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In order to study the influence of the stress direction during underground engineering excavation on the strength of weakly cemented soft rock, the soft rock samples were obtained from the coal mine roadway engineering soft rock and the GDS(Global Digital Systems) hollow cylindrical test system was used to determine the average fixed principal stress. The deviation stress under the action of the rotation of the principal stress axes, the stress path of the soft rock mechanics, and rotation angle of the principal stress axes of weakly consolidated soft rock specimen were studied, and the stress-strain characteristics as well as the influence of the strength characteristics was determined. Based on the test results and power function model, an empirical equation of the shear strain of weakly consolidated soft rock under the principal stress axis was established. The results show that the deformation of weakly consolidated soft rock accumulates with the rotation of the principal stress axis, and with the rotation angle of the principal stress axis increases, the axial strain and shear strain rates increase rapidly, and the cumulative deformation also increases.The rotation of the principal stress axis accelerates the accumulation of the axial strain of the specimens.The shear formular can effectively predict the shear strain variation law of weakly consolidated soft rock under the stress axis rotation. Additionally, it can be applied in the deformation prediction of practical engineering. [ABSTRACT FROM AUTHOR]

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This experimental study describes the elastic and yielding response of compacted high-plasticity clay in conjunction with deformation characteristics at relatively larger strains for different stress paths and stress states. K0 consolidated undrained triaxial compression and extension stress path tests were conducted on compacted Nagpur cohesive soil with bender element measurements to explore the elastic and yielding response of soil. The experimental results showed that the stress path direction had a significant impact on the stress–strain–strength properties of soil as the peak deviatoric stress was found to increase with the increase in stress path angle. The measured small-strain shear modulus (Gmax) values were observed to be a function of the mean effective stress (p′) with slight dependency on deviatoric stress (q). Gmax was found to reduce with increase in strain level during shearing. An attempt was made to formulate an empirical correlation for compacted cohesive soil based on the elastic shear stiffness in pre-yield conditions. This correlation was further used to determine the yielding criteria of compacted cohesive soil along with the other two well-established criteria of stress–strain and strain energy. [ABSTRACT FROM AUTHOR]

It is extremely difficult to accurately predict the rock damage evolution during the underground space development or the deep excavation activity. In this paper, based on the statistical damage mechanics, a plastic strain-induced damage model of porous rock was established to describe the damage evolution and the constitutive behavior of porous rock under different stress paths. In the proposed model, the modified porosity was introduced which considered the effect of the generalized plastic shear strain. Besides, the proposed damage evolution function was also controlled by the generalized plastic shear strain. To validate the proposed damage model, the sandstone is selected as the experimental specimen due to it is a typical porous rock, and a series of conventional tri-axial compressive experiments (CTC) and confining pressure unloading experiments under constant deviatoric stress (UCP-CDS) were carried out. Furthermore, the confining pressure unloading experimental data under increscent deviatoric stress (UCP-IDS) was referenced to further validate the applicability of the proposed model. The results showed that the deviatoric strain-damage curves were an "S" shape, moreover, the relationship between the damage variable with the unloading ratio was exponential function. The proposed damage model could better reflect the void volume change and the radial dilation during the unloading process. Moreover, the model could successfully capture the damage evolution law and the mechanical behavior of sandstone by matching a set of tri-axial compressive experiments under different stress paths. Finally, it is found that the strength, strain-hardening and strain-softening characteristics were controlled by the Weibull distributed parameters m0 and F0. Highlights: The modified porosity was introduced which considered the effect of the generalized plastic shear strain. The relationship between the damage variable with the unloading ratio was exponential function. The proposed model could reflect the void volume and the radial dilation. The proposed model could reflect the stress-strain behavior under different stress paths. [ABSTRACT FROM AUTHOR]

It is important to determine the volumetric change properties of hydrate reservoirs in the process of exploitation. The Skempton pore pressure coefficient A can characterize the process of volume change of hydrate-bearing sediments under undrained conditions during shearing. However, the interrelationship between A value responses and deformation behaviors remain elusive. In this study, effects of hydrate saturation and effective confining pressure on the characteristics of pore pressure coefficient A are explored systematically based on published triaxial undrained compression test data of hydrate-bearing sand and clay-silt sediments. Results show that there is a higher value of the coefficient A with increasing hydrate saturation at small strain stage during shearing. This effect becomes more obvious when the effective confining pressure increases for hydrate-bearing sand sediments rather than hydrate-bearing clayey-silt sediments. An increasing hydrate saturation leads to a reduction in A values at failure. Although A values at failure of sand sediments increase with increasing effective confining pressure, there are no same monotonic effects on clayey-silt specimens. A values of hydrate-bearing sand sediments firstly go beyond 1/3 and then become lower than 1/3 at failure even lower than 0, while that of hydrate-bearing clayey-silt sediments is always larger than 1/3 when the effective confining pressure is high (e.g., >1 MPa). However, when the effective confining pressure is small (e.g., 100 kPa), that behaves similar to hydrate-bearing sand sediments but always bigger than 0. How the A value changes with hydrate saturation and effective confining pressure is inherently controlled by the alternation of effective mean stress. [ABSTRACT FROM AUTHOR]

Based on the changes of total stress and pore water pressure in the spring model, the mechanism of vacuum preloading consolidation is studied by theoretically discussing the consolidation behavior of soil in vertical and horizontal directions with different boundary conditions. The variation of effective stress and development of deformation are analyzed for the soil under vacuum preloading under stress paths of K0 consolidation and isotropic consolidation based on the boundary conditions, respectively. Meanwhile, the theoretical earth pressure coefficient (K0MC) at rest for soft soil is deduced based on modified Cam-clay model. With engineering applications, the correctness and effectiveness of the mechanism is confirmed by the application in large area of ground improvement for soft soil and statistical analysis of some engineering cases. It is found that the drainage boundary condition and the scale of treated area have effects on deformation of soil mass. For the middle part of the treated zone under large-area vacuum preloading, the variation of effective stress is not isotropic, but following the pattern of one-dimensional compression. According to the statistic results from some site investigations, one-dimensional compression equation (Cc-Cr method) can be applied to predict the settlement at the center of the treated zone when the one dimensional condition is found in the center of treated zone as the ratio between treated breadth and length of vertical drains larger than 4-5.

A new constitutive model for sand is formulated by incorporating two new constitutive ingredients into the platform of a reference critical state compatible bounding surface plasticity model with kinematic hardening, in order to address primarily the undrained cyclic response. The first ingredient is a memory surface for more precisely controlling stiffness affecting the plastic deviatoric and volumetric strains and ensuing excess pore pressure development in the pre-liquefaction stage. The second ingredient is the concept of a semifluidised state and the related formulation of stiffness and dilatancy degradation, aiming at modelling large shear strain development in the post-liquefaction stage. In parallel, a modified flow rule aimed at providing a better description of non-proportional monotonic and cyclic loading is introduced. With a single set of constants, for which a detailed calibration procedure is provided, this new model successfully simulates undrained cyclic torsional and triaxial tests with different cyclic stress ratios, separately for the pre- and post-liquefaction stages, as well as liquefaction strength curves based on r u and shear strain criteria for initial liquefaction. The successful reproduction of the sand element response under undrained cyclic shearing contributes to future applications in realistic and thorough seismic site response analysis. [ABSTRACT FROM AUTHOR]

In this paper, the compression drive mechanism, which can be considered as the most related oil drive mechanism with the reservoir geomechanical properties, has been investigated. The constant total vertical stress on the reservoir and uniaxial reservoir compaction with zero lateral strain are two main assumptions in the conventional reservoir compaction modeling. These assumptions are not considering the stress arching, which leads to a reduction in the total vertical stress and also affects the distribution of vertical and horizontal stresses. In this paper, it was tried to investigate the effects of three keys elastic parameters, including Young's modulus, Poisson's ratio, and Biot coefficient on the reservoirs stress paths and compaction drive mechanism using numerical modeling with ABAQUS software. Based on the modeling results the reservoirs stress paths is very different from simplifying assumption in uniaxial compaction models while the total compaction of reservoir obtained using numerical modeling and uniaxial model in some cases can be equal. Also, among the three geomechanical parameters, the ratio of Young's modulus of the reservoir and surrounding rocks has a significant effect on the reservoir stress paths and compaction. The results of modeling indicate when a reservoir rock is softer than its surrounding environment, due to the arching occurrence, the total vertical stress applied to the reservoir rock reduced even to half of the initial vertical stress, which leads to the oil recovery by the compression drive mechanism will be less than 50% of the predicted value. [ABSTRACT FROM AUTHOR]

In this study, we numerically investigate the influence of hysteretic stress path behavior on the seal integrity during underground gas storage operations in a depleted reservoir. Our study area is the Honor Rancho Underground Storage Facility in Los Angeles County (California, USA), which was converted into an underground gas storage facility in 1975 after 20 years of oil and gas production. In our simulations, the geomechanical behavior of the sand reservoir is modeled using two models: (1) a linear elastic model (non-hysteretic stress path) that does not take into consideration irreversible deformation, and (2) a plastic cap mechanical model which considers changes in rock elastic properties due to irreversible deformations caused by plastic reservoir compaction (hysteretic stress path). It shows that the irreversible compaction of the geological layer over geologic time and during the reservoir depletion can have important consequences on stress tensor orientation and magnitude. Ignoring depletion-induced irreversible compaction can lead to an over-estimation of the calculation of the maximum working reservoir pressure. Moreover, this irreversible compaction may bring the nearby faults closer to reactivation. However, regardless of the two models applied, the geomechanical analysis shows that for the estimated stress conditions applied in this study, the Honor Rancho Underground Storage Facility is being safely operated at pressures much below what would be required to compromise the seal integrity.

The critical damage value of rock is an important parameter of stability analysis when engineering rock mass. Based on the previous single test method, this paper, combined with the deformation characteristics of rock residual phase under different stress paths, proposed a single specimen repeated loading method to measure the critical damage value of rock based on different damage variables. The paper also proposed to modify the critical damage value based on residual constitutive energy and to improve the definition of damage variables based on constitutive energy dissipation. According to the microstructure characteristics of the rock sample, the mechanism of the single specimen method to determine the critical value of rock sample damage is revealed. The results show that: 1) Comparing the results of triaxial failure test of rock under different stress paths and single specimen repeated loading test, the residual strength of rock sample is mainly controlled by the confining pressure of loading, which is not an obvious relationship with initial confining pressure and stress path; 2) The number of repeated compressions has little effect on the internal structure, particle flatness, and particle surface smoothness of their specimens, which is reflected at the macroscopic level that the residual strength of sandstone tends to achieve a stable value; and 3) The corrected critical damage value based on residual strain energy is closer to the theoretical value. Test methods in this paper can provide useful references for determination of critical damage value.

Stress path tests in plane strain (PS) conditions with varying stress ratios have been carried out using a newly developed flexible boundary (FB) multiaxial testing device. It employs electro-pneumatic control and allows the application of independent principal stresses on each face of a 96-mm cubical soil specimen through a closed-loop feedback control system. Different stages of testing have been automated for both deformation and stress-controlled loading. It also has provisions for local deformation field estimation from the images acquired on the transparent rigid PS boundary. The friction angle for different stress paths in compression and extension mode is found to have minimal variation in PS conditions. The experimentally observed yield points match well with a simplified two-dimensional (2D) yield criterion resembling the shape of a "tear-drop." Local shear strain profiles under FB-PS conditions imply nearly uniform material behavior, while the emergence of instabilities is found to be minimal and mostly at the specimen corners. [ABSTRACT FROM AUTHOR]

Geotechnical investigation and design show that when evaluating the settlement of ground with soft soil ground, the use of a single stress path results in a larger error than in the case of a multistress path. To understand the influence of a multistress path on the deformation properties and microstructure characteristics of soil, a two-stage stress path (corresponding to three stress states: the in situ state, unloading-loading state, and grouting pressure state) of soil was examined by analytical solution. Then, a series of triaxial tests was conducted to analyze macroscopic deformation properties considering four influencing factors, i.e., the relative distance between the soil and tunnel, stress paths, type of silty clay, and drainage conditions. The results show that axial strain of silty clay caused by the two-stage stress path increases by 31.6% with the relative distance decreasing from 1.67R (R means tunnel radius) to 1.25R on tunnel side, while the volume strain increases by 80% with the relative distance decreasing from 1.21R to 1.06R on tunnel bottom. In addition, the deformation caused by the two-stage stress path (or reconstituted soil or undrained condition) is larger than that caused by the one-stage stress path (or undisturbed soil or drained condition). Finally, through scanning electron microscopy and mercury intrusion porosimetry, the pore size, distribution, shape, and arrangement of silty clay on the bottom and sides of the tunnel were investigated. The research shows that under the two-stage stress path, the number of large pores decrease and that of small pores increase. The probability entropy of pore orientation ranges from 0.834 to 0.971 when silty clay is subjected to a two-stage stress path. This illustrates that the pore order of the soil on tunnel side is better than that on the tunnel bottom. [ABSTRACT FROM AUTHOR]

Bio-mediated techniques have the potential to be an eco-friendly and sustainable solution for engineering problems in the presence of unfavorable soil conditions. During the microbial induced carbonate precipitation (MICP) process, calcium carbonate is the byproduct of a series of biological and chemical reactions in the soil media. Although the shear response of MICP-treated sands with different calcium carbonate content has been extensively investigated, the behavior of this material subjected to varying stress paths with different levels of cementation and particle sizes is still unknown. In this study, the material behavior of MICP-treated sands under axisymmetric compression, radial extension, constant p', and constant q stress paths at moderate and heavy level of cementation is evaluated by conducting drained triaxial tests on specimens with relative density of about 40%. Shear wave velocity was measured during the course of treatment and shearing to monitor cementation and degradation processes. In addition, the effect of stress relaxation and compression after bio-treatment on shear response is evaluated. A previously proposed nonlinear failure envelope for MICP-treated sands is also verified by comparing the shear and normal stresses at failure with those predicted by the nonlinear failure envelope. [ABSTRACT FROM AUTHOR]

Isotropically and anisotropically consolidated undrained triaxial compression shear tests are conducted on 87 specimens of eight kinds of clays reconstituted over a wide spectrum of initial water contents. These data in conjunction with those of 55 specimens previously published by the authors and their research team are used to investigate the shear strength–compressibility interrelation for reconstituted clays. The effects of the initial void ratio and the consolidation stress path on both the consolidated undrained shear strength–consolidation effective stress relationship and the compression curves in terms of the void ratio against the consolidation effective stress are investigated. It is found that the consolidated undrained shear strength–void ratio relationship is independent of the initial void ratio and the consolidation stress path, and a quantitative expression is proposed for determining the relationship as a function of the void ratio at the liquid limit. Meanwhile, the compression curves of reconstituted clays at different values of the initial void ratio and the void ratio at the liquid limit from different consolidation stress path tests can be normalised into a unique line with the application of the intrinsic compression framework. Accordingly, a quantitative expression can be established to determine the relationship between the consolidated undrained shear strength and the effective consolidation stress. The validity and accuracy of the proposed equations are discussed based on data from independent researchers and from the research team. [ABSTRACT FROM AUTHOR]

Tunnel excavations in rock usually cause initial stress release, which is different from the stress path in classical triaxial compression tests. To analyze the permeability variations in excavation disturbed zone, two different stress paths, e.g. classical triaxial compression and confining pressure unloading, are applied on granite, and the permeability at different damage levels is measured using gas as transport medium. The differences in deformation and failure mechanism and their impacts on gas permeability variation are discussed. The test results show that lateral deformation of samples in confining pressure unloading tests is obviously larger than that in classical triaxial compression tests. Unlike the permeability decrease-increase transition in classical triaxial compression tests, the permeability in confining pressure unloading tests shows an overall progressively increase trend. The permeability in both tests increases significantly near failure and achieves the same order of magnitude. The differences in permeability evolutions under the two stress paths are directly attributed to their different deformation mechanisms. The test results could provide a new understanding of permeability properties during the underground excavation. [ABSTRACT FROM AUTHOR]

During deep buried hard-brittle rock tunnel excavation, the surrounding rock experiences a complicated stress path and stress adjustment process. Once the adjusted stress exceeds the ultimate bearing capacity of rockmass, a rock failure mode defined as stress-cracking type will occur. In order to investigate the effect of stress paths on failure mechanism and progressive damage of deep-buried rockmass, the cyclic loading-unloading, loading-unloading, uniaxial, conventional and unloading triaxial compression tests on samples of hard-brittle sandstone were conducted. According to the experimental results, increase in the confining pressure was beneficial to improve the mechanical parameters of rock, but it will reduce the brittle failure features. Compared with conventional triaxial compression, the sandstone under unloading state had more remarkable stress drop and unstable failure characteristics. Meanwhile, it was found that the energy dissipation and energy release in the whole process of rock deformation were the internal power of driven rock progressive damage. With the increase of confining pressure, the energy hardening and energy accumulation features of rock were weakened, while the progressive damage evolution characteristics could be enhanced. In unloading state, more energy could be converted into elastic energy in the energy softening phase (σeb-σP), so that the pre-peak damage rate of rock was lower than that of conventional triaxial compression state. Thus, the energy dissipation rate of rock after peak strength decreased linearly with the increase of confining pressure under conventional triaxial compression state, while in unloading state it showed the opposite law. [ABSTRACT FROM AUTHOR]

The post-peak response of elasto-plastic material is dependent upon the stress path it is subjected to. This paper analyses and compares the stress path and strain response of advancing tunnel, using two-dimension:1al plane strain (2D PS) and full three-dimensional step by step excavation and support (3D SBS) finite difference numerical model (FDM). Stress paths for 3D SBS model indicate higher stress relaxation and straining at unsupported spans as well as elastic recompression behind applied supports; these features are not evident in 2D PS models. Implications of these differences on tunnel response are discussed. The complex 3D stress evolution with tunnel construction is greatly simplified by considering major and minor principal stresses only, for the purpose of stress path generation. To verify that the formulation of stress path adopted here is sufficient to describe stress–strain evolution of 3D SBS tunnel model, a numerical simulation of "stress path following triaxial unloading test" is conducted using "simplified loading conditions." Results show that the suggested test methodology has potential to map stress path of 3D SBS tunnel model for ideal geomaterial behavior. Also, improvement in measured strain response, as compared to 2D PS models, is observed. [ABSTRACT FROM AUTHOR]

Various mechanical responses of rocks under a complex stress state have attracted the attention of engineers. A hollow cylinder torsional apparatus (HCA) was constructed using the discrete element method (DEM). A series of conventional triaxial compression tests under different confining pressures and a complex stress path test with torsion were conducted. The numerical results are consistent with laboratory and theoretical findings, which indicates that the microscopic parameters of the DEM sandstone specimens were correct and reasonable. Based on the HCA DEM specimen, the macro–micromechanical response of sandstone specimens under four typical unloading stress paths were studied, and variations in the strain energy density, strain energy, and dissipated energy of sandstone specimens were obtained. These results could provide a reference for the study of the mechanical properties of unloaded sandstone in geotechnical engineering. [ABSTRACT FROM AUTHOR]

An analytical solution for elastoplastic responses of an underground lined rock cavern for compressed air energy storage under initial hydrostatic pressure and high internal air pressure is proposed. In the analytical solution, the stress paths during cavern excavation and operational stages are considered. A numerical simulation is performed by using the finite element software Abaqus to verify the analytical solution. The concept of "high-pressure plastic zone" is proposed and the evolution of "high-pressure plastic zone" is analyzed. Furthermore, taking the occurrence of "high-pressure plastic zone" and failure of the sealing layer as the critical states, the calculation methods for critical internal pressure p c r 1 and ultimate internal pressure p u are proposed. Sensitive analyses are conducted to study the key parameters affecting the "high-pressure plastic zone" and the critical internal pressure. The research indicates that the proposed analytical solution can accurately predict the elastoplastic responses of an underground lined rock cavern. The radius of "high-pressure plastic zone" are mainly related to the surrounding rock quality, magnitude of air internal pressure and buried depth of cavern. The critical internal pressure p c r 1 and ultimate internal pressure p u are mainly related to the surrounding rock quality, cavern size, buried depth and thickness of concrete layer. [Display omitted] • An analytical solution for elastoplastic responses of an underground lined cavern for CAES is proposed. • The evolution of "high-pressure plastic zone" is analyzed. • The calculation methods for the critical internal pressure and ultimate internal pressure are proposed. [ABSTRACT FROM AUTHOR]

In order to study the deformation and non-coaxial characteristics of fiber cement modified iron tailing sand (FCIT) under different stress paths. The evolution law of the dynamic stress ratio (η) on the cumulative deformation and non-coaxial angle of FCIT under different tension-compression amplitude ratios (α) was explored through hollow torsional shear tests. The deformation behavior of FCIT is analyzed by using the stability theory. The research results show that: 1) The increase of dynamic stress level will deepen the cumulative deformation of FCIT, while the increase of α will make the deformation mode of FCIT develop into bulging failure-shear failure-tensile failure. 2) According to the development trend of cumulative strain and cumulative strain rate of FCIT, it can be divided into three stages: plastic stability, plastic creep and incremental collapse, and the cumulative strain rate development prediction model is established, and the prediction error distribution is within ±20%. 3) The stress path determines the non-coaxiality of FCIT to a certain extent, and the non-coaxial angle fluctuates and decreases with the increase of the principal stress direction angle. When α =0.125, the maximum and minimum values of the non-coaxial angle of FCIT are 1.5 rad and 0.4 rad respectively. 4) Cement hydration reaction generates calcium alumina and hydrated calcium silicate (C-S-H) to make FCIT structure more compact, fiber-matrix interfacial connections more dense, and play a role in filling the pores. • Effects of dynamic stress levels and stress paths on cumulative deformation and failure modes of FCIT. • Decomposition of deformation behavior of FCIT by stabilization theory. • Quantifying the change in the non-coaxial angle to explore the non-coaxiality of FCIT. [ABSTRACT FROM AUTHOR]

This work attempted to understand the behavior of the Upper Cretaceous Nezzazat and Lower Cretaceous–Carboniferous Nubia sandstone reservoirs in response to production-induced depletion and fluid injection for enhanced hydrocarbon recoveries from the October oil field, Gulf of Suez, Egypt. Pore pressure (PP), vertical stress (Sv) and minimum horizontal stress (Shmin) magnitudes were modeled based on well logs, drilling data and subsurface measurements. The latest measurements indicated 11.7–12.7 MPa pressure drop (ΔPP) in the Nezzazat reservoirs, while the Nubia sandstone reservoir was depleted by 19–21 MPa. Revised PP and Shmin gradients offer a narrow mud weight window of 9–10.7 PPG (pore pressure gradient) if the entire Lower Miocene–Carboniferous section was planned to be drilled with a single casing in the infill/injector wells. A more conservative approach will be to drill the depleted reservoirs with 5.5–9.3 PPG mud window and case separately, although that may incur an additional cost. Based on the PP–Shmin poro-elastic coupling, stable stress path values of 0.61 and 0.65 are interpreted in the Upper and Lower reservoirs, indicating depletion-induced normal faulting is unlikely to occur at the present rate of depletion. The reservoir stability threshold during pressurization was assessed for fluid injection optimization to sustain production and curtail the bypassed oil. The maximum allowable pressure build-up during injection was estimated using various possible pore pressure–stress coupling scenarios at their maximum depletion state. Based on the PP–Shmin coupling approach, maximum pressure increments of 23 and 27 MPa can be permitted in the depleted Nezzazat and Nubia sandstone reservoirs during injection, without exceeding the lower limit of caprock Shmin, as applicable for both the reservoirs. This will ensure the geomechanical stability of the reservoirs as well as the caprock integrity. This geomechanical study provides crucial comprehensions regarding the optimization of drilling, production, and fluid injection by reducing the risk of reservoir instabilities and formation integrity. [ABSTRACT FROM AUTHOR]

Sand roughness is now accessible to measurement. Incorporating this parameter into sand models using the discrete element method (DEM) is known to improve bulk small strain response. In this work we explore the effect on problems where particle crushing takes place. A well-established DEM particle crushing model and a rough Hertzian contact model are here combined to incorporate both effects in a single contact model. Including contact roughness results in stronger particles whilst all other material parameters being equal. The model is then used to simulate high pressure oedometric compression tests on a strong silica sand. It is shown that including realistic values of surface roughness enables to correctly capture both load-unload behaviour and particle size distribution evolution while using realistic values of elastic bulk properties for the sand grains. Roughness is then a model refinement that may result in simpler, more objective DEM calibrations. [ABSTRACT FROM AUTHOR]

This study experimentally investigates the influences of grain shape and gradation on stress-dilatancy parameters. For this purpose, consolidated-drained triaxial tests are conducted on five different granular materials. Specimens are sheared following two different stress paths; axial compression and lateral extension, in order to monitor the influence of stress path on the investigated problem. For each soil, grain shapes, sizes and stress-dilatancy parameters are experimentally determined. Moreover, available data in literature for other sands are compiled with the data of this study. This allows the investigation of the dependence of stress-dilatancy parameters on gradation and grain shape characteristics. Especially with this study, variations of stress-dilatancy constants with grain characteristics are examined. Statistically significant relationships are obtained for predicting stress-dilatancy parameters as functions of average grain shapes. On the other hand, influences of stress path and gradation on stress-dilatancy response are experimentally considered. Finally, compiled and determined data used in the development of the relationships are provided in the form of tables. [ABSTRACT FROM AUTHOR]

It is important to determine the volumetric change properties of hydrate reservoirs in the process of exploitation. The Skempton pore pressure coefficient A can characterize the process of volume change of hydrate-bearing sediments under undrained conditions during shearing. However, the interrelationship between A value responses and deformation behaviors remain elusive. In this study, effects of hydrate saturation and effective confining pressure on the characteristics of pore pressure coefficient A are explored systematically based on published triaxial undrained compression test data of hydrate-bearing sand and clay-silt sediments. Results show that there is a higher value of the coefficient A with increasing hydrate saturation at small strain stage during shearing. This effect becomes more obvious when the effective confining pressure increases for hydrate-bearing sand sediments rather than hydrate-bearing clayey-silt sediments. An increasing hydrate saturation leads to a reduction in A values at failure. Although A values at failure of sand sediments increase with increasing effective confining pressure, there are no same monotonic effects on clayey-silt specimens. A values of hydrate-bearing sand sediments firstly go beyond 1/3 and then become lower than 1/3 at failure even lower than 0, while that of hydrate-bearing clayey-silt sediments is always larger than 1/3 when the effective confining pressure is high (e.g., >1 MPa). However, when the effective confining pressure is small (e.g., 100 kPa), that behaves similar to hydrate-bearing sand sediments but always bigger than 0. How the A value changes with hydrate saturation and effective confining pressure is inherently controlled by the alternation of effective mean stress.

Multidirectional shearing of granular soils involves shearing more than one shear stress component, and thus may result in more-complex soil responses than unidirectional shearing. This study developed an advanced numerical procedure that can impose an arbitrary loading path on a granular assembly in three-dimensional space as a basis for discrete-element simulations to investigate the behavior of granular assemblies under undrained cyclic multidirectional shearing conditions. The tests included varying stress trajectories on the deviatoric stress plane, including straight-line, circular, figure-8, and teardrop shapes. The numerical results indicated that the liquefaction resistance of a sample for a given cyclic shear stress ratio (CSR) depends on the stress orbits, and that of the teardrop shape was the highest and that of the figure-8 shape was the lowest. The microstructure of the granular assembly was quantified by the contact-normal-based fabric tensor, and its evolution trend correlation with stress–strain responses was explored. The evolution of fabric and stress–strain relationships from discrete-element modeling under multidirectional shearing conditions can provide useful insights into the behavior of granular soils. [ABSTRACT FROM AUTHOR]

The stresses generated from rock expansion in a confined underground environment will exert additional loads to the existing support system resulting in severe damage during tunnel construction. In this study numerical investigations were carried out for Zagros tunnel in Iran, where swelling has been reported to be an inherent formation property. The presence of expandable clay was investigated using X-ray diffraction mineralogical analyses. The swelling volume strain was used to estimate the amount of stress increased due to swelling. The stress path method is used to obtain the location of swelling stresses. The stress relief zone (SRZ) was considered as the criterion to define the area around the tunnel where swelling stresses exist. In doing so, in the areas where the first stress invariant after excavation becomes lesser than its corresponding value before excavation, the swelling pressure can be built up. Then tunnel stability was investigated by numerical simulations using FLAC2D. The swelling stresses calculated from the free swelling test were applied to those elements around the tunnel within the stress relief zone. A comparison between the results of numerical investigations with field observation indicates that the proposed approach is successful in simulating swelling stress in swelling rocks. [ABSTRACT FROM AUTHOR]

Granite residual soil is one of the most frequently encountered problem soils in tropical regions, whose mechanical behavior heavily depends on the pattern of variation of mean effective stress (p') during shearing, which can be classified into three categories: increasing-p', constant-p', and decreasing-p'. Unfortunately, so far, the stress–strain strength characteristics of granite residual soils have been studied mainly under increasing-p' stress paths, although it is very likely to encounter stress paths with decreasing p' in practice, especially in excavation engineering. Moreover, most pertinent research has focused on remolded granite residual soils, whereas undisturbed specimens have not yet received enough attention. In this paper, stress path triaxial tests considering different patterns of variation of mean effective stress were conducted on an undisturbed granite residual soil. Subsequently, a variable termed loading angle was introduced to quantitatively represent stress path. The influences of stress path on the Mohr–Coulomb strength parameters, deformation characteristics, ductility, and shearing stiffness were analyzed, with an emphasis on the role of pattern of variation of mean effective stress. The experimental results show that friction angle of the soil increases while cohesion decreases with the increase in loading angle. The increase in loading angle leads to less volume contraction and smaller failure strain. During shearing, the soil exhibited a less brittle response under stress paths with smaller loading angles. The initial secant shear modulus first decreased and then increased as the loading angle increased, with the minimum shearing stiffness occurring at a certain loading angle lying between 90° and 123.7°. [ABSTRACT FROM AUTHOR]

Soil collapse is a phenomenon triggered by wetting a loaded soil, the structure of which contains large pores. This type of soil response has been studied since the 1970s and models that can predict its occurrence and magnitude were proposed from the 1990s. In particular, the concept of loading collapse (LC) curves has been developed in the framework of unsaturated soil mechanics and it has been validated using low-reactivity soils. Several publications have highlighted that the microstructure of expansive soils evolves significantly during swelling, a phenomenon that may affect the location of the LC curve. With that perspective, some studies mention a need to shift the LC curves for experimental data and model predictions to agree. This paper brings new insight into the significance of microstructure for the collapse of a reactive soil. A series of tests was conducted to characterise the microstructure of soil specimens compacted under different initial conditions. Then, three pairs of wetting tests under constant load and under controlled suction were conducted in order to identify the zone of onset of collapse. It was found that the conventional LC curves cannot adequately predict the occurrence of collapse. In contrast, analysing the swelling response in the light of microstructural characterisation led to the conclusion that, for Maryland clay, occurrence of collapse can be tracked by the evolution of the void ratio associated to the macro porosity during wetting. [ABSTRACT FROM AUTHOR]

Soils subjected to the loading or unloading in geotechnical engineering applications like foundation excavation, embankment, and tunnel construction will reconsolidate under different stress paths. This paper presents an experimental investigation on the effect of stress path on shear strength characteristic of reconstituted Zhenjiang clay. Specimens were isotropically or anisotropically consolidated and sheared without drainage in triaxial shear tests. Test results showed that undrained shear strength (Su) under various stress paths is markedly influenced by the stress ratio (η), which is defined as the ratio of the deviator stress to the mean effective stress (p′). Su almost linearly increases with p′ or η at a given η or p′. The value of Su for specimens consolidated under different stress paths can be predicted as a function of p′ and η, i.e., Su = (0.13η + 0.37)p′. It is concluded that Su of specimen under anisotropic consolidation is higher than that under isotropic consolidation at a certain p′. Moreover, specimen anisotropically consolidated to the same p′ at higher η results in an increase in Su compared with that at lower η. Duncan-Chang model can be employed to characterize the strength behavior of reconstituted clays under various consolidation stress paths. The variation of c or c′ is insignificant under different η, indicating that the effect of η on cohesion is negligible. φ increases linearly with η while φ′ shows an insignificant change. [ABSTRACT FROM AUTHOR]

A reservoir geomechanical modeling has been attempted in the hydrocarbon-bearing Miocene formations in the offshore Badri field, Gulf of Suez, Egypt. Pore pressure established from the direct downhole measurements indicated sub-hydrostatic condition in the depleted mid-Miocene Hammam Faraun and Kareem reservoirs. Vertical stress (Sv) estimated using bulk density data yielded an average of 0.98 PSI/feet (22.17 MPa/km) gradient. Magnitudes of minimum (Shmin) and maximum (Shmax) horizontal stresses were deduced from the poro-elastic model. Relative stress magnitudes (Sv ≥ Shmax > Shmin) reflect a normal faulting tectonic stress in the Badri field. Pore pressure and stress perturbations (ΔPP and ΔSh) in the depleted reservoirs investigated from actual measurements recognized 'stress path' values of 0.54 and 0.59 against the Hammam Faraun and Kareem Formations, respectively. These stress path values are far away from the normal faulting limit (0.68), indicating induced normal faulting or fault reactivation to be unlikely at the present depletion rate. [ABSTRACT FROM AUTHOR]

Shale is classified as a poroelastic material as the shale matrix has comparable stiffness with its constituent solid grains. Also, shales at large depth are often fully saturated. Due to their low permeability, which can be in the nano-Darcy range, shales remain undrained during typical loading conditions. Additionally, shales are quasi-brittle materials which can exhibit very pronounced peak shear stress followed by pronounced strain softening during shearing. All these unique aspects of shale behavior must be considered in developing a model that predicts the change of the pore pressure, the effective stress response and the undrained shear strength of shales during loading. This study presents a coupled plastic-damage constitutive model which considers poroelastic effect with the objective to predict the mechanical behavior of saturated shales under undrained condition. The proposed model couples the plastic mechanism, which is formulated on the effective stress space, and the damage mechanism which is responsible for the elasticity degradation and the post-peak softening of shale. Validation of the proposed model against triaxial est results demonstrates that the proposed model can predict the stress-strain behavior, the effective stress path and the pore pressure evolution of shale specimen during shearing. [ABSTRACT FROM AUTHOR]

Coal permeability is akey component that affects gas migration. Considering seepage behavior of gas during mining is critical for improving the production profile of coal-bed methane (CBM) wells, and preventing coal and gas outburst. Aseries of seepage tests have been carried out to simulate the coal permeability evolution law during mining-induced actions. Coal damage has been assessed by using atri-axial servo seepage testing system. The results show that the coal permeability under PCM, Top-coal Caving Mining (TCM) and NM are all kept constant before the deviatoric stress reaches the peak strength, then increases mildly and finally increases rapidly. The effective stress decreases during confining pressure relief, and the permeability increases exponentially with the decrease of effective stress, which is quite different from the traditional triaxial compression test. According to continuum damage mechanics, adamage constitutive model of coal was proposed to characterize coal mechanical characteristics by mining-induced activities. Furthermore, according to the matchstick model, anew coal permeability model is proposed, which could accurately reflect seepage behavior under different mining layouts. [ABSTRACT FROM AUTHOR]

Columnar jointed basalt is widely distributed in the dam foundation and underground cavern of the Baihetan hydropower station, in which columnar jointing and microfractures are developed, the integrity of the rock mass is poor, and the material easily relaxes after excavation. Therefore, it is important to obtain the real mechanical parameters of columnar jointed basalt in rock mechanics problems. In the test cave at the dam site, six in situ samples with a size of 50 cm × 50 cm × 100 cm were successfully made, and in situ true triaxial test were carried out. This paper presents a method for obtaining the strength parameters of the Mohr–Coulomb criterion and Hoek–Brown criterion by true triaxial testing. The peak strength parameters under three states and yield strength parameters under one state were obtained by setting various stress paths for the undisturbed samples and failure samples. The strength characteristics of columnar joint basalt and the crack characteristics of the specimens are analyzed, and the results of the triaxial test and direct shear test are compared. By means of acoustic emission (AE) techniques and surface crack descriptions, the failure process and failure pattern of the samples were obtained. The abundant test results provide strong support for the selection of engineering rock mechanics parameters. [ABSTRACT FROM AUTHOR]