Principal Stress Ratio Research Articles

Stress Zoning Characteristics of Goaf Side and Quantitative Evaluation of Surrounding Rock Stability

Aiming to maintain the stability of the mining roadway and the next working-face roadway in the goaf side of a coal mine, a systematic study was carried out, through theoretical analysis, numerical simulation, field case analysis, and other methods, of the synergistic change mechanism in the stress distribution and plastic zone development morphology of the roadway’s surrounding rock in the goaf side under mining disturbance. It was revealed that, under the influence of mining, the goaf side will form a high-deviatoric-stress environment, which directly affects the shape and stability of the plastic zone of the roadway’s surrounding rock. Based on the characteristics of the principal stress ratio and the morphological development law of the plastic zone of the surrounding rock, the side of the goaf is divided into four regions: unloading zone, high-deviatoric-stress zone, low-deviatoric-stress zone, and original-stress zone. And the corresponding mine pressure behavior zoning is proposed: fracture zone, butterfly failure zone, elliptical failure zone, and circular failure zone. A quantitative evaluation method for the stability of the roadway surrounding rock based on the plastic zone morphology criterion was established, and combined with the division of mine pressure, the stability of the surrounding rock on the side of the goaf was quantitatively evaluated.

The Effects of Loading Angles on the Failure of Cross-Ply Notched Bio-Basalt Composites

A cross-ply basalt V-notched butterfly specimens were subjected to pure tension, combined tension-shear, and shear stress-strain state using a modified Arcan test fixture with loading angles from 0° to 90° with 15° increment. Multiaxial stress and strain states were studied using the principal stress and strain ratio, from which the principal angle was determined and used to represent principal states in the gauge section. The quasi-elastic to non-linear transition stresses were determined for each loading angle. In biaxial stress-strain states and pure shear, the deformation and consequently the shear-induced damage start to accumulate significantly. Also, in biaxial stress-strain states between 15°-75°, the shift from tension-shear to pure shear was observed after the transition to the non-linear part of the stress-strain curve. The digital image correlation (DIC) images and microscopic evaluation show that a large extent of damage is the consequence of the shear deformation after the rotation of 0° clamped fibres, while the 90° fibres maintained their original straight form. In off-axis tests, the principal strain axis rotates towards the weakest material axis even at small off-axis angles. This causes a transition from a tension-shear biaxial state in the linear loading part to shear in the non-linear part, leading to irreversible damage beyond the transition point.

Three-Dimensional Coupled Temporal Geomechanical Model for Fault-Reactivation and Surface-Deformation Evaluation during Reservoir Depletion and CO2 Sequestration, Securing Long-Term Reservoir Sustainability

Assessing reservoir subsidence due to depletion involves understanding the geological and geophysical processes that lead to ground subsidence as a result of reservoir fluid extraction. Subsidence is a gradual sinking or settling of the Earth’s surface, and it can occur when hydrocarbons are extracted from underground reservoirs. In this study, a time-integrated 3D coupled geomechanical modeling incorporating the fourth dimension—time—into traditional 3D geomechanical models has been constructed utilizing seismic inversion volumes and a one-dimensional mechanical Earth model (1D MEM). The 3D geomechanical model was calibrated to the 1D MEM results. Geomechanical rock properties were derived from the density and sonic log data that was distributed with conditioning to the seismic inversion volumes obtained from running pre-stack inversion. The standard elastic parameter equations were used to generate estimates of the elastic moduli. These properties are dynamic but have been converted to static values using additional equations used in the 1D MEM study. This included estimating the Unconfined Compressive Strength. In situ stresses were matched using different minimum horizontal principal stress gradients and horizontal principal stress ratios. The match is good except where the weak carbonate faults are close to the wells, where the Shmin magnitudes tend to decrease. The SHmax orientations were assessed from image log data and indicated to be 110° in the reservoir section. A time-integrated 3D coupled simulation was created using the finite-element method (FEM). The effective stresses increase while there is depletion in all directions, especially in the Z direction. The predicted compaction in the reservoir and overburden was 350 mm. Most of the compaction occurs at the reservoir level and dissipates towards the surface (seabed). Furthermore, the case displayed no shear failure that might cause or fault reactivation in the reservoir interval (Kangan–Dalan Formations) located in the simulated area. In this study, we applied an integrated and comprehensive geomechanical approach to evaluate subsidence, fault reactivation and stress alteration, while reservoir depletion was assessed using seismic inversion, well logs, and experiment data. The deformation monitoring of geological reservoirs, whether for gas storage or hazardous gas disposal, is essential due to the economic value of the stored assets and the hazardous nature of the disposed materials. This monitoring is vital for ensuring the sustainability of the reservoir by maintaining operational success and detecting integrity issues.

Finite element analysis on non-contact lap splice connections in ultra-high-performance concrete

This study investigates the stress characteristics and failure modes of the lap splice region for non-contact lap-spliced rebar embedded in Ultra-High-Performance Concrete (UHPC), as well as the influence of lap parameters on these aspects. A series of refined finite element models of non-contact lap-spliced rebars embedded in UHPC were established and validated. The influences of UHPC mechanical properties, splice length, and clear spacing on lap region and the failure modes of specimens were studied. As the lap clear spacing increases from 0.5d to 4d, the ratio of principal compressive stress to principal tensile stress in the lap region of the elastic model decreases from 2.36 to 0.36. This trend indicates that with increasing clear spacing, the force transfer between rebars shifts from being dominated by compressive stress to tensile stress within the UHPC lap region. Consequently, the maximum load of the specimen transitions from being governed by compressive strength to being governed by tensile properties. With a clear spacing of 2d, the maximum load of the specimen increases with the splice length. The maximum load of 8d splice length specimen was increased by at least 27 % compared to the specimen with 0d splice length.

Research on Multiple-Factor Dynamic Constitutive Model of Poured Asphalt Concrete.

This study conducted dynamic triaxial tests on a typical poured asphalt concrete material of core walls in Xinjiang, exploring the dynamic characteristics of poured asphalt concrete under various confining pressures, principal stress ratios, and vibration frequencies. On this basis, the dynamic constitutive relationship of poured asphalt concrete was investigated using the Hardin-Drnevich model. The results indicate that under different confining pressures, principal stress ratios, and vibration frequencies, the variation patterns of the backbone lines of dynamic stress-strain of poured asphalt concrete are basically identical, consistent with a hyperbolic curve. The confining pressure and principal stress ratio significantly affect the backbone line of dynamic stress-strain. By comparison, frequency has a minimal effect. The changing trends of dynamic elasticity modulus and damping ratio of poured asphalt concrete under various factors are almost the same. When the material has high dynamic stress and strain, the hysteresis loop is large. When the curve of the damping ratio becomes flat, the asymptotic constant can be used as the maximum damping ratio. The relationship between the reciprocal of the dynamic elasticity modulus and the dynamic strain of poured asphalt concrete exhibits a linear distribution. Under different ratios of confining pressure to principal stress, there are large discrepancies between the calculated values from the formula and the experimental fitting values of the maximum dynamic elasticity modulus, and the maximum relative errors reach 16.65% and 18.15%, respectively. Therefore, the expression for the maximum dynamic elasticity modulus was modified, and the calculated values using the modified formula were compared with the experimental fitting values. The relative errors are significantly reduced, and the maximum relative errors are 3.02% and 2.04%, respectively, in good agreement with the fitting values of the experimental data. The findings of this article render a theoretical basis and reference for the promotion and application of poured asphalt concrete.

Gradient failure mechanism and control technology of deep roadways under the action of deviatoric stress field

Deformation control of deep roadways is a major challenge for mine safety production. Taking a deep roadway with a burial depth of 965 m in a mine in North China as the engineering background, on-site investigation found that significant creep deformation occurred in the surrounding rock of the roadway. The original supporting U-shaped steel support failed due to insufficient supporting strength. The rock mass near the roadway experienced a transition from triaxial stress conditions to biaxial and even uniaxial stress states as a result of excavation and unloading, leading to a gradient stress distribution in the surrounding rock. From the perspective of the roadway's deviatoric stress field distribution, we investigated the gradient failure mechanism of the roadway and validated it through theoretical analysis and numerical simulations. The study found that the ratio of horizontal principal stress and vertical principal stress determines the distribution shape of the surrounding rock deviatoric stress field. The gradient distribution of the stress field in the roadway will cause time-related deformation of the roadway, which will lead to large deformation and failure of the roadway. Based on this, the control mechanism of roadway gradient failure was studied, and then a combined support technology of CFST supports with high bearing capacity was proposed.

Enhancing Load-Bearing Capacity of Calcareous Sands through Gel Stabilization: A Mechanical and Material Characterization Study.

Calcareous sands often display wide ring grain configurations, high intragranular porosity, a complex structure, and low grain hardness. These attributes typically do not meet the strength criteria necessary to sustain overlying infrastructure in civil engineering applications. This study investigates gel stabilization techniques, blending gel material with calcareous sand at concentrations ranging from 5% to 22%, followed by curing periods of 3 to 28 days to evaluate the load-bearing capacity. Subsequently, an unconfined compressive test is performed to determine the gel material content in stabilized specimens and investigate the influence of gel material types. The gel material-to-sand ratios employed are set at 5%, 10%, and 16% for Portland cement and 13%, 16%, and 22% for gypsum. After that, a triaxial consolidated undrained test is conducted to assess mechanical behavior, pore water pressure, and mechanical properties. The findings reveal increased dilation, stress-strain hardening, and softening post-yield, regardless of gel material type. Principal stress ratios, secant modulus, and cohesion show a positive correlation with maintenance duration and binder content, with implications for improved load-bearing capacity. The study also elucidates the qualitative relationship between secant modulus E50 and confining pressure.

A stability evaluation method for deep-seated toppling in the upper Lancang river, Southwestern China

Deep-seated toppling in the upper reaches of the Lancang River, southwest China involves deformations exceeding 100 m in depth. The slope deformation is initiated by river downcutting and evolves distinctive characteristics with a depth of river incision. In this study, we propose a system for evaluating the stability of deep-seated toppled slopes in different evolutionary stages. This system contains identification criteria for each evolutionary stage and provides the corresponding stability evaluation methods. Based on the mechanical and kinematic analysis of slope blocks, the specific stage of slope movement can be identified in the field through outcrop mapping, in situ tests, surface displacement monitoring, and adit and borehole explorations. The stability evaluation methods are established based on the limiting equilibrium theory and the strain compatibility between the undisturbed zone and the toppled zone. Finally, several sample slopes in different evolution stages have been investigated to verify the applicability and accuracy of the proposed stability evaluation system. The results indicate that intense tectonic activity and rapid river incision lead to a maximum principal stress ratio exceeding 10 near the slope surface, thus triggering widespread toppling deformations along the river valley. When considering the losses of joint cohesion during the further rotation process, the safety factor of the slope drops by 7%–28%. The self-stabilization of toppling deformation can be recognized by the layer symmetry configuration after the free rotation of the deflected layers. Intensely toppled rock blocks mainly suffer sliding failures beyond the layer symmetry condition. The factor of safety of the K73 rockslide decreased from 1.17 to 0.87 by considering the development of the potential sliding surface and the toe-saturated zone.

Propagation and complex morphology of hydraulic fractures in lamellar shales based on finite-discrete element modeling

We explore the controls of stress magnitude and orientation relative to bedding on the resulting morphology/topology of hydraulic fractures using a combined finite-discrete element method (FDEM). Behavior is shown conditioned by the ratio of principal stresses λ=σ3/σ1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda ={\\sigma }_{3}/{\\sigma }_{1}$$\\end{document} and relative inclination of the bedding. When the lateral pressure coefficient (λ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda$$\\end{document}) is less than 0.67, hydraulic fractures predominantly initiate as tensile fractures along the wellbore, aligning with the maximum principal stress direction. Conversely, for λ≥0.67\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda \\ge 0.67$$\\end{document}, shear cracks are favored to initiate for the minor stress difference, leading to a less predictable initiation and extension direction. Simultaneously, diminished stress differences correspond to elevated reservoir breakdown pressures, displaying a linear correlation with lateral pressure coefficients and little influenced by equivalent bedding orientation. Bedding plane orientation significantly impacts the mode and morphology of hydraulic fracture propagation. Bedding parallel to the direction of the minimum principal stress (σ3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\sigma }_{3}$$\\end{document}) favors layer-penetrating and bifurcated fractures, whereas inclined bedding facilitates the emergence of numerous steering-type and capture-type fractures. Especially at steeper inclinations (β=60∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\beta =60^\\circ$$\\end{document}), hydraulic fractures readily extend along the bedding surface, inducing macroscopic shear slip failure. Under high-stress disparities, the breakdown pressure exhibits greater sensitivity to bedding inclination, and its influence pattern aligns with the variations in tensile strength, typically reaching maximum and minimum values at bedding inclination angles of 0° and 60°, respectively.

Micromechanical modeling of hollow cylinder torsional shear test on sand using discrete element method

Previous studies on the hollow cylinder torsional shear test (HCTST) have mainly focused on the macroscopic behavior, while the micromechanical responses in soil specimens with shaped particles have rarely been investigated. This paper develops a numerical model of the HCTST using the discrete element method (DEM). The method of bonded spheres in a hexagonal arrangement is proposed to generate flexible boundaries that can achieve real-time adjustment of the internal and external cell pressures and capture the inhomogeneous deformation in the radial direction during shearing. Representative angular particles are selected from Toyoura sand and reproduced in this model to approximate real sand particles. The model is then validated by comparing numerical and experimental results of HCTSTs on Toyoura sand with different major principal stress directions. Next, a series of HCTSTs with different combinations of major principal stress direction (α) and intermediate principal stress ratio (b) is simulated to quantitatively characterize the sand behavior under different shear conditions. The results show that the shaped particles are horizontally distributed before shearing, and the initial anisotropic packing structure further results in different stress–strain curves in cases with different α and b values. The distribution of force chains is affected by both α and b during the shear process, together with the formation of the shear bands in different patterns. The contact normal anisotropy and contact force anisotropy show different evolution patterns when either α or b varies, resulting in the differences in the non-coaxiality and other macroscopic responses. This study improves the understanding of the macroscopic response of sand from a microscopic perspective and provides valuable insights for the constitutive modeling of sand.

Characterising mechanical properties and failure criteria of steel slag aggregate concrete under multiaxial stress states

To advance the application of steel slag aggregate concrete (SAC) in civil engineering, this study introduces a comprehensive experimental program to explore the failure mode, strength, deformation, and failure criteria of SAC under multiaxial stress conditions. In this investigation, SAC specimens containing 50% stabilised steel slag as a coarse aggregate were examined for mechanical behaviour (e.g. stress–strain relationships) under diverse loading scenarios, including uniaxial tensile and compressive loadings, biaxial compression, and true triaxial compression loadings. Subsequently, three distinct failure criteria (i.e. the Ottosen criterion [1], William–Warnke criterion [2], and Guo criterion [3]) for SAC were formulated and authenticated based on the experimental outcomes. The results showed varied failure modes in the SAC, such as prism type, splitting tensile, sheet-like tensile, and oblique shear, which depended on the stress state of the specimen during failure. A significant elevation in the ultimate strength and deformation of SAC specimens was observed, with an increment in the first-to-third principal stress ratio (σ1/σ3). Furthermore, the failure criterion introduced by Guo was validated as adept at reproducing all geometric characteristics of the concrete failure surface and precisely forecasting SAC’s ultimate strength of the SAC. Overall, this investigation proposed a reliable failure criterion based on true triaxial test data that can not only describe the multiaxial strength characteristics of SAC but also provide design advice for the applications of SAC in engineering structures.

Research on the deformation mechanism and ACC control technology of gob‐side roadway in an extra‐thick coal seam with varying thickness

AbstractTo improve the extraction of coal resources, gob‐side entry driving is gradually being promoted and applied. However, as the thickness of the coal seam increases, the deformation control of the roadway becomes more difficult. Aiming at this problem, a combination of theoretical calculation, numerical simulation, and field test was used to analyze the aspects of the stress environment, surrounding rock properties, and support forms. The phenomenon and mechanization of intensified deformation and failure of roadway with increasing coal seam thickness were revealed. The specific results include: (1) The ratio of principal stress at the excavation position controls the maximum failure depth of roadway, and the direction of principal stress determines the location of the maximum failure depth; (2) As the thickness of the coal seam increases, the properties of the surrounding rock at the excavation position decrease, the principal stress ratio increases, and the deflection angle of the principal stress increases, which lead to an intensification of the deformation and failure of the roadway. Based on the deformation and failure characteristics of the roadway and the shortcomings of the original support form, a control strategy and support scheme based on a new support structure called “Anchor Cable with C‐shaped Tube” is proposed, which has achieved better deformation control effect in on‐site application.

Experimental study on strain accumulation characteristics of saturated remolded loess under pure rotation of the principal stress axis

The study conducted a pure rotation test of 720° for saturated remolded Q2 loess under principal stress axis (PSA) rotation when deviatoric stress and the intermediate principal stress ratio are different, also paying attention to examining the changes of strain accumulation and pore pressure of such loess. According to the test, as the PSA rotates continuously, the pore water pressure exhibited by saturated remolded loess presents a normal cyclic accumulation elevation, and under the same deviatoric stress conditions, the pore water pressure accumulation trend of different principal stress ratios is the same; however, the magnitude is different. The increment of pore water pressure becomes smaller as the number of cycles increases. When the deviatoric stress level is low, the material hardening is in a cyclic stable state, the strain component remains stable with the continuous rotation of the PSA, and the strain path area is reduced, together with a stable final size. In case of higher deviatoric stress, the material strength cycle becomes weaker and the strain component accumulates slowly as the PSA (the spiral line of the strain path) continuously rotates in the γzθ−(ϵz−ϵθ) plane, and gradually expands until failure. The increased intermediate principal stress is accompanied by accelerated strain development speed and advanced failure.

Dynamic Characteristics of Asphalt Concrete as an Impervious Core in Embankment Dams under Varying Temperatures and Stress States.

To reveal the dynamic characteristics of asphalt core embankment dams (ACEDs), we carried out a dynamic triaxial experiment on hydraulic asphalt concrete (HAC) under different temperatures (T = 4 °C, 10 °C, 16 °C, and 22 °C) and stress states (Kc = 1.0, 1.2, 1.4, and 1.6; σ3 = 0.5, 0.6, 0.7, and 0.8 MPa). The results indicate that HAC's maximum dynamic elastic modulus increased with decreasing temperature, increasing principal stress ratio, and increasing confining pressure. However, the damping ratio showed the opposite trend. Moreover, in order to study the deformation capacity of HAC, 300 cyclic loads were applied to some specimens. At a temperature of 22 °C, the specimens had a tendency to deform axially, but not significantly. With a decrease in temperature, the axial deformation tendency of the specimen gradually weakened or even disappeared. However, a small number of cracks appeared in the aggregate and between the asphalt and the aggregate of the specimen. In order to quantify the dependence of dynamic parameters on temperature, the temperature influence factor of the maximum dynamic elastic modulus and the temperature sensing factor of the damping ratio were defined. The variation in the temperature influence factor of the maximum dynamic elastic modulus with temperature can be described by a logistic function. The temperature sensing factor of the damping ratio increased with an increasing principal stress ratio and peripheral pressure. Finally, maximum dynamic elastic modulus and damping ratio computational models for the interaction of temperatures and stress states were developed using the normalization method. Upon comparison, the dynamic parameters were observed to be very close to those listed in the literature, which verifies the applicability of the computational models of the maximum dynamic elastic modulus and damping ratio.

Numerical investigation of hydraulic fracture propagation interacting with bedding planes

Bedding planes are abundant in the continental shale oil reservoirs, but their impact on hydraulic fracturing is still not fully understood. To investigate the effects of stress condition, bedding plane’s properties and layer lithology on hydraulic fracture propagation in layered formation, a 3D hydro-mechanical coupled model is presented. The bedding planes are explicitly represented by cohesive elements with degraded properties compared to the rock. The results show that increasing the stress difference ratio of the vertical principal stress to horizontal principal stress promotes the hydraulic fracture propagating across bedding planes, and the critical value of vertical principal stress required for hydraulic fracture crossing rises with the decrease of bedding plane’s critical traction. Furthermore, when the critical traction ratio of bedding plane to rock drops below 0.7, the fracture propagation pattern changes from crossing to deflecting. Additionally, the extension pressure increases slightly, the fracture length decreases and the area of opening bedding planes rises. As the permeability of bedding plane increases, the fracture cannot cross the bedding planes and even stops extension. The layer lithology contrast also affects fracture propagation patterns. This study examines the fracture propagation behaviors under different patterns, including the opening morphology of bedding plane and the evolution of injection pressure and strain energy. The simulation results may provide useful insights for hydraulic fracturing design in layered formations.

Experimental study on bending fatigue strength of adhesively bonded joints

This paper focuses on the establishment of repair or strengthening method of steel structures by externally bonded patch plates. The bending static tests and bending fatigue tests of adhesively bonded joints were conducted and the fatigue life is evaluated. The Konishi E258R is used as an adhesive, and the bending fatigue tests were conducted, varying the applied stress ranges and the stress ratio, the ratio of minimum and maximum stress of fatigue test. The result indicates that the fatigue life of adhesively bonded joints can be evaluated in the function of principal stress ratio, the ratio of principal stress range against principal stress at debonding by the bending test, with constant linearity regardless of the stress ratio.

Large scale land reclamation and the effects on hydro-mechanical behavior in loess and loess-derived fill

To discover the complex hydro-mechanical evolution on Loess Plateau that accompany large-scale land reclamation project, we tracked changes of moisture, matric suction, anisotropic displacements and stresses in geological interfaces of natural loess and deep man-made fill during a comprehensive process of construction and the subsequent three years rest period. Results indicate that vertical stress increases during emplacement of thick fill, while the lateral earth pressure first increases and then gradually decreases with elapsed time. There was a non-linear increase in lateral earth pressure with depth of infill during construction. Whereas lateral earth pressure decreased with depth in lengthy rest period. The fill vertically compressesed and laterally extended during construction, while the geological interface of fill and loess laterally rebounded during construction pause. The primary vertical displacement occurred at the contact zone between the original loess slope and fill. The lateral earth pressure coefficient of infill is influenced by lateral compression and extension of contact zone during land reclamation process. It mirrors the observation that variations in lateral earth pressure with depth are dependent on deformation of natural slope materials. Interestingly, vertical stress and lateral earth pressure periodically fluctuated during rest period, correlating to creeping and shrinkage in loess influenced by moisture content change under different environmental relative humidity. The volumetric moisture contents in fill and contact zone significantly increased during construction. The moisture content of contact zone is 10% higher than the overlying higher density fill after construction, which is attribute to the process of matric suction equilibrium. Considering a constant moisture content, the ratio of historical mean principal stress to deviatoric stress in fill and interface are under the principal stress failure line during construction. However, water diffusion potentially threats the stability of this man-made geological structure.

Influence of intermediate principal stress ratio on strength and deformation characteristics of frozen sandy soil under different moisture contents

Frozen ground often contain non-uniform moisture content and are under true triaxial stress state and their practical engineering application are still explored. The present study is carried out to investigate the strength and deformation characteristics of frozen soil with different moisture contents under true triaxial stress state conditions. A series of true triaxial tests were conducted on frozen soil specimens having constant minimum principal stress (σ3) and constant intermediate principal stress ratio (b). The experimental test results showed that the stress-strain curves represent the strain-hardening characteristics under different test conditions. The intermediate principal stress ratio (b) has a dual effect of strengthening and weakening on the strength of frozen sandy soil. When 0 ≤ b ≤ 0.5, the strengthening effect was found to be dominant in the frozen soil specimen, whereas, the weakening effect was dominant when 0.5 < b ≤ 1. The strength of frozen soil under true triaxial stress state is 8.18% to 36.33% higher under different water content conditions, compared to that under generalized triaxial compression stress state. The difference in deformation between the direction of the intermediate principal stress and the minimum principal stress increased with the increment in the intermediate principal stress ratio value. The volumetric strain in the soil sample initially exhibited the characteristics of shear contraction and then shear dilation.

Spatial and Temporal Stress Variations before and after the 2008 Wenchuan Mw 7.9 Earthquake and its Implications: A Study based on Borehole Stress Data

AbstractIn situ stress measurement data was analyzed to estimate the temporal and spatial stress variations at shallow depths in the Longmenshan fault zone (LMSF), prior to and following the 2008 Wenchuan earthquake (WCEQ). Analysis of the stress field related to fault strength and behavior is useful for understanding geodynamic processes and conducting hazard assessments. The shallow stress changes after the WCEQ show clear along‐strike variations. Degrees of stress orientation rotations have a negative correlation with the horizontal principal stress ratios and the WCEQ apparently reduced the magnitude difference between horizontal principal stresses. Taking stress magnitudes and orientation distribution relative to the fault strike into account, we propose an intermediate‐strength of LMSF, with a friction coefficient generally constrained between 0.35 and 0.6. In addition, high‐pressure fluids in the fault zone reduce the effective normal stress and to a certain degree weaken the fault strength. The accumulated stress over a certain period following release of the WCEQ indicates the start of another earthquake cycle. The changing crustal stress field makes the LMSF stable or slipping optimally during geodynamic processes. The segmentation feature of the shallow crustal stress field in the LMSF may imply a different tectonic loading and seismic release processes along the fault. The southwestern section to the epicenter of the WCEQ favors the occurrence of future earthquakes, as high μm in a state of critical failure was present in this area, which indicates that the Wenchuan and Lushan earthquakes did not release the accumulated stress to a sufficient extent there.

Coalbed methane accumulation, in-situ stress, and permeability of coal reservoirs in a complex structural region (Fukang area) of the southern Junggar Basin, China

The enrichment of coalbed methane (CBM), in-situ stress field, and permeability are three key factors that are decisive to effective CBM exploration. The southern Junggar Basin is the third large CBM basin in China but is also known for the occurrence of complex geological structures. In this study, we take the Fukang area of the southern Junggar Basin as an example, coalbed methane accumulation and permeability, and their geological controls were analyzed based on the determination of geological structures, in-situ stress, gas content, permeability, hydrology and coal properties. The results indicate that gas contents of the Fukang coal reservoirs are controlled by structural framework and burial depth, and high-to-ultra-high thickness of coals has a slightly positive effect on gas contents. Perennial water flow (e.g., the Baiyanghe River) favors gas accumulation by forming a hydraulic stagnant zone in deep reservoirs, but can also draw down gas contents by persistent transportation of dissolved gases to ground surfaces. Widely developed burnt rocks and sufficient groundwater recharge make microbial gases an important gas source in addition to thermogenic gases. The in-situ stress field of the Fukang area (700–1,500 m) is dominated by a normal stress regime, characterized by vertical stress &amp;gt; maximum horizontal stress &amp;gt; minor horizontal stress. Stress ratios, including lateral stress coefficient, natural stress ratios, and horizontal principal stress ratio are all included in the stress envelopes of China. Permeability in the Fukang area is prominently partitioned into two distinct groups, one group of low permeability (0.001–0.350 mD) and the other group of high permeability (0.988–16.640 mD). The low group of permeability is significantly formulated by depth-dependent stress variations, and the high group of permeability is controlled by the relatively high structural curvatures in the core parts of synclines and the distance to the syncline core. Meanwhile, coal deformation and varying dip angles intensify the heterogeneity and anisotropy of permeability in the Fukang area. These findings will promote the CBM recovery process in China and improve our understanding of the interaction between geological conditions and reservoir parameters and in complex structural regions.