Increase In Deviatoric Stress Research Articles

Energy characteristics of saturated Jurassic sandstone in western China under different stress paths

AbstractTo study the energy evolution and failure characteristics of saturated sandstone under unloading conditions, rock unloading tests under different stress paths were conducted. The energy evolution mechanism of the unloading failure of saturated sandstone was systematically explored from the perspectives of the stress path, the initial confining pressure, and the energy conversion rate. The results show that (1) before the peak stress, the elastic energy increases with an increase in deviatoric stress, while the dissipated energy slowly increases first. After the peak stress, the elastic energy decreases with the decrease of deviatoric stress, and the dissipated energy suddenly increases. The energy release intensity during rock failure is positively correlated with the axial stress. (2) When the initial confining pressure is below a certain threshold, the stress path is the main factor influencing the total energy difference. When the axial stress remains constant and the confining pressure is unloading, the total energy is more sensitive to changes in the confining pressure. When the axial stress remains constant, the compressive deformation ability of the rock cannot be significantly improved by the increase in the initial confining pressure. The initial confining pressure is positively correlated with the rock's energy storage limit. (3) The initial confining pressure increases the energy conversion rate of the rock; the initial confining pressure is positively correlated with the energy conversion rate; and the energy conversion rate has a high confining pressure effect. The increase in the axial stress has a much greater impact on the elastic energy than the confining pressure. (4) When the deviatoric stress is small, the confining pressure mainly plays a protective role. Compared with the case of triaxial compression paths, the rock damage is more severe under unloading paths, and compared with the case of constant axial stress, the rock damage is more severe under increasing axial stress.

Creep behavior of dry and saturated medium-grain sandstone and its relationship with conventional mechanical properties

With the large scale mining of coal and the increase of abandoned goafs under weakly cemented aquifer strata in Western China, it is urgent to study the time dependent mechanical properties of water sensitive aquifer strata. In this paper, creep behavior of dry and saturated medium-grain sandstone, which represent two limit states affected by water, were studied and compared. The results showed that water greatly weakens the compressive strength of medium-grained sandstone, but the difference in axial strain between dry and saturated rock samples decreases with the increase of confining pressure. The creep compression volume of rocks decreases with the increase of deviatoric stress, and only under low confining pressure does the creep volume of rocks exhibit expansion. There is an order of magnitude difference in creep strain between medium grained sandstone and other common rocks. The instantaneous elastic modulus has a negative exponential relationship with deviating stress. The creep rate has a positive exponential relationship with deviating stress. Based on Burges model with exponential damage variables, the law of the influence of confining pressure on creep model parameters has been discussed. The linear relationship between elastic modulus of medium-grained sandstone and parameters of Burgers model with damage was found. The research results are conducive to the convenient prediction of creep behavior of medium grained sandstone engineering and the long-term stability control of the surrounding rock.

Creep behavior of layered salt rock under triaxial loading and unloading cycles

Abstract This paper presents an extensive laboratory investigation of layered salt rocks under triaxial loading and unloading cycles aiming to better understand their viscoelastic plasticity strain characteristics. Specifically, creep curves were obtained from the triaxial loading and unloading tests and analyzed using the Burgers model. The microstructure of the layered salt rocks before and after the creep test was comparatively analyzed using the dry grinding optical diagram. The results show that the creep deformation of layered salt rocks is mainly viscoelastic under a low deviatoric stress and contains both viscoelastic and viscoplastic deformations when the deviatoric stress is high. With the increase of deviatoric stress, both the instantaneous elastic and plastic strains increase, and the increasing rate becomes lower as the deviatoric stress increases. The creep curves of layered salt rocks can be well represented with the Burgers model. Since the creep leads to fracturing and interlayer rupturing and producing new fluid seepage channels in layered salt rocks, this study has great value for understanding the long-term performance of gas and hydrocarbon liquid storage caverns in layered salt rock formation.

Strong Effect of Stress on the Seismic Signature of the Post‐Stishovite Phase Transition in the Earth's Lower Mantle

AbstractThe stishovite to post‐stishovite phase transition may modify the scattering of seismic waves by stishovite‐bearing rocks in the Earth's lower mantle. A series of continuous compression experiments on sintered polycrystalline stishovite was performed to study the effect of stress on the phase transition. The experimental results show that the phase transition shifts to lower pressures as the magnitude of deviatoric stress increases. Our results further show that the bulk modulus of sintered polycrystalline stishovite differs from that derived from single crystal measurements and decreases at the phase transition. In cold regions, such as subducted slabs, stresses may accumulate and shift the phase transition to a shallower depth. In hot regions with less stress, such as rising plumes, the phase transition is shifted to a greater depth. In addition, the phase transition may have varying seismic signatures depending on the behavior of the grain boundaries in mantle rocks and the micro‐stresses present in neighboring grains.

Effect of the Field-Stress State on the Subgrade Resilient Modulus for Pavement Rutting and IRI

The new Mechanistic-Empirical Pavement Design Guide (MEPDG) uses the subgrade resilient modulus (MR) as the key input parameter to represent the subgrade soil behavior for pavement design. The resilient modulus increases with an increase in confining pressure, whereas, for an increase in deviatoric stress, it increases for granular soils and decreases for fine-grained soils. The value of MR is highly stress dependent, with the stress state (i.e., bulk stress) a function of the position of the materials in the pavement structure and applied traffic loading. Applying excessive vertical stress at the top of the subgrade without knowing the appropriate stress state can result in permanent deformation. In situ stress must be calculated so the correct resilient modulus can be determined. To facilitate the implementation of MEPDG, this study develops a methodology to select the appropriate subgrade resilient modulus for predicting rutting and IRI. A comprehensive research methodology was undertaken to study the effect of in situ or undisturbed subgrade MR on pavement performance using the MEPDG. Results show that MR obtained from in situ stress is approximately 1.4 times higher than the MR estimate from NCHRP-285. Thus, the in situ stress significantly affects the calculation of subgrade MR and, subsequently, the use of MR in the predicted rutting, with IRI using the AASHTOWare pavement mechanistic-empirical design. Results also show that the pavement sections were classified as in “Good” and “Fair” conditions for rutting and IRI, respectively, considering in situ MR.

Study on Hydro-Mechanical Coupling Failure and Permeability Enhancement Mechanisms for Sandstone with T-Shaped Fractures.

The rise in the connectivity of the fractures is a key task in oil/gas and geothermal exploitation systems. Natural fractures widely exist in underground reservoir sandstone, while the mechanical behavior of rock with fractures subjected to hydro-mechanical coupling loads is far from clear. This paper employed comprehensive experiments and numerical simulations to investigate the failure mechanism and permeability law for sandstone specimens with T-shaped faces subjected to hydro-mechanical coupling loads. The effects of crack closure stress, crack initiation stress, strength, and axial strain stiffness of the specimens under different fracture inclination angles are discussed, and the evolution processes of permeability are obtained. The results show that secondary fractures are created around the pre-existing T-shaped fractures through tensile, shear, or mixed modes. The fracture network causes an increase in the permeability of the specimen. T-shaped fractures have a more significant effect on the strength of the specimens than water. The peak strengths of T-shaped specimens decreased by 34.89%, 33.79%, 46.09%, 39.32%, 47.23%, 42.76%, and 36.02%, respectively, compared with intact specimen without water pressure. With the increase in deviatoric stress, the permeability of T-shaped sandstone specimens decreases first, then increases, reaching its maximum value when macroscopic fractures are formed, after which the stress suddenly decreases. When the prefabricated T-shaped fracture angle is 75°, the corresponding permeability of the sample at failure is maximum, with a value of 15.84 × 10-16 m2. The failure process of the rock is reproduced through numerical simulations, in which the influence of damage and macroscopic fractures on permeability is discussed.

Study on the Accuracy of Fracture Criteria in Predicting Fracture Characteristics of Granite with Different Occurrence Depths

The fracture network of a deep geothermal reservoir forms the place for heat exchange between injected fluid and rock mass with high temperature. The fracture resistance ability of reservoir rocks will affect the formation of fracture-network structure, heat exchange and transmission characteristics, and reservoir mechanical stability. However, there are few reports on the fracture toughness and trajectory prediction of geothermal reservoirs with different depths. In this paper, the modified maximum tangential stress criterion (MMTS) is analyzed. The results show that the experimental data are significantly different from the theoretical estimate of MMTS under the influence of different occurrence depths. It is found that the fracture process zone (FPZ) seriously affects the accuracy of predicting fracture initiation angle and mixed-mode (I+II) fracture toughness by MMTS. The FPZ value, considering the influence of different occurrence depths, is modified, and the accuracy of MMTS in predicting the fracture mechanical characteristics of granite is improved. In addition, the mechanical test results show that the Brazilian splitting strength (σt) of granite fluctuates increase with the increase in temperature. With the increase in deviatoric stress, the Brazilian splitting strength and the Brazilian splitting modulus of rock show a trend of first increasing, then decreasing, and then increasing.

NMRI online observation of coal fracture and pore structure evolution under confining pressure and axial compressive loads: A novel approach

Quantitative characterization of the spatial distribution, content and heterogeneity of fracture and pore structure (FPS) in coal reservoirs under confining pressures and axial compressive loads is significant for the engineering of coal bed methane. A novel online observation approach that combines nuclear magnetic resonance imaging with triaxial loading techniques is employed to achieve the visualization and full-scale quantitative characterization of the evolution of FPS in coals in the laboratory. The relationship between the stress states and FPS evolution was formulated. The results show that the spatial distribution of the FPS evolution process of coal samples can be divided into four stages: initial pore and fracture compaction closure, pore and fracture stable growth, pore and fracture unstable growth, and failure stages. As the deviatoric stress increases, the content of the adsorption pores, the heterogeneity of the adsorption space, and the gas adsorption capacity of coal samples gradually increase. In contrast, the seepage pore and fracture content as well as the permeability of coal samples decrease first and then increase. The heterogeneity of the seepage space of coal samples initially increases and then decreases. The maximum compression of seepage space and increase of adsorption space are 4.742% and 14.743%, respectively.

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.

A Review on Factors Affecting the Resilient Modulus of Subgrade Soils

The subgrade layer is the lowest pavement layer, which carries the loads transferred from the upper layers. Different researchers have studied the resilient modulus (Mr) of different subgrade soils for the fine-grained and coarse-grained soil types. The layer's resilient response mechanism was found to be different for those fine and coarse materials, and it is vital for improving the pavement performance and life constructed over it. The different parameters related to the soil that can affect the resilient modulus include moisture content, stress level, compaction degree, loading frequency, and matric suction characteristics. Due to the variability of the Mr result, a study is needed for each soil type and input parameter. The effects of these parameters on the Mr are reviewed and discussed in this paper. The results show that the water content beyond the optimum level and the increase in deviatoric stress decreased Mr. In contrast, the increase in confining stress, compaction degree, loading frequency, and matric suction was found to improve the Mr. During the wetting and drying of the soil, the Mr was improved in the drying process.

Nonlinear Elasto-Visco-Plastic Creep Behavior and New Creep Damage Model of Dolomitic Limestone Subjected to Cyclic Incremental Loading and Unloading

For many rock engineering projects, the stress of surrounding rocks is constantly increasing and decreasing during excavating progress and the long-term operation stage. Herein, the triaxial creep behavior of dolomitic limestone subjected to cyclic incremental loading and unloading was probed using an advanced rock mechanics testing system (i.e., MTS815.04). Then, the instantaneous elastic strain, instantaneous plastic strain, visco-elastic strain, and visco-plastic strain components were separated from the total strain curve, and evolutions of these different types of strain with deviatoric stress increment were analyzed. Furthermore, a damage variable considering the proportion of irrecoverable plastic strain to the total strain was introduced, and a new nonlinear multi-element creep model was established by connecting the newly proposed damage viscous body in series with the Hookean substance, St. Venant body, and Kelvin element. The parameters of this new model were analyzed. The findings are listed as follows: (1) When the deviatoric stress is not more than 75% of the compressive strength, only instantaneous deformation, transient creep, and steady-state creep deformation occur, rock deformation is mainly characterized by the instantaneous strain, whereas the irrecoverable instantaneous plastic strain accounts for 38.02–60.27% of the total instantaneous strain; (2) Greater deviatoric stress corresponds to more obvious creep deformation. The visco-elastic strain increases linearly with the increase of deviatoric stress, especially the irrecoverable visco-plastic strain increases exponentially with deviatoric stress increment, and finally leads to accelerated creep and delayed failure of the sample; (3) Based on the experimental data, the proposed nonlinear creep model is verified to describe the full creep stage perfectly, particularly the tertiary creep stage. These results could deepen our understanding of the elasto-visco-plastic deformation behavior of dolomitic limestone and have theoretical and practical significance for the safe excavation and long-term stability of underground rock engineering.

Experimental and Numerical Investigation on Undrained Behavior of Geogrid Reinforced Pond Ash

Geogrid reinforced pond ash is an effective composite material, which can be utilized as a construction material for various geotechnical engineering purposes. In this study, both experimental and numerical investigations have been carried out on shear strength characteristics of unreinforced and geogrid reinforced pond ash samples under undrained condition. Consolidated undrained (CU) triaxial tests have been performed on 100 mm x 200 mm cylindrical ash samples under static compressive loading. The investigations include the influence of confining pressure and number of geogrid layers on stress–strain characteristics of pond ash. Finite element package PLAXIS 2D was used for numerical simulation of unreinforced and geogrid reinforced pond ash models under undrained condition. Both experimental and numerical results show that the peak deviatoric stress increases as confining pressure increases for both unreinforced and reinforced ash samples. For a particular confining pressure, with increase in number of geogrid layers the value of peak deviatoric stress increases and its corresponding strain decreases. The increase in peak deviatoric stress is about 27%–93% for one-layer geogrid reinforced sample and 37%–117% for two-layer geogrid reinforced sample compared to unreinforced ash sample. Finite element analysis shows a good match in the stress–strain plot with the experimental result.

Porosity and Permeability Evolution with Deviatoric Stress of Reservoir Sandstone: Insights from Triaxial Compression Tests and In Situ Compression CT

Porosity and permeability are the two most important characteristics of underground gas storage in sandstone reservoirs. Injection of gas into reservoir rocks will cause rock deformation. The deformation will influence the porosity and permeability properties of the rocks. We investigate the evolution of these two properties of storage sandstone by triaxial compression tests and a uniaxial in situ compression CT test. As the deviatoric stress increases, the sandstone is compressed firstly (porosity reduction) and then dilates (porosity enhancement). With the increase in confining stress, the occurrence of volumetric dilation will be delayed. Trapped porosity of this sandstone at different deviatoric stresses is very small (0.122%-0.115%) which indicates that nearly all pores are connected. During the compression stage, the decrease in permeability is related to compression of pores and microcracks. During the volumetric dilation stage, it is related to increase in tortuosity. This interpretation can be confirmed by observations of in situ compression CT. The permeability evolution estimated by pore network modeling is consistent with macroscopic testing results.

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.

Effect of stress condition on the creep behaviour of Beishan granite

In China, a deep granite formation in Beishan area is currently considered as a potential host rock for high-level radioactive waste repositories. Creep tests were conducted by two modes, namely single step loading mode on several samples and multi-step loading mode on single sample. According to the experiment results, the granite specimens failed almost at the same axial strain and volumetric strain at a given confining pressure, despite of the differences in deviatoric stress, stress ratio or strain ratio. The increase in stress condition (deviatoric stress, stress ratio, or strain ratio) could accelerate the cracking process and creep strain rate, and consequently resulted in a shorter time to failure. The stress effect on the cracking process was also confirmed by the acoustic emission (AE) energy rate. Significant increases in AE energy rate and the percentage of AE events with high energy could be observed with the increase of deviatoric stress. It was indicated from the results that, the stress ratio and strain ratio were more reasonable when discussing the influence of stress condition, perhaps because they were defined by considering both the applied stress level and the mechanical properties of rock material.

Investigation of factors affecting the behaviour of subgrade soils resilient modulus using robust statistical methods

When designing pavements, there are three fundamental external design parameters to evaluate (1) the characteristics of the subgrade upon which the pavement is placed, (2) the applied loads and (3) the environment. This study is based on laboratory testing of cohesive subgrade materials to characterise their resilient modulus properties and to identify the main controlling factors of this property. Experimental results showed a slight increase in resilient modulus values with an increase in confining pressure and a noteworthy decrease in resilient modulus accompanied by an increase in deviatoric stress. This finding was verified statistically. Based on the independent samples t-test analysis, it was revealed that soil type and water content caused statistically significant difference in resilient modulus values. According to one-way ANOVA, deviatoric stress turned out to be influential while confining pressure did not. In addition, it was demonstrated using the two-way ANOVA that none of the two-factor combinations was significant in changing resilient modulus values of A-4a and A-6a soils. Finally, three-way ANOVA showed that changing the deviatoric stress, soil type and water content all together had a noticeable effect on resilient modulus behaviour. Furthermore, all predictive models were found statistically significant at the 0.05 significance level with high predictive power. Confining pressure and deviatoric stress are the two predictors whereas bulk stress was not included by the stepwise regression method. The non-linear regression relatively enhanced the predictive power of all the models.

Influence of mining activities on the reactivation of a footwall fault

The presence of geological faults intersecting with steeply dipping orebodies is not uncommon in hardrock mining. More often than not, such a scenario leads to seismic activities as ore extraction approaches the intersection point. This is particularly true for deep mining. This paper examines the influence of mining direction, i.e., overhand (bottom-up) versus underhand (top-down) mining, on the reactivation of a footwall fault in a sublevel stoping system. To do so, the static and dynamic behaviour of a footwall fault intersecting with a steeply dipping orebody is investigated. A model parametric study is undertaken with respect to the dip angle of the fault and the location of the fault intersection point with the orebody. The static analyses demonstrate that the volume of ore extracted before mining activity reaches the fault intersection is an influential factor, which impacts the degree of unclamping of the fault. It is thus suggested that mining directions be assessed in such a way as to minimize the volume of mined ore prior to intersection with the fault. It is also shown that the effect of mining direction on seismicity along the fault significantly decreases as the fault dip angle decreases. A dynamic analysis is then carried out and it confirms the same trends as the static analysis, i.e., fault-slip occurs over a larger fault surface area as the volume of extracted ore prior to intersecting the fault increases. However, there is no noticeable difference in the intensity of near-field ground motion during fault-slip for the two mining directions. A case study of the Lucky Friday Mine is then reviewed, where bedding faults intersect with a steeply dipping vein from the footwall side. It is reported that shear movements and seismicity on the fault are not the direct cause of rockbursts that took place around a stope. Rather, the damage was caused by the stress change induced by the slip. In order to elucidate the mechanism, the development of deviatoric stress induced by the shear movement along the footwall fault is analyzed while comparing with the case at Lucky Friday Mine. In order to clarify the influence of shear movements, the deviatoric stress is compared in models with and without faults. It is shown that the shear movement significantly contributes to the increase in deviatoric stress around the fault, thus suggesting damage to the rockmass and generation of seismically active zones. The results are found to be in agreement with the case at Lucky Friday Mine.

Stick-slip behaviours of dry glass beads in triaxial compression

Consolidated undrained triaxial compression tests are conducted on standard cylindrical samples of nearly monodisperse glass beads, initially assembled in a dense state by a funnel pouring technique. Deviator stress, volumetric strain and induced pore air pressure are measured as functions of vertical strain under different confining stress and vertical strain rates. We find that in stick phase the glass beads accumulate energy and regain strength, which can lead to slow sliding in volumetric strain curve without stress drop in deviatoric stress curve for confining stress of 1000 kPa. Particularly after the previous slip event, as the deviatoric stress increases, the volume of sample repetitively compacts and then dilates before reaching the next slip events under high confining stress, which never happens at the lower. In slip phase, a stress drop coincides with a jump of volumetric strain and vertical strain, which make the pore air pressure grow sharply. The effects of confining stress and vertical strain rate on stick-slip instabilities are discussed in dry dense glass beads samples.

Experimental Study of the Stress Dependence of the Permeability of Rock

The seepage evolution characteristic of rock materials is very significant for the stability and safety of rock engineering. In this research, triaxial compression and water seepage tests were carried out on sandstone specimens. Based on the experimental results, the relationship between permeability and deformation is analyzed in detail. The results show that the permeability decreases with the increase of deviatoric stress at the initial compaction and elastic deformation stages, while the permeability increases with the increase of deviatoric stress at the stages of yield deformation, stress sharp drop and residual stress. The permeability evolution in rock is inherently coupled with the deformation, damage and failure process. It is highly related to the initiation and propagation of cracks. The compressive strength is found to decrease when the pore water pressure is higher and to increase when the confining pressure is higher.

Elastoplastic modelling the creep behaviour of cataclastic rock under multi-stage deviatoric stress

The objective of this paper is to realise the elastoplastic modelling of the creep behaviour of cataclastic rock under multi-stage deviatoric stress. The multi-stage triaxial creep tests show that the cataclastic rock exhibits pronounced irreversible time-dependent deformations which enlarge with the increase in deviatoric stress. The plastic shearing mechanism of the rock can be identified based on the observation that the rock shows typical plastic strain and large strain rate during the creep tests. Towards this, a unified creep model is developed to describe both the instantaneous and the time-dependent elastoplastic behaviour of the cataclastic rock. The elastoplastic model for describing the instantaneous behaviour is developed based on classic plasticity using a revised Drucker–Prager criterion and a non-associated flow rule, and the time-dependent deformation is described in terms of evolution of microstructure in the context of irreversible thermodynamics. The time-dependent deformation is considered as a macroscopic consequence of progressive degradation of material structure in microscopic scale. Finally, the proposed model is applied to predict the material responses in short-term triaxial compression tests and multi-stage creep tests. Comparisons between experimental and simulated results show that the proposed model is able to describe the main features of the creep behaviours observed in this material.