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Copyright of Hydrogeology & Engineering Geology / Shuiwendizhi Gongchengdizhi is the property of Hydrogeology & Engineering Geology Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Although rock mechanical behaviour has a long record of study, attempts to understand the role of fractures on rock deformation still have unresolved issues. Due to technical and/or economic challenges, natural rock fractures are often dealt with crudely, without detailed consideration of fracture geometry and heterogeneity in many geoscience applications. Veined rocks that are ubiquitous in the upper Earth crust fall in that category where sustained efforts are needed to offer key information for rock mechanics and geomechanics applications. Following on from a recent study on the rupture of veined rocks (DOI: 10.1029/2019JB019052), we further examine stress path constraints on the deformation of veined rocks (i.e., stress-path-dependent behaviour of veined rocks) under polyaxial conditions. The Discrete Element Method is used to establish a calcite veined model where constant mean stress (σm) and constant least principal stress (σ3) paths that are representative in the subsurface activities are considered. The results reveal the stress-path dependency of brittleness for models under different loading paths. Models tested under constant-σm conditions exhibit no brittleness, compared to cases where constant-σ3 is applied. Sliding along the strike of an inclined vein is evident under constant-σm deformation, irrespective of the level of stress. Shear bands along the dominated (inclined) veins exhibit apparent particle trajectory anisotropy for the constant-σm deformations which is demonstrated by the evident colour contrast of the adjacent rock matrix and the displacement dispersion of the particles forming the shear bands. We envisage that the reactivation of veins is of relevance to Enhanced Geothermal Systems (EGS) development in terms of seismicity mitigation and multiphysics control of fracture and reservoir permeability. [ABSTRACT FROM AUTHOR]

Copyright of China Sciencepaper is the property of China Sciencepaper and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A novel test method for analyzing the influence of stress path and roughness of structure on the strength characteristics of saturated sand-structure interface by using triaxial apparatus is presented. By making a triaxial specimen with a slope structure with a certain inclination angle, the soil slides along the preset failure surface during the shearing process, and the slope surface roughness of the structure can be controlled and adjusted. A triaxial instrument is used to control the changes in axial pressure,confining pressure to achieve loading in different stress paths. The soil slides along the structure surface with a certain angle during shearing when preformed failure surface exists. The roughness of the structure can be controlled and adjusted. The triaxial apparatus is used to control the confining pressure change of the sample to realize loading under different drainage conditions and stress paths. To verify the effectiveness of the method, direct shear test and triaxial shear tests are carried out on the specimen composed of Fujian sand and steel structures with different roughness and different stress paths. The results show that at the beginning of sliding, the friction angle of the interface in the interfacial constant normal stress path is smaller than that in the conventional shear path. Under smooth conditions, the interface friction angle obtained by direct shear test is 30% to 40% lower than that obtained by triaxial test. Under rough conditions, the interface friction angle obtained by direct shear test is slightly smaller than that obtained by triaxial test, and closer to the friction angle in normal stress shear paths.

This paper presents analytical solutions for the finite expansion problems of a spherical or cylindrical cavity, using a simple yet novel graphical approach recently proposed by Chen & Abousleiman in 2022, in both original Cam Clay (OCC) and modified Cam Clay (MCC) soils under undrained conditions. It is shown that, for a soil mass subjected to isotropic in situ stress conditions, the stress paths in the deviatoric plane for the spherical and cylindrical cavity expansions turn out to be two straight lines, which correspond to Lode angles equal to 11π6$\frac{{11\pi }}{6}$ and 5π3$\frac{{5\pi }}{3}$, respectively. The desired limiting cavity pressure therefore can be directly and accurately evaluated through simple numerical integration with respect to the mean effective stress, while the relationship between the internal cavity pressure and the cavity radius, the cavity expansion curve, may be equally conveniently determined. Numerical results obtained from the current graphical method, for a range of the values of over consolidation ratio considered, compare extremely well with those from the conventional semianalytical formulations of the undrained cavity problem that involve solving a system of coupled governing differential equations. It is interesting to note that the representative and approximate solution developed by Collins & Yu in 1996 indeed is a correct one for the spherical cavity expansion problem, and, with minor modifications, will be applicable for the accurate calculation of the responses of the cylindrical cavity as well. [ABSTRACT FROM AUTHOR]

The formulation and calibration of constitutive models for geomaterials require material behaviour from experiments under a wide range of triaxial loading conditions. However, failure of geomaterials usually involves localisation of deformation that leads to very strong inhomogeneous behaviour. Therefore, the experimentally measured macro (specimen) behaviour is a mix between very different responses inside and outside the localisation zone and thus should not be used as a true representation of the material responses. This paper proposes a theoretical framework that provides links between mechanical responses inside and outside the localisation band, alongside their contributions toward the overall behaviour of a specimen undergoing localised deformation. This meso–macro connection allows the quantification of behaviour inside the localisation band, which is the main source of material inelasticity, from experimentally measured specimen behaviour. Correlation between the thickness of the localisation band and its behaviour is shown, bounded by a unique stress–deformation relationship describing the behaviour of an idealised zero-thickness localisation band. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Engineering Geology / Gongcheng Dizhi Xuebao is the property of Journal of Engineering Geology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

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.

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.

In order to study the influence of stress path on the true triaxial macroscopic and mesoscopic deformation characteristics of rocks, a calibrated granular flow numerical specimen was established using the discrete element method, and the loading planes were selected as meridian plane, n-plane and the fixed-axis plane. Numerical experiments with different stress paths were conducted on the numerical specimen to analyze the true triaxial deformation evolution of rocks from the macroscopic and mesoscopic viewpoints, and the numerical simulation results were compared with the D-P yield criterion to verify the correctness of the results. The research results showed that: for the same stress increment, the anisotropy of strain increment was the lowest when loading along the meridian plane, and the stress reached the yield plane fastest when loading along the n-plane, resulting in the largest strain increment and the two-way unloading in fixed-axis plane produced the largest strain increment in all stress path loading processes. The elastic-plastic deformation characteristics were analyzed using the relative imaging of stress space and yield surface, defining deformation anisotropy parameters; the correlation between stress and strain increment paths was quantitatively analyzed at the mesoscopic view level, and it was concluded that the number of contact force chain transformations and strain increment deflection angle were negatively correlated, and the numerical model produced the largest number of microcracks with the most obvious internal damage when loaded along the n-plane, and the smallest number of microcracks with the least internal damage when loaded along the fixed-axis plane. [ABSTRACT FROM AUTHOR]

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]

Copyright of Journal of Zhejiang University: Science A is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

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]

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.

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]

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]

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]

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]

Studying train-induced response characteristics is essential for safely operating permafrost railway subgrades. A three-dimensional thermal-mechanical coupling nonlinear dynamic model of train-track-subgrade-ground relationships was established to analyse the train-induced dynamic stress, acceleration and stress path characteristics of a permafrost railway subgrade, and field monitoring data were used to verify this model. The differences between the 2D and 3D models are also discussed, along with the seasonal changes, train speed, axle load, and train type affecting permafrost subgrades. The main results are as follows. First, the vibration load significantly impacts the subgrade 6 m below the sleeper, producing distinct vertical dynamic stress waves due to the wheels and bogies. Dynamic compression stress dominates the subgrade and is influenced by the train structure, speed, and sleeper spacing. While the 2D model tends to underestimate the dynamic stress in shallower layers, it concurs with the 3D model in deeper subgrade dynamics within a 10% margin of error. Then, the principal stress axis of the subgrade soil rotates synchronously with train movements, exhibiting regular stress paths in the YZ plane (longitudinal section) with depth-dependent variations in the stress cycles and deviatoric stress. Finally, predominantly originating from sleeper-induced vibrations, the subgrade vibration acceleration varies with the train speed, sleeper spacing, and season and is most pronounced in the vertical direction. This study provides theoretical guidance for the vibration response of permafrost subgrades on the Qinghai-Tibet Railway (QTR). • Proposed a 3D train-track-subgrade-ground dynamic model of permafrost subgrade. • Compared 2D and 3D simulation results. • Discussed seasonal and train effects on permafrost subgrade vibration. [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]

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]

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]

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 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]

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.