigation on hydro-mechanical behaviour of sandy mudstone

The hydro-mechanical behaviour and the constitutive model of sandy mudstone were investigated in this article. Two groups of hydro-mechanical triaxial compression tests were conducted. The results indicated that the inelastic deformation, damage, permeability and failure of the sandy mudstone were affected by the pore pressure and confining pressure. The hydro-mechanical behaviour was characterised by the coupled elastoplastic damage model. This model was based on the effective stress principle and numerical return mapping algorithms. Sensitivity analysis was also performed to examine the effects of four key parameters on the modelling of elastoplastic and damage behaviour. This study can provide a foundation for physical understanding and mathematical modelling of sandy mudstone under coupled loads.