Variation Of Effective Stress Research Articles

A Semi-Analytical numerical non-stationary creep analysis of a rotating disk made of Polyamid66 under mechanical and thermal loads

A semi-analytical numerical non-stationary creep analysis of constant and variable thickness rotating disks made of polyamide 66 under mechanical and thermal loads is presented using Mendelson’s method of successive approximation. The material properties are time, temperature, and stress-dependent, which vary along the disk’s radius due to effective stress variation and temperature distribution and are represented by Burger’s model. A non-homogeneous differential equation in terms of radial displacement with variable and time-dependent coefficients was established using equilibrium, strain-displacement, and stress-strain relations. First, the initial thermo-elastic stresses at zero time were calculated using the Galerkin method and Hermitian elements. The history of stresses and strains was then calculated using Burger’s viscoelastic model and Prandtl-Reuss relations. To validate the results, an analysis of a disk with constant thickness made of PVDF material was also carried out using the same method employed in this study. Very good agreement of the results was observed in both elastic and creep conditions. Creep analysis was performed for PA66 disks of constant and variable thickness conditions, and the results were compared together. It has been found that the radial displacements and effective stresses are increasing with time for both disks. However, effective stresses and radial displacements of variable-thickness disks are much lower than constant-thickness disks.

Comparative Analysis of Foundation Systems in Expansive Soil: A Three-Dimensional Model Approach to Moisture Diffusion and Volume Changes

This study compares the performance of various foundation systems in expansive soils, such as mats, granular anchor piles, and concrete piles. Expansive soils experience volumetric changes due to moisture fluctuations, which can lead to structural damage. Abaqus software, in conjunction with the SCV approach, is used to analyze soil-foundation interactions. A custom subroutine enhances simulation accuracy by incorporating empirical data on unsaturated clay behavior, matric suction, and variations in effective stress. The method’s accuracy is validated by comparing simulation results to field and laboratory experiments. The findings indicate that increasing the applied load on mats decreases overall heave but increases the differential heave. Additionally, higher soil permeability dereases the differential heave of mats. Granular anchor piles outperform concrete piles by more than 50% in highly expansive soils, suggesting a preference for these foundations. This study provides insights into the behavior of expansive soils, which will assist engineers in designing resilient foundation systems for structures.

A Simplified Model to Predict the Repeated Shear Strain during the Cyclic Triaxial Test by Using an Elastic Coefficient-Damping Ratio System

Some researchers in past years have tried to develop a simplified method for analyzing soil liquefaction. However, the correctness of the pore water pressure in the model will affect the results. In addition, the formulas derived are not easy, and the exact parameters of the model are difficult to obtain. This study used a mass-spring-damping system to simulate the repeated strain of liquefaction cyclic triaxial tests. Because the model is simple and the parameters are easy to understand and obtain, it also shows the extensibility of this model. During the parameter study, damping coefficient c and spring coefficient k parameters decreased with the increasing cyclic number. Preliminary results of the research show that this model can further simulate the repeated strain obtained by cyclic triaxial tests without considering the variation of effective stress during cyclic loading. Four samples were used to verify the model’s correctness, and their boring sites were found in Yunlin areas, Taiwan. Simulation results show that the spring-damping system is feasible for simulated cyclic triaxial tests because the simulated results correlate to the testing results in trend. Generally, the first cycle number simulation will be less accurate because the pore water pressure of the specimen changes rapidly when the performance has just started. In contrast, the increase in subsequent cycles may be biased due to cyclic stress variation and soil plasticity during simulation. In the future, pure sand specimens created in the laboratory will be suggested for simulation.

Benchmark for Nonlinear Consolidation of a Soil Deposit by Considering Depth-Dependent Initial Effective Pressure

Analytical solutions for 1D nonlinear consolidation of soils are widely applied to calculate consolidation settlement for their simplicity and convenience. Furthermore, analytical solutions can provide valid benchmark for numerical solutions of nonlinear consolidation. However, the existing analytical solutions for 1D nonlinear consolidation assume the initial effective stress to remain constant in the whole deposit. In practice, the initial effective stress increases with depth in a soil deposit. The existing analytical solutions for 1D nonlinear consolidation are lack of consideration about the variation of initial effective stress along with depth, so there is no benchmark for evaluating the corresponding numerical calculation. In this study, an analytical solution for nonlinear consolidation is developed under a special case, considering the linear increase of initial effective stress with the depth, which provides a benchmark for the corresponding numerical calculation of nonlinear consolidation. To facilitate the verification of the reliability of the numerical calculations of nonlinear consolidation, the pore pressure dissipation curves and settlement curves with time for conditions of PTIB (the case for a permeable top surface and an impermeable bottom surface is referred to as PTIB) and PTPB (the case for a permeable top surface and a permeable bottom surface is named as PTPB) are provided in this paper, thus providing some validation data for future calculations of nonlinear consolidation considering a linear increase in the initial effective stress with depth.

Highly variable friction and slip observed at Antarctic ice stream bed

The slip of glaciers over the underlying bed is the dominant mechanism governing the migration of ice from land into the oceans, with accelerating slip contributing to sea-level rise. Yet glacier slip remains poorly understood, and observational constraints are sparse. Here we use passive seismic observations to measure both frictional shear stress and slip at the bed of the Rutford Ice Stream in Antarctica using 100,000 repetitive stick-slip icequakes. We find that basal shear stresses and slip rates vary from 104 to 107 Pa and 0.2 to 1.5 m per day, respectively. Friction and slip vary temporally over the order of hours, and spatially over 10s of metres, due to corresponding variations in effective normal stress and ice–bed interface material. Our findings suggest that the bed is substantially more complex than currently assumed in ice stream models and that basal effective normal stresses may be significantly higher than previously thought. Our observations can provide constraints on the basal boundary conditions for ice-dynamics models. This is critical for constraining the primary contribution of ice mass loss in Antarctica and hence for reducing uncertainty in sea-level rise projections.

Experimental Investigation on Jacked Pile-Sinking and Capacity-Induced Pore Pressure and Effective Radial Stress at the Pile-Clayey Soil Interface

Based on research into how static pressure piles sink in saturated cohesive soil, alterations caused by a rise in pore stress during the load process between the pile and the earth as well as the effects of a variation in valid radial thrust on the project are thoroughly studied. These changes are of great importance for the application and practice of practical projects. Microtest components are embedded on the surfaces of open and closed piles, and test research is conducted using a large-scale system and double-deck open and closed pipe piles. Finally, complete consideration is given to how variations in effective radial stress. Through this test, it was determined that as the pile’s buried depth gradually grows, so do the valid radial thrust and excess pore pressure. However, the quantity of the superfluous pore water stress for a closed pile is higher than for an open pile. And the higher half has substantially lower effective radial thrust and overall surplus pore stress than the lower part does. The pile penetration depth is constant, and the value of h/L of the pile shaft is also rising, the occurrence of “lateral pressure degradation” of earth load between pile and earth is becoming increasingly prominent. The maximum ratio of the excess pore stress to the valid soil pressure between the pile and earth contact is 62.1% for an exposed pile and 52.1% for a sealed pile. At the interface, the effective radial stress is about 3.76∼5.46 times that of the remaining pore water stress. Therefore, under changing pile penetration depths, the value of h/L will have an impact on the pile’s and the soil’s additional pore water pressure or excellent soil pressure. The test results have a significant impact on how static pressure piles are built and how bearing capacity is designed in real-world applications.

Gas permeability variation during methane hydrate dissociation by depressurization in marine sediments

Accurate acquisition of permeability change in hydrate-bearing sediments (HBSs) is the key to predicting reservoirs' multiphase fluids distribution and production capacity. Hydrate saturation and effective stress present significant effects on reservoir permeability. Thus, the influence of hydrate saturation and effective stress on gas permeability change of remolded core during decompression was studied. The results showed that effective stress compression and hydrate decomposition determine the flow channel structure within hydrate sediments. Under unconsolidated and low effective stress(σ' = 1.2 MPa) conditions, the gas permeability increased gradually as the decomposition of hydrate, and the magnitude of permeability variation before and after decomposition increased with increasing hydrate saturation, indicating that the effect of hydrate saturation on permeability dynamic change dominated at low effective stresses. When σ′ was increased to 3.2 MPa and 5.2 MPa, the results were contrary, indicating that the effective stress variation on gas permeability dynamic change dominated at higher effective stresses. In addition, it was found that compression led to higher heat transfer efficiency in high-saturation samples. When hydrate saturation was increased from 23.37% to 48.7%, the effect of increasing effective stress on gas production changed from inhibition to facilitation.

Cyclic compression of sands at constant stress ratio—experimental investigations and constitutive modelling

Several constitutive models for sands are based on an additive split of the effective stress rate into two terms. One term accounts for the deviatoric stress ratio varvec{r}, the other for the mean effective stress {p}. While sophisticated techniques are available to account for monotonic and cyclic variations of varvec{r}, the {p} term is usually treated by means of rather rudimentary constitutive mechanisms. In particular, no plasticity sand model seems to exist whose functions take into account cyclic changes of {p} in a realistic manner. However, cyclic changes of {p} frequently occur in geotechnical boundary value problems and can cause irrecoverable deformation in non-cohesive soils. Present sand models significantly under- or overestimate these deformations. This issue is tackled in the paper. First, results from a comprehensive series of triaxial tests on Toyoura Sand are presented. The samples were loaded with cyclic changes of {p} at constant stress ratio in order to study the effects of mean effective stress variations exclusively. The test results show that the samples accumulate significant irrecoverable strains throughout successive loading cycles. The results furthermore allow to investigate the effects of pressure level, pressure amplitude, stress ratio level and density on the strain evolution. Second, new constitutive functions for an existing Bounding Surface Plasticity model are proposed in the paper. They are intended to improve the simulation results obtained with the model for monotonic and cyclic changes of {p} at constant stress ratio. The extended model is then carefully calibrated and validated using the experimental data from the triaxial tests. The results of the validation prove that the new constitutive functions have the desired effect.

A 2D hybrid NMM-UPM method for waterflooding processes modelling considering reservoir fracturing

This study presents a methodology to integrate two simulation codes, i.e., cohesive element-based numerical manifold method (Co-NMM) and unified pipe network method (UPM), for 2D coupled two-phase seepage-stress analysis of waterflooding processes in fractured rock masses. The Co-NMM is an extension of the numerical manifold method (NMM) and designed for efficiently addressing continuity-discontinuity problems of rock especially complex multiple-fracture propagation, while the UPM is a relatively novel approach for underground seepage analysis with single−/multiphase fluid flow in both fractured and fractured porous reservoirs. To link the Co-NMM and UPM properly, a two-phase seepage-stress coupling model is proposed for considering interactions between the fractured rock mass and two-phase fluids. The mechanical behaviour of the fractured rock mass responds to the variation in skeletal effective stress induced by the average two-phase fluid pressure in both the rock matrix and fractures, including deformation of the rock matrix, change in fracture aperture and the initiation and propagation of fractures. Accordingly, the induced changes in hydraulic behaviours such as the porosity, permeability and capillary pressure are captured by nonlinear stress-dependent coupling functions. Furthermore, an implicit sequential solution is adopted to execute the two solvers, consisting of two iteration loops, i.e., the two-phase flow solving loop and seepage-stress coupling solving loop. The relative performance of the developed 2D hybrid NMM-UPM method is illustrated for four cases related to the waterflooding process in porous and fractured porous mediums.

Competing Controls of Effective Stress Variation and Chloritization on Friction and Stability of Faults in Granite: Implications for Seismicity Triggered by Fluid Injection

AbstractFluids injection for hydraulic stimulation and fracturing, typical in the development of enhanced geothermal systems (EGS) in granites, can reactivate deep faults and induce seismicity. Such faults typically contain chlorite coatings as an alteration product that may impact styles of deformation—aseismic through seismic. We performed low velocity shear experiments on simulated granite fault gouges under conditions typifying a geothermal reservoir at ∼4‐km depth with a confining pressure of 110 MPa, a temperature of 150°C, fluid pressures of 21–80 MPa, and chlorite contents of 0–100%, to investigate the influence of variation in effective stress and mineral composition on fault strength and stability. Our results show a transition from velocity‐strengthening to velocity‐weakening behavior in simulated granite gouge when the effective confining pressure was reduced from 89 to 30 MPa, characterized by a transition from fault compaction to dilation—as revealed by microstructural observations—with implications in enabling unstable failure. Conversely, increasing chlorite content stabilizes slip but reduces frictional strength. The microstructures of these mixed gouges exhibit shear localized on chlorite‐enriched planes and promoting fault sliding. These results suggest that earthquake ruptures occurring during fluid injection can be facilitated by effective stress variations and that both controlling fluid overpressures (effective stresses) and being aware of the presence of alteration minerals are both important controls in mitigating such injection‐induced seismic risks.

Influence of Wall-lumen Ratio of Umbilical Arteries on the Stress Distribution in Wharton's Jelly.

The umbilical cord is the link between fetus and the placenta. It consists of one vein and two arteries, encased inside Wharton's jelly. In this study, the influence of morphological parameters of umbilical arteries, namely the wall-lumen ratio and lumen diameter, on the stress distribution in Wharton's jelly is analyzed using a 3D finite element model. The lumen diameter of the arteries is varied from 0.4 mm to 2.0 mm in steps of 0.4 mm. The variation of average and maximum effective stresses in the Wharton's jelly with wall-lumen ratio is analyzed. Further, differences in stresses between the placental and fetal ends of umbilical cord are analyzed. Results show that, the average and maximum effective stresses at both ends of the umbilical cord vary nonlinearly with the wall-lumen ratio. For all the considered lumen diameters, the average effective stress is found to decrease with an increase in wall-lumen ratio at both the ends. An increase in the lumen diameter is found to be associated with a nonlinear decrease in average stress ratios. Clinical Relevance- The results of this study could be useful for the early diagnosis of fetal abnormalities and might be helpful to develop better treatment strategies.

Poroelastic effects on gas transport mechanisms and influence on apparent permeability in shale

Poroelasticity and gas slippage on the walls of nanopores are two major mechanisms that regulate shale permeability concurrent with hydrocarbons depletion. Nonlinear Klinkenberg plots reported in experiments indicate that gas slippage has a complex relationship with the variation of effective stress; however, a theoretical framework that explains the underlying physics is still missing. In the present contribution, a series of experiments were designed and conducted on intact Utica shale samples to study the simultaneous effects of gas slippage and poroelasticity under constant confining and constant effective stress conditions. In addition, an apparent permeability model was developed considering the effects of rock poroelasticity and a non-constant velocity gradient for gas flow at the pore wall. The model was calibrated and tested against the presented experimental results for Utica shale as well as previously published data. Results show that the concavity in Klinkenberg plots is caused by two separate parameters, each dominant within different ranges of pore pressures: second-order gas slippage and effective stress coefficient. The model demonstrates that gas slippage is inherently coupled with effective stress at low pore pressures. However, the evolution of apparent permeability with gas pressure deviates from the classical Klinkenberg theory only when the pore diameter is sufficiently small. When the effective stress coefficient is smaller than unity, the permeability evolution deviates from the linear Klinkenberg trend only at high pore pressures. In addition to the pore size, gas slippage is found to be closely correlated with the variation of gas viscosity with pressure and temperature.

Characterization of gas transport in shale: A multi-mechanism permeability modeling approach

Accurate characterization of gas transport in shale is crucial for many industrial processes and scientific issues, such as CO2 sequestration in depleted shale and shale gas production. Gas transport in shale involves complex chemical, hydraulic, and mechanical effects and is generally characterized by permeability. Laboratory observations show some “abnormal” permeability evolution behavior which cannot be explained by classical shale permeability models. In this research, we develop a generic shale permeability model that couples multiple gas transport mechanisms and explains complex permeability evolution. Shale is idealized as an improved sugar-cube structure with inorganic flow channels and composite flow channels penetrating organic and inorganic zones. In organic zones, gas rarefaction effects, multilayer adsorption, multilayer-adsorption-induced swelling, stress sensitivity, and surface diffusion are considered. For inorganic zones, gas rarefaction effects and stress sensitivity control gas transport. Based on this conceptual geometry, the variation of flow channel size, effective stress, porosity, tortuosity, and permeability is coupled. The permeability model can be degraded into different forms that are suitable for constant confining pressure, constant effective stress, and constant average pore pressure conditions. The model is verified against experimental data involving different permeability evolution trends. The comparison among this model and several classical models further demonstrates its advantage and uniqueness. Simulation results show that shale permeability is determined by the competitive effects of gas rarefaction phenomenon, sorption-induced organic matter swelling, and effective stress variation. Before and after pore pressure reaches a switch pressure value, the dominant mechanisms for permeability evolution are different. We use the switch pressure to drawn two controlling factor diagrams that demonstrate the dominant mechanisms in different realms of permeability evolution during gas injection or depletion.

Creep behavior of cemented sand investigated under cyclic loading

The influence of stress state on the creep behavior of an artificial cemented sand resembling soft rocks was evaluated. The stress state was characterized by a mean stress and a stress ratio. The cyclic stress-induced creep test was adopted in this study, where the cyclic loading involved a constant deviator stress and a cyclic mean stress (or confining pressure) of the same amplitude and period; the test indicated similar trends as the conventional creep test with a shorter time to creep failure at less creep strain. Results showed that when the creep strain is large enough, the greater the creep strain accumulates, the smaller is the post-creep strength. Under the same number of cyclic loads, with the same stress ratio, the creep strain and the steady-state strain rate in the secondary creep stage increase with increasing mean stress; with the same mean stress, the two said parameters also increase with increasing stress ratio. It was also found that the time to reach creep failure decreases with decreasing mean stress and increasing stress ratio. The stress ratio is proposed to account for the tendency of a stress state to cause failure, and the cyclic variation of mean stress, which is equivalent to the effective mean stress with pore water pressure being zero in the tests conducted, reflects the effective stress state of a geomaterial under fluctuations of groundwater table. Under a fixed deviator stress, a soft porous geomaterial subjected to cyclic variation of effective mean stress may yield contraction and could lead to failure if the stress ratio is high. The findings can help explain the mechanism of ground subsidence or slope failure subjected to cyclic fluctuations of groundwater table.

THE EFFECTS OF WATER-SATURATED SAND-FRACTIONS ON THE SAND-BOILING PHENOMENON

Understanding the stress distribution behaviors along a soil cross-section have been the challenging topics in geotechnical engineering for the last two decades. Stress analysis by considering saturated sand without upward flow of water has been well studied. However, stress analysis in saturated sand-fraction with upward flow of water, which causes the sand-boiling phenomenon still needs a better understanding. In this paper, the relative density is used to indicate the denseness and looseness of sand fraction. The sand-boiling behaviors are then investigated through two scenarios. The first scenario uses a fine sand-fraction, called the FS scenario. The second scenario uses a mixture of coarse and medium sand-fractions, called the CS-MS scenario. For the FS scenario, the fine sand-fraction only consists of one layer, while for the CS-MS scenario, it consists of two layers where the coarse sand-fraction is above the medium one. The use of the relative density values of the fine sand, medium sand, and coarse sand in both scenarios are 7.6%, 12.4%, and 5%, respectively. As a result, all sand fractions used are categorized as very loose sand because their relative densities are less than 15%. The FS scenario shows that the effective stress is equal to zero obtained within 10 seconds at a depth of 0.12 m under the ground surface. This indicates that the sand boiling has occurred in the FS scenario. Whereas in the CS-MS scenario, the variation of effective stress with depth for a soil layer with upward seepage has a value greater than zero. This indicates that the sand boiling never occurred in the CS-MS scenario. Furthermore, both scenarios also show that the addition of coarse sand-fraction above medium sand-fraction could prevent the sand-boiling phenomenon.

Effect of adsorption-induced matrix swelling on coal permeability evolution of micro-fracture with the real geometry

Gas transport in coal induces effective stress variation, matrix swelling/shrinkage, and significantly affects matrix and fracture deformation, resulting in porosity and permeability evolution. However, the heterogeneity and anisotropy of coal are neglected in dual porosity models, which can lead to the deviation from the real physical mechanisms. To uncover the permeability evolution, especially the influence of dynamic matrix-fracture interaction for real fracture distribution, advanced virtual simulation is proposed. In this study, real fracture geometry is taken into account in the physical model based on the CT-scan image, while the mathematical models for coal deformation and gas flow are established. Our calculations are verified against a long-term experimental data under the same boundary condition. Accordingly, the real matrix-fracture interaction caused by adsorption-induced matrix deformation has been visually exhibited, and some new insight into the behavior of fracture permeability in real materials is offered. The results indicate the non-uniform distribution of fracture geometry is responsible for the nonmonotonic change of permeability. It also found that injection pressure, Langmuir strain constant and initial matrix permeability have important influences on the fracture permeability evolution. This research provides valuable insight into the understanding of the permeability change for the real fracture spatial distribution in coal.

Implications of the in situ stress distribution for coalbed methane zonation and hydraulic fracturing in multiple seams, western Guizhou, China

With 59 sets of well testing data from 32 wells, 70 coal seam gas data from 9 wells, and production data from 17 wells, the in situ stress distribution within depths of 136-1244 m and its implications for coal permeability (0.0001-1.56 mD), gas content (5-22 m 3 /t) and gas productivity in western Guizhou were investigated. Three major depth intervals with different stress regimes were identified. At depths of 800-1244 m, the horizontal stresses increased to high values with depth due to the compressional zone near the axis of the syncline. Permeability changes with depth were consistent with the effective stress variations. The 500–800 m depth interval with a normal faulting stress regime was favorable for good permeability (0.008-0.57 mD, mean 0.2 mD) and a stable pressure gradient (approximately 1 MPa/100 m), and the gas content generally increased with depth to a peak value at 800 m. For the 200-500 m and 800-1244 m depth intervals, extremely low permeability (0.0001-0.17 mD, mean 0.03 mD) resulted in discontinuous changes in gas content and pressure gradient (0.47-1.71 MPa/100 m). Overall, the stress release zone at depths of 500–800 m was favorable for coalbed methane extraction, which agreed with the measured production data. Low horizontal stress anisotropy in western Guizhou contributes to complex hydraulic fracture networks, individual seam fracturing with low proppant concentration and high fracturing fluid volume is suggested for multilayer commingled production. • In-situ stress distribution characteristics of coals in the Western Guizhou. • Permeability and resource distribution show similar belting as with stress regimes. • Stress release zone at 500–800 m depths are favorable for CBM extraction. • Low proppant concentration and high fluid volume are suggested for hydrofracturing. • Individual-seam fracturing is the most effective productivity enhancement method.

Stress–Strain Strength Characteristics of Undisturbed Granite Residual Soil Considering Different Patterns of Variation of Mean Effective Stress

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

Forecasting the long-term deterioration of a cut slope in high-plasticity clay using a numerical model

This paper details development of a numerical modelling approach that has been employed to forecast the long-term performance of a cut slope formed in high plasticity clay. It links hydrological and mechanical behaviour in a coupled saturated and unsaturated model. This is used to investigate the influence of combined dissipation of excavation-generated excess pore water pressures and seasonal weather-driven near-surface cyclic pore water pressures. Deterioration of slope performance is defined in terms of both slope deformations (i.e. service) and factor of safety against shear failure (i.e. safety). Uniquely, the modelling approach has been validated using 16 years of measured pore water pressure data from multiple locations in a London Clay cut slope. Slope deterioration was shown to be a function of both construction-induced pore water pressure dissipation and seasonal weather-driven pore water pressure cycles. These lead to both transient and permanent changes in factor of safety due to effective stress variation and mobilisation of post-peak strength reduction over time, respectively, ultimately causing shallow first-time progressive failure. It is demonstrated that this long-term (90 year) deterioration in slope performance is governed by the hydrological processes in the weathered near surface soil zone that forms following slope excavation.

Experimental Research on Dynamic Variation of Permeability and Porosity of Low-Rank Inert-Rich Coal Under Stresses.

The effective stress variation can cause permeability change during coalbed methane (CBM) production. At the same time, macerals have an important influence on the stress sensitivity of coal reservoirs. To investigate the low-rank inert-rich coal permeability dynamic response to effective stress during the production of CBM, eight low-rank coal samples were collected from the Huanglong coalfield, China, and the dynamic changes in helium permeability and porosity were tested under conditions in which the effective stress changed from 0, 15, to 0 MPa. Then, the permeability dynamic change, stress sensitivity control mechanism, and their influence on the development of low-rank CBM are discussed. The results show that with the increase in effective stress, the permeability and porosity of coal samples decrease in the form of negative exponents. During the process of effective stress increase, the permeability variation of low-rank coal includes three stages: rapid loss stage (0–3 MPa), slow loss stage (3–9 MPa), and stable loss stage (>9 MPa). In the process of effective stress reduction, the irreversible damage rate linearly increases. Fracture permeability is easier to recover than pore permeability during pressure relief. Coal macerals have an important influence on the initial permeability and stress sensitivity of low-rank coal. The ratio of vitrinite to inertinite of low-rank coal is positively correlated with initial permeability, but negatively correlated with the permeability loss rate and the fracture compression coefficient (Cf). Cf shows a decreasing trend of the negative index with an increase in effective stress and is highly positively correlated with the average permeability loss rate. The porosity–permeability index model and the permeability–effective stress-sensitive model of low-rank coal under effective stress are established. Research shows that inertinite is helpful to improve the stress sensitivity of low-rank coal. Accordingly, to reduce the damage of coal reservoir permeability, the pressure should steadily change either in the process of fracturing or drainage.