Unloading Stress Paths Research Articles

Failure and deformation characteristics of shale subject to true triaxial stress loading and unloading under water retention and seepage.

A multi-functional true triaxial fluid-structure coupling system was used to conduct water retention and seepage tests of shale under true triaxial loading and unloading stress paths. The stress–strain evolution of shale specimens under different experimental conditions was obtained, and the corresponding deformation and strength were analysed. The evolution and failure characteristics of cracks in shale were obtained by CT scanning images before and after the experiment. The results show that the volumetric strain of shale specimen increases first, then decreases and finally continues to increase with an increase in deviatoric stress under water retention, indicating that the volumetric change has experienced a compaction-expansion-compacting. The σ-ε1 curve of the sample increases first and then decreases, while the deformation in the σ2 direction shows the repeated compression and expansion. In the seepage test, the permeability–strain curve can be divided into two parts before and after fracture according to the σ-ε1 curve. Before fracture, the compression velocity of the specimen in the loading direction exceeds the expansion velocity in the unloading direction, resulting in a decrease in volume and a decrease in permeability. With an increase in deviatoric stress, fractures occur inside the particles and continue to spread from the tip until the fractures break through the shale specimen. The pore fissure area increases and the permeability of the sample increases rapidly. In terms of fracture evolution, for the water retention test, dense tensile and shear cracks appear on the failure plane perpendicular to the σ1 and σ3 directions, and complex shear fracture network appears on the failure plane perpendicular to the σ2 direction. For the seepage test, heavy shear failure occurs throughout the original fracture of the sample. With an increase in the penetration depth, the fracture shape on the failure surface perpendicular to the σ2 direction gradually changes from single to complex.

Instability of gold mine tailings subjected to undrained and drained unloading stress paths

The drained instability of two gold mine tailings under lateral stress relief is investigated in this study. Constant deviator stress (CDS) unloading tests were performed using a triaxial apparatus to examine instability imposed by unloading in a drained condition. Instability was induced by enforcing a constant deviator stress while simultaneously reducing the mean effective stress. Stress paths and shear strengths of CDS tests were compared with undrained triaxial tests on comparable specimens which were anisotropically consolidated to the same initial stress ratios of the CDS tests. Several techniques were employed to determine the onset of instability in the CDS tests. The effects of unloading rate and initial stress ratio on the behaviour and the triggering of instability in the CDS tests were further investigated. It was observed that specimens consolidated to the same void ratio and initial stress ratio undergo instability at similar stress ratios or friction angles in both CDS and undrained shear tests. This suggests that the instability characteristic of tailings prone to stress relief can be predicted using undrained triaxial tests on anisotropically consolidated specimens. Critical states and state parameters of specimens subjected to CDS and unloading stress paths were also compared and analysed.

Unloading effect of the shear resistance of rock joints

To investigate the stress path dependent of rock joints, a series of tangential loading tests and normal unloading tests were conducted using 32 cement mortar replicas of artificially split rock joints with 4 different joint roughness coefficients (JRC). The critical normal stresses of the unloading tests were recorded, and a comparative study was conducted by referencing the results under different loading paths and joints conditions. The test results indicate that the shear resistance has a distinct unloading effect, which is mainly influenced by the morphologic characteristics, normal stress levels and stress paths. By analyzing the results, it can be concluded that the variation trend of shear/normal stress ratio against the normal stress and JRC of the two test conditions were identical. However, under low normal stress condition, the stress ratio of the joints under normal unloading stress path is the higher one; while under higher normal stress, the relationship becomes converse. Compared to that of the tangential loading condition, shear/normal stress ratio of the unloading stress path reduces rapidly as the increasing of normal stress, and the influence of the morphology is masked under lower normal stress.

An Experimental Study on the Effects of True Triaxial Loading and Unloading Stress Paths on the Mechanical Properties of Red Sandstone

Loading and unloading stress paths play critical roles in investigating the deformation and failure of roadway excavation. In this study, tests under four different loading and unloading stress paths were conducted on red sandstone samples, with the aid of a self-developed true triaxial test system. Meanwhile, the deformation and failure characteristics of the samples were monitored during the tests. The following research conclusions were obtained: The octahedral shear stress is linearly correlated with the average effective stress, and the correlation coefficient R2 is 0.9825. The Mogi–Coulomb strength criterion is superior to the Drucker–Prager strength criterion in reflecting strength failure characteristics of red sandstone during loading and unloading. Shear failure tends to occur under uniaxial compression, whereas shear–tensile composite failure occurs under loading and unloading conditions. Compared with the true triaxial loading test, loading and unloading tests produce a larger strain in the unloading direction. Under loading and unloading stress paths, with the increase in intermediate principal stress (IPS), the strain in the direction of IPS gradually changes from expansion to compression, and the peak strength gradually increases. The state of IPS affects the failure strength of the sample and reflects the strengthening effect of IPS. This paper boasts a certain value and significance for research on the deformation and failure characteristics of sandstone in the actual in situ stress environment with triaxial dynamic changes.

Stability prediction and analysis for large underground cavern controlled by weak interlayer zone under high geostress

Due to the special engineering geological characteristics of weak interlayer zone (WIZ) with the initial shear, the pre-consolidation, the water softening and clay generation, that occurs between different rock strata (e.g., tuff and basalt), it tends to represent potential threats to the overall stability of large underground cavern under high geostress, such as the large deformation of upper and lower rock masses, structural stress-induced collapse and the time-dependent plastic squeezing-out failure, which could never be neglected, and requires more widespread concern. Focusing on the prediction and analysis of such stability issues induced by WIZ, a novel constitutive model for WIZ, based on the unearthed macro-meso mechanical response of WIZ under complicated unloading stress paths, has been first established, and well embed into the effective numerical calculation platform, to realize the quantitative descriptions of the mechanical effects of stress path, the particle breakage as well as the mechanical parameter evolution of WIZ. Then the rock failure index for WIZ (RFDwiz) was derivated, to help determine the cracking scale, depth and degree of surrounding rock masses with WIZ. Based on the proposed model, the RFDwiz and the cracking-restraint design method, the prediction of the failure position and degree, as well as the dynamic optimization design of the reasonable supporting parameters of rock masses with WIZ in underground cavern, were ultimately realized. This study will provide theoretical basis for the analysis and prediction of the instability of deep underground projects controlled by rock masses with WIZ.

Effects of high temperatures and unloading confining pressures on granite

The mechanical and permeability properties of rocks under unloading stress path and high temperature are critically important for hot dry rock. A series of unloading confining pressure and gas permeability tests were conducted on granite samples after heating and rapid cooling treatment. Five levels of temperature and three levels of confining pressures were used. The stress–strain curves of unloading confining pressure were obtained. The strength parameters and deformation parameters were further discussed and analysed. When the heat treatment temperature is higher than 600°C, the granite samples experience a significant degree of thermal damage. With the increase in heat treatment temperature and the decrease in confining pressure, the gas permeability increases. Beyond 600°C, the gas permeability of the samples increases significantly. With the increase in temperature, the internal friction angle decreases slightly from 25 to 400°C, while it increases beyond 400°C. The cohesion exhibits an inverse evolution. The initial unloading elastic modulus and deformation modulus decrease gradually during relief of confining pressure. The decrease rate of the deformation modulus increases with the increase in heating temperature. The experimental results could improve knowledge about the effect of thermal shock and stress on the physico-mechanical properties of granite.

Macro- and Micromechanical Behaviors and Energy Variation of Sandstone under Different Unloading Stress Paths with DEM

Abstract 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...

The effects of unloading on drained cyclic behaviour of Sydney sand

New excavation or tunnelling affects the stress state of soils in ground. The change of stress state due to excavation may affect the cyclic behaviour of soils. Cyclic loading, such as traffic and earthquake loading, induced ground deformation may be greater than expected if such effect is not considered. A series of cyclic triaxial tests were performed on Sydney sand with different relative densities. The effect of unloading sequence on deformation of the sand under cyclic loading was simulated by reducing lateral stress in steps between loading cycles. The dependence of strain accumulation on the magnitude of confining pressure reduction and on unloading stress paths was studied. The results indicate that the sand has a memory of stress history and the stress history of such unloading enlarges the strain accumulation during the subsequent cycles, and the greater the reduction of lateral stress, the greater the accumulated strain. Under cyclic loading, the accumulated axial strain could increase nonlinearly or linearly with the ratio of unloading magnitude to initial mean effective stress, depending on the stress state before cyclic loading. The unloading stress paths have limited effects on the final accumulated strain if the initial and final stress states are the same. The variation of strain accumulation direction attributes to the change of average stress ratio resulting from lateral stress reduction, but hardly depends on relative density and unloading stress paths. The strain accumulation direction after unloading roughly agrees with the modified Cam Clay flow rule.

Study on the Permeability Change Characteristic of Gas-Bearing Coal under Cyclic Loading and Unloading Path

Using the self-developed three-axis servo fluid-solid coupling system with gas-solid coupling of gas-bearing coal, the variation law of the permeability of gas coal under the stress cycle loading and unloading path was studied. The qualitative and quantitative relationships between permeability, axial force, and radial stress of gas-bearing coals were established, and the variation law of permeability of gas-bearing coals was discussed. The results show that (1) different cyclic loading and unloading stress paths correspond to the permeability characteristics of different gas-bearing coals. (2) Permeability of gas-bearing coal decreases with the increase of axial stress and radial stress, and it has a logarithmic function with axial stress and radial stress. This shows that axial stress and radial stress are important factors affecting the permeability characteristics of gas-bearing coal. (3) Under the same stress loading and unloading conditions, the axial stress is less than radial stress on the permeability of gas-bearing coal. In the cyclic loading and unloading axial stress process, the permeability of the gas-bearing coal varies by a smaller extent than the cyclically unloaded confining force. (4) The cumulative damage rate of gas-bearing coal under axial stress gradually increases with the increase of the number of cycles of loading and unloading, and the rate of the cumulative damage rate of permeability is less than the corresponding rate of radial stress.

An Insight into the Excavation-Induced Stress Paths on Mechanical Response of Weak Interlayer Zone in Underground Cavern Under High Geostress

A weak interlayer zone (WIZ) is a poor zonal geotechnical system with loose structure, weak mechanical properties, variable thickness, random distribution, and strong extension, that occurs between different rock strata (e.g., tuff and basalt), due to the intense tectonic movement, representing a potential threat to the overall stability of rock masses with WIZs in large underground cavern excavations. Focusing on the excavation-induced hazards in the weak interlayer zone (WIZ) occurring in underground cavern under high geostress, the mechanical response of WIZ under different loading and unloading stress paths has been well investigated and unearthed, by a series of automatic stress path controlled triaxial tests, as well as through scanning electron microscope (SEM) analysis. Results show that the mechanical characteristics of WIZ are closely related to the initial confining pressure and stress paths. Sudden increases in the circumferential strain and strong dilations of WIZ occur from the start of unloading, among which the unloading total strains are most strongly promoted by stress path II (i.e., axial pressure loading and confining pressure unloading), whereas the unloading stress path IV (i.e., axial pressure and confining pressure unloading) has the greatest promotion on enhancing the plastic volumetric dilatation. The deformation modulus and the Poisson’s ratio follow a deterioration law under the unloading process of confining pressure, and it is the unloading stress path II that most influences the damage of elastic parameters. The ultimate bearing strength, the internal friction angle, and the cohesion of WIZ appear extremely obvious degradation in course of unloading, among which the damage effect of stress path IV is the most remarkable. The macroscopic and mesoscopic analyses reveal that the failure mechanism of WIZ under complicated unloading stress paths lays in the accumulation and propagation of axial and circumferential cracks and fractures, as well as pervasive particle breakage, with the intergranular fractures, transcrystalline fractures, and the shear scratches on a meso level. The research could provide an effective basis and good lessons for the mechanical response and failure mechanism of WIZ under unloading stress paths in underground cavern under high geostress.

Mechanical Properties of Soft Soil considering the Influence of Unloading Stress Paths

The mechanical properties of soft soil are crucial for the design and construction of underground space excavation; however, the current design and numerical analysis of underground spaces consider the loading mechanical parameters, ignoring the influence of the unloading stress path resulting in frequent construction accidents in practice. Here, soft soil in Shenzhen, China, is taken as the research subject, and a series of consolidated‐undrained unloading tests are performed. First, K0 consolidation is conducted. Then, unloading tests are performed with different unloading ratios to simulate different unloading stress paths. The test results show that the soft soil deformation characteristics are closely related to the stress path and unloading ratio. Under different unloading ratios, soft soil will undergo compression deformation or rebound deformation. Under unloading conditions, the deviator stress‐strain curve satisfies a hyperbolic function and can be normalized with the average consolidation confining pressure. With the increase in the unloading ratio, the initial tangent modulus first decreases and then increases, the cohesion decreases, and the internal friction angle does not change significantly. The loading mechanical parameters are not suitable for numerical calculation in unloading engineering. In this paper, more unloading paths are considered, such as UU1.0 and UU0.5. The results of the study provide a theoretical basis for the calculation of the numerical analysis of the soil body at different depths in rich soft soil pits.

Experimental Investigation on Deformation and Strength Behavior of Marble with the Complex Loading-Unloading Stress Path

The excavation of deep tunnel in rock mass undergoes complex loading and unloading stress paths, resulting in rib spalling, flaking, and even severe rockburst disasters. Based on the variation law of the stress path of the surrounding rock, laboratory tests of rock mechanics are designed, and the deformation and strength behavior of marble with different initial confining pressure and unloading rates are systematically studied. By introducing strain increment, the characteristic stress, and the dilatancy index, the rock’s dilatancy and brittleness under different unloading conditions are quantitatively analyzed. During unloading, the energy transformation mechanism of rock is described, and the law of deformation and failure is discussed based on characteristic energy. The rock failure strength fitting formula is given by applying the Mogi–Coulomb criterion and elastic strain energy criterion. The advantages of the elastic strain energy criterion are theoretically explained. This study shows that comprehensive consideration of the complex stress paths, confining pressure levels, and the loading-unloading rates of surrounding rock is an effective way to accurately study unloading rock characteristics. The results can provide theoretical basis for stability analysis of high-stress underground engineering.

STUDY ON CREEP CHARACTERISTICS OF SALT ROCK UNDER UNLOADING CONDITIONS

The underground salt karst caverns are considered as an excellent oil and gas reserve site due to the dense low permeability and excellent creep. The creep characteristics of salt rocks under frequent gas injection and gas production activities are the focus of salt cavern gas storage. The 700h loading and unloading creep tests were performed on salt samples taken from Jintan, Jiangsu Province, China, and the creep behaviors of rock salt under different loading and unloading stress paths were compared and analyzed. The results showed that the steady-state creep strain rate increases with the increase of deviatoric stress during multi-stage creep test, The deviatoric stress has a great influence on the steady-state creep strain rate, and the steady-state creep strain rate increases obviously with the increase of the deviator stress in the loading stage, but in the unloading stage, steadystate creep strain rate increases with deviatoric stress increase, and the increase range is small. The Fractional Derivative model, the Nishihara model and the Burgers model are used to fit the experimental data of the initial creep stage and the steady-state creep stage during loading stage, the results show that the Fractional Derivative model can better describe the creep curve. The creep deformation curve is almost straight and the creep curve is fitted by a linear function in unloading stage, the results show that the fitting curve is consistent with the experimental results.

Experimental Study on the Gas Flow Characteristics and Pressure Relief Gas Drainage Effect under Different Unloading Stress Paths

Coal seam gas is a critical substance because it can be a source of a large quantity of clean energy as well as a dangerous source of risk. A pressure relief gas drainage is an effective and widely used method for coal seam gas recovery and gas disaster control in coal mines. A series of pressure relief gas drainage experiments were conducted using large-scale coal samples under different unloading stress paths in this study to explore the unloading stress paths. From the experimental results, the dynamic evolutions of gas pressure, coal temperature, and gas production were analyzed. The trends of gas pressure and coal temperature during pressure relief gas drainage were similar: dropping rapidly first and then slowly with time. Correspondingly, gas production was fast in the early stage of pressure relief gas drainage and became stable thereafter. Meanwhile, gas flow characteristics were significantly affected by the unloading stress paths. Gas pressure and coal temperature had the maximum descent by unloading stress in three directions simultaneously, and the unloading stress of the Z direction had the minimal impact when only unloading in one direction of stress. However, the influence of unloading stress paths on gas production was complex and time dependent. The difference coefficient parameter was proposed to characterize the influence degree of unloading stress paths on the pressure relief gas drainage effect. Eventually, the selection of unloading stress path under different situations was discussed based on time, which is expected to provide the basis for pressure relief gas drainage.

Impact of loading-unloading on deformation and permeability of sandstone containing methane

ABSTRACT In this paper, based on the evolution characteristics of the stress field of coal and rock masses during the mining process of the 2 + 3# coal seam of the Sichuan Coal Group, a three-way loading and unloading stress path is designed, and the mechanical and permeability characteristics of the sandstone samples tested at room temperature and after experiencing different temperature effects, the following conclusions are drawn: During the loading and unloading process, the specimen exhibits axial compressive deformation, and the radial direction only exhibits shrinkage deformation during the process of increasing the confining pressure. Under the same confining pressure, the peak strength of the sample gradually decreases with increasing gas pressure, and the fitted gas pressure and the peak strength of the sample conform to a power function relationship. The use of an axial strain change rate and radial strain change rate are proposed to represent the axial strain change characteristics and radial strain change characteristics during loading and unloading. After undergoing different temperature effects, in the process of providing a constant confining pressure and increasing axial pressure, as well as the process of providing a constant confining pressure and increasing confining pressure, the permeability of the sample decreases nonlinearly with the increase in the deviatoric stress. In the process of simultaneously loading the axial stress and unloading the confining pressure, the permeability of the sample increases nonlinearly with the increase in the deviatoric stress. The relationship curve between the Poisson’s ratio, deformation modulus and axial strain of the sample and the relationship curve between the permeability, Poisson’s ratio, and deformation modulus are discussed.

3-D Numerical Study on Progressive Failure Characteristics of Marbles under Unloading Conditions

3-D particle-based discrete element method (PB-DEM) was employed to numerically study the mechanical and progressive failure characteristics of pre-fissured marble specimens under conventional triaxial unloading conditions. The microscopic parameters of PB-DEM for marble materials were calibrated using comparison with the previous experimental data. To systematically investigate the mechanical properties and the progressive failure characteristics of pre-fissured marble specimens under the unloading conditions, a series of numerical specimens were simulated. The effects of fissure geometric conditions, initial confining pressures, and unloading rates on the mechanical and failure behaviors were explored via simulations. The present numerical results indicate that peak strength increased as the initial confining pressures increased or the unloading rate decreased. Crack coalescence types and the ultimate failure modes in the pre-fissured marble specimens were significantly affected by the unloading stress paths. The present numerical results provide a better understanding of unloading mechanical and failure characteristics to scientists and engineers in rock mechanics and rock engineering.

Influence of Loading and Unloading Stress Paths on the Deformation and Failure Features of Jinping Marble Under True Triaxial Compression

To study the difference in the failures of laboratories #7 and #8 of the China Jinping Underground Laboratory Phase II (CJPL-II), different loading and unloading stress path tests were carried out under a true triaxial compression condition based on the measured in situ stress of CJPL-II. The strength, deformation and failure mechanisms of Jinping marble under different stress paths were thoroughly investigated. The results showed that under the same stress state as that at sample failure, the bearing capacity and deformation of the unloading stress path were greater than those of the loading stress path. The bearing capacity and deformation in the σ2 and σ3 directions of stress path II and stress path III were similar, and the others decreased from stress path IV to stress path V under unloading conditions. The failure mode results reflect the violence of the unloading failure. The failure features and mechanisms of different stress paths were analysed though the acoustic emission (AE) ring, cumulative energy and strain energy trend. This research may provide a reference for the excavation and support of deep underground works.

Study on Transient Unloading Response Characteristics of Brittle Coal Samples Under Different Stress Paths

During the unloading process of engineering excavation, the high energy storage in brittle coal is induced to release, which leads to the failure of surrounding rock. The response characteristics of brittle coal samples under instantaneous unloading are analyzed by numerical simulation of different unloading stress paths in this paper. The results show that under the condition of limited lateral displacement, there is a certain linear relationship between the lateral stress and the vertical stress in a certain range before unloading. With the increase of the level of vertical stress loading, the maximum displacement increases obviously after the instant unloading and the failure is more serious. The critical impact stress value of the brittle coal sample is 17 MPa. Under the condition of equal lateral pressure on both sides, the maximum displacement and the vertical stress increase in the form of quadratic polynomial after instant unloading. The relationship between vertical stress and lateral stress on the critical failure envelope is a fourth-order quartic polynomial. With the increase of lateral stress, the impact liability of brittle coal samples is changed from being controlled by vertical stress to being controlled by lateral stress. Under the condition of unequal lateral pressure on both sides, the burst liability of brittle coal samples after instantaneous unloading is most affected by the vertical stress, followed by the minimum stress, and least affected by the intermediate stress.

Study on the damage characteristics of gas-bearing shale under different unloading stress paths.

In order to understand the influence of unloading on the mechanical properties of shale rock, triaxial unloading tests under different stress paths were conducted. In this paper, three types of tests are completed, including: 1) Conventional triaxial compression test;2) Pre-peak constant maximum principal stress-unloading confining pressure test with different initial confining pressures and rates;3) Increasing axial stress-unloading confining pressure test. The deformation and rupture modes characteristics of shale sample under different unloading stress paths were obtained. Research results show that: 1) The confining pressure effect is obvious and the peak strength increases with the increase of initial confining pressure, under conventional triaxial compression test, the samples show obvious elastic-plastic characteristics; Under unloading confining pressure test, it shows obvious elastic brittleness characteristics.2) Compared with conventional triaxial compression test, unloading confining pressure is more prone to deformation and rupture, and the damage is more serious. Under same initial stress level, the brittle characteristics in unloading confining pressure are more obvious and the expansion is more intense. 3) Under same unloading stress path, the higher the initial confining pressure is, the more severe the sample failure is. With the increase of unloading rate, the rupture degree of the sample becomes more complex.4) The brittle rupture characteristic increases with the increase of unloading rate and initial confining pressure. Increasing axial stress-unloading confining pressure, various types of tensile and shear fractures with different mechanisms are well developed. These conclusions reveal loading and unloading mechanical properties of gas-bearing shale under different stress paths; it provides theoretical basis for horizontal drilling, fracturing design and long-term fracturing effect analysis of shale gas reservoirs.

Investigation on fractal characteristics of salt rock under different tensile conditions

In order to study the characteristics of fractal dimension of salt rock and the change of its physical properties under different tensile conditions, the experiment of indoor indirect tensile, direct stretching, three-point bending and tensile pressure alternately was carried out on the pure salt rock, and the whole process was monitored by acoustic emission test system and the fracture scanning was performed after the test. The results showed that: (1) the fractal characteristics of the rock fracture roughness under different tensile conditions are related to the mechanical behavior. The higher the tensile strength, the smaller the fractal dimension of the cross section, and the smaller the roughness of the section. (2) under the four tensile conditions, the fractal dimension of the acoustic emission of salt rock shows a dimension reduction with the increase of stress and the release of energy, and the dimension reduction process is related to the mechanical characteristics of salt rock. Under the alternating stress path, the fractal feature of the acoustic emission location point of the salt rock appears more late, and the fractal dimension of the peak time is smaller than that under the direct tensile loading and unloading stress path, indicating that under the alternating stress path of tension and pressure, the internal damage of the salt rock is more aggregated; (3) the fractal dimension of the roughness and the fractal dimension of the roughness under the same tension condition. The results of acoustic emission fractal dimension calculation are highly similar.