Increase In Axial Strain Research Articles

Study on the influence of hole defects with different shapes on the mechanical behavior and damage law of coal and rock.

To study the effect of different shapes of hole defects in coal and rocks on their mechanical behavior and macro damage law, the microscopic mechanical parameters required for particle flow code (PFC) simulation were calibrated with laboratory test data, and then the evolution process of crack and stress field in coal and rocks with circle, square, triangular and trapezoidal holes under uniaxial compression were researched. The findings indicate that: the existence of hole defects lowers the elastic modulus, peak stress, peak strain and other mechanical parameters of coal and rock, and the reduction degree is influenced by the shape of defect. Meanwhile, the existence of hole defects promotes the generation and evolution of meso-cracks in coal and rock. For coal and rock with hole defects, the crack initiation stress and expansion stress are less than those of intact coal and rocks. The crack initiation stress and expansion stress of coal and rocks with trapezoidal hole defects are the smallest, and the coal and rocks with circular hole defects are the largest. The existence of hole defects weakens the damage degree of coal and rocks to some extent. With the increase of axial strain, the evolution curve of the number of meso-cracks shows stage characteristics, which consists of the calm period before the crack initiation point, the stable growth stage between the crack initiation point and the dilatation point, and the accelerated growth stage after the dilatation point. Before the initiation of crack, the concentration zone of compressive stress is located on the left and right sides of the hole defect, and the concentration zone of tensile stress is located on the upper and lower sides of the hole defect. The concentration of tensile stress is the main reason for the initiation and propagation of cracks, while the existence of compressive stress chain among macroscopic cracks is the cause of the residual strength of coal and rocks after failure.

Study on the Failure Mechanism and Permeability Characteristics of Gas Containing Sandstone Intercalated Coal.

The deformation failure mechanism and permeability characteristics of loaded coal sandstone samples under triaxial stress loading were analyzed based on the experimental platform of the coal-rock fluid solid coupling triaxial servo system. The results show that the stress–strain curve of coal intercalated sandstone can be divided into five stages: compaction closure, elastic deformation, stable fracture expansion, unstable fracture expansion, and post-peak strain softening. With the increase of confining pressure, the peak strength and residual strength of the sample increase linearly with the confining pressure. The permeability of the sample decreases slowly with the increase of axial strain, increases rapidly after reaching the stage of unstable fracture propagation, and changes in “V” shape. The relationship between initial permeability and confining pressure can be fitted with exponential function. Based on the fracture volumetric strain model, the progressive failure mechanism of damage expansion of coal mixed sandstone is discussed. The fracture volume strain of each sample under different stress states is given, and the failure mode and crack distribution characteristics of the sample are obtained. The research results can provide reference for reasonable, safe, and efficient coal mining and gas disaster control.

Effect of strain rate on triaxial extension behavior of silty clay

Traditional methods of suction caisson foundation design only consider the soil is under either purely drained or purely undrained conditions and ignore the effects of the excess pore water pressure (EPWP). A series of triaxial extension stress path tests were conducted in this article to evaluate the effects of the loading rate on the development of EPWP and soil strength. The soil specimens were consolidated under pressures of 60, 80, and 100 kPa and then unloaded at seven different loading rates. It is found that the EPWP and soil strength are heavily influenced by the loading rate. The EPWP tends to be negative at small axial strain and then increases to be positive with the increase of axial strain. The normalized deviatoric stress versus loading rate curves are in the backbone curve. The minimum deviatoric strength happens when the normalized loading rate V ref=5 and linearly increases with the increase of consolidation pressure with the slope of −0.715. The magnitude of the slope of the critical state line in p′ – q space is heavily influenced by the loading rate.

Experimental study on static characteristics of the Yellow River silt under (triaxial) consolidated undrained conditions

The siltation of reservoirs and rivers in the Yellow River Basin and the occurrence of flood disasters are mainly caused by silt transportation. The Yellow River silt being applied to practical production will help solve the problem of siltation in the Yellow River. In this paper, the consolidated undrained (CU) test of the Yellow River silt is carried out by using GDS triaxial apparatus. The effects of confining pressure, relative density and loading rate on the stress-strain relationship, strength parameters, generalized shear stress ratio and pore water pressure variation of the Yellow River silt are explored. The results show that, under the same axial strain, the deviator stress increases with the increase of confining pressure and relative density, and decreases with the increase of loading rate; compared with the relative density, the effect of the confining pressure and loading rate on the generalized shear stress ratio is insignificant. The friction angle is affected by relative density and confining pressure in varying degrees, but less by loading rate. The secant modulus and initial tangent modulus increase with the increase of confining pressure and relative density. The pore water pressure first increases to the positive peak value, and then decreases to a negative value gradually with the increase of axial strain under most conditions. The pore water pressure variation is affected by the confining pressure and relative density obviously, while affected by the loading rate insignificantly.

Relaxation behaviour of Jinping marble under true triaxial stresses

After being excavated for a certain time, deeply buried rock mass is still likely to suffer time-dependent failure. Long-term time-dependent failure may lead to the stability failure of engineering structures. As a special case of time-dependent behaviour, the stress relaxation of hard rock be of great research significance. In this paper, true triaxial relaxation tests were carried out. The test results showed that Jinping marble presented obvious stress relaxation behaviours under true triaxial stresses. Before reaching the peak strength, the stress relaxation degree increased monotonically with the increase of axial strain . Under true triaxial unequal stresses, the ε 2 and ε 3 are different during the stress relaxation test. With the increase of σ 2 , the difference between ε 2 and ε 3 increased. Interestingly, the time-dependent deformation of ε 3 was always in a state of dilation while that of ε 2 changed from dilation to compression with the increase of σ 2 . The reason for this is that the crack growth was inhibited in the σ 2 direction under high σ 2 . During relaxation test, the decrease of axial stress led to rebound of elastic deformation in the σ 2 direction. This rebound of elastic deformation exceeded the irreversible viscoplastic deformation in the σ 2 direction induced by time-dependent cracking, thus finally causing the time-dependent deformation of ε 2 to exhibit compressive characteristics. Under high axial stresses, the dilation deformation triggered by the crack growth in the σ 3 direction exceeded the compressive deformation induced by the decrease of axial stress. Thus, the deformation in the σ 3 direction was always dilated.

Study on Energy Evolution and Damage Constitutive Model of Siltstone

In order to study the energy evolution characteristics and damage constitutive relationship of siltstone, the conventional triaxial compression tests of siltstone under different confining pressures are performed, and the evolution laws of input energy, elastic strain energy and dissipative energy of siltstone with axial strain and confining pressure are analyzed. According to the test results, the judgment criterion of the rock damage threshold is improved, and an improved three-shear energy yield criterion is proposed., The damage constitutive equation of siltstone is established based on the damage mechanics theory through the principle of minimum energy consumption and by considering the residual strength of rock, and lastly, the rationality of the model is verified by experimental data. The results reveal that (1) both the input energy and dissipative energy gradually increase with the increase of axial strain, and the elastic strain energy first increases and then decreases with the increase of axial strain, and reaches its maximum at the peak. (2) The input energy and dissipation energy increase exponentially with the increase of the confining pressure, and the elastic strain energy increases linearly with the increase of confining pressure. (3) According to the linear relationship between the sum of shear strain energy and hydrostatic pressure, an improved three-shear energy yield criterion is established. (4) The model curve can better describe the strain softening stage and the residual strength characteristics of siltstone. The relative standard deviation between the model results and the test results is only 4.35%, which verifies the rationality and feasibility of the statistical damage constitutive model that is established in this paper.

Experimental study the viscoelastic properties of deep shale gas reservoirs in Sichuan Basin

In recent years, the deep shale gas reservoirs has been a major shale gas target in Sichuan Basin. Usually, the burial depth of this target formation is more than 3500m, which is receiving increasing interests to get industrial natural gas. The deep shale gas reservoirs, characterized by fine-grained nature and low matrix permeability as the common shale, require good understanding of geomechanical properties to optimize drilling and fracturing strategies. Meanwhile, the shale reservoirs are anisotropic and inhomogeneous, exhibiting a nonlinear feature under stress loading. However, very limited studies are devoted to characterization of the mechanical properties of deep shale, as opposed to other shale formations. Due to an increasing demand for fundamental properties of this formation to meet the needs of development of this deep shale gas, a comprehensive characterization studies is needed. One of the most important investigation is to obtain the mechanical properties based on an integration of unconfined/triaxial compression tests and creep tests. In this study, the time-dependent, elastic, hysteresis, and strength properties of deep shale specimens, retrieved from a deep well in Sichuan Basin, are evaluating by performing a series of triaxial creep experiments. With respect to mineral composition, the shale cores are found to be organic-rich. The response of these shale specimens are found to be influenced by the planes of weakness and the presence of micro-cracks. The results of creep experiments show that shale tends to significantly creep under the applied stress. The time-dependent axial strain increase linearly with differential stress. And the Power-Law creep model can be used to describe the time-dependent behaviour of shale well. Moreover, the existence of bedding planes results in the creep anisotropy of shale. It is observed that the overall time-dependent axial strain normal to bedding is higher than the parallel to bedding. But the axial strain at 45° angle with respect to the bedding plane is highest. These research results are of great significance for safe and fast drilling in deep shale gas reservoirs. Also, it can be used to guide hydraulic fracturing.

Fractal dimension and its variation of intact and compacted loess

To investigate the fractal dimension and its variation of intact and compacted loess, consolidated-drained (CD) triaxial tests were performed to obtain intact and compacted loess specimens at different stress-strain states, their pore-size distribution (PSD) curves were measured by using the mercury intrusion porosimetry (MIP) technique. Based on the PSDs, three models (namely, Menger sponge model, Neimark model and the model based on the law of thermodynamics) were used to determine the fractal dimensions of specimens. The results show that there have 3 or 4 fractal intervals (or fractal dimensions, i.e., Ds1, Ds2, Ds3, Ds4,) when using Menger sponge model, only Ds3 satisfies the definition of fractal dimension. Neimark model can be used to determine the fractal dimensions of inter-aggregate pores and intra-aggregate pores, respectively (i.e., DNmacro and DNmicro). Most DNmacro values are meaningless physically, DNmicro increases with the increase of axial strain during shearing, suggesting that the surfaces of pores become complex with the increase of axial strain. The model based on the law of thermodynamics is the most appropriate if only one fractal dimension (i.e., Df) is expected in the whole measurable range of pore diameter. Df increases linearly with the growth of confining stress during consolidation or axial strain during shearing for both loess soils, indicating that the roughness of the pore surfaces increases with the increase in stress or strain. Besides, the fractal dimensions of intact and compacted loess were compared. According to the variations of PSD and Df, the mechanism for the evolution of the loess soil pore structure is proposed. These findings are expected to provide new insight into establishment of the connection between microstructure and macro stress-strain state of loess.

Experimental research on dilatancy characteristics of coarse-grained soils by triaxial compression test

Dilatancy is a basic behavior of soils which is closely related to strength and deformation behaviors of soils and makes soils significantly different from normal elastic materials. A large-scale triaxial compression testing machine was used to study the dilatancy characteristics of coarse-grained soils of earth rock dam. The results show that the confining pressure has a significant effect on the dilatancy of coarse-grained soils, the soil first shrinks and then expands under low confining pressure, and the soil mainly produces shear shrinkage deformation under high confining pressure. When the soil changes from shear shrinkage to dilatancy, the strain ratio of soil changes abruptly. Rowe dilatancy model can reflect the dilatancy deformation characteristics of coarse-grained soils. The parameter Kf of the model tends to be constant with the increase of axial strain, and the parameter Kf of coarse-grained soils is smaller when the confining pressure is larger.

Anisotropic characteristics of the energy index during the shale failure process under triaxial compression

Energy conversion is an essential driving force of shale anisotropic deformation, damage initiation and evolution. In this study, theoretical calculations of five energy indexes (total input energy Ut, releasable energy Ue, dissipated energy Ud, dilatational energy UV and distortional energy UD) during the shale failure process considering the inherent anisotropy of shale are presented in detail for the first time. The anisotropic features of the five energy indexes are systematically discussed. The results indicate that bedding structure considerably influences these energy indexes. Ue, UV and UD all decrease to a minimum as the bedding orientation reaches 60° and then increase with the bedding degree. The relationships between Ue, UV and UD and the confining pressure can be described by linear functions, similar to the variation in the compressive strength with either bedding inclination angle or confining pressure. As the axial strain increases, the rates of increase in Ue, UV and UD first increase, then remain constant, and then decrease before failure; however, the rates of increase in Ut and Ud continue to increase until failure. Ue, UD and UV have strong relationships with the peak compressive strength but weak relationships with the peak axial strain, whereas Ut and Ud exhibit the opposite relationships with the peak strength and strain. The anisotropic characteristics of the energy indexes can help to further elucidate the failure mechanisms of shale and are of great significance to hydraulic fracturing design.

Deformation characteristics of medium-dense sand-clay mixtures under a principal stress rotation

A moving wheel load induces a principal stress rotation (PSR) in pavement foundation geomaterials including subgrade/subbase soils. Simulating such a stress condition is not possible with stress path tests conducted with conventional cyclic triaxial (CT) equipment. More complex stress paths such as a heart-shaped stress path are required to determine the deformation characteristics of these under a PSR. A heart-shaped stress path can be simulated on a soil specimen in cyclic hollow cylinder (CHC) tests via user-defined waveforms for its stress components (axial stress, and torsional shear stress). In this study, a series of CT and CHC tests were performed to analyze the impact of a PSR on strain behaviors of medium-dense sand-clay mixtures. The specimens contained 0%, 5%, 10%, and 20% clay by weight and were prepared at an initial relative density of 50%. All specimens were anisotropically consolidated under K0≈0.5. It was determined that all CT specimens underestimated the strain performances (both axial strain and shear strain) of the sand-clay mixtures. On the other hand, a heart-shaped stress path was simulated successfully in CHC tests and thus, all specimens yielded more accurate strain results. At low clay content (≤10%), the impact of a PSR on strain performances of the sand-clay mixtures was observed to be less (axial strain (εz) < 0.12%, and shear strain (γzθ) < 0.8% after number of load cycles (N) = 5000) due to the low stress ratios (CVSR = 0.15 and η = 1/3). On the other hand, despite the low stress ratios, a PSR caused a rapid increase in axial strain and shear strain (εz = 5%, and γzθ > 0.8%) of the specimen containing 20% clay, which resulted in the failure of the specimen at N = 478. Results of this study clearly indicated that the effect of a PSR should be taken into consideration while evaluating the strain characteristics of the sand-clay mixtures that contain clay particularly at high contents (≥20%) under traffic loads.

Particle breakage and micromechanical characteristics of calcareous sand during shearing

Based on the discrete element method, the numerical simulation of the shear test for calcareous sand was carried out to explore the particle breakage and micromechanical mechanism of calcareous sand in the shear process. The results show that the particle breakage can reduce the peak strength of calcareous sand, and make the bulk deformation change from dilatancy to shrinkage. The particle breakage is concentrated in the shear band; with the increase of axial strain, the degree of particle breakage increases, which is also positively correlated with confining pressure and axial strain. The contact forces are redistributed when the particles are broken; the dominant direction of the contact forces in the strong force chain is consistent with σ1, always in the vertical direction, while the dominant direction of the contact forces in the weak force chain is consistent with σ3, in the horizontal direction, and it gradually deflects to the vertical direction with increasing axial strain; the anisotropy of the contact normal also decreases significantly after particle breakage. The magnitudes of the contact forces in the strong and weak force chains conform to the Weibull distribution. After particle crushing, the magnitudes of the contact forces decrease significantly, and the numbers of the contact forces increase significantly, and the percentage of the weak forces increases significantly, which leads to the decrease of the strength of the sample.

A Nonlinear Constitutive Model for Disintegrated Carbonaceous Mudstone Based on Logarithmic Functions

To study the mechanical characteristics of the disintegrated carbonaceous mudstone (DCM), consolidated drained triaxial tests were conducted on the DCM with three degrees of compaction (i.e., 90%, 93%, and 96%). Then, the nonlinear constitutive model suitable for the DCM was established based on test results using a logarithmic function. The stress-strain characteristics of the DCM were analyzed. The results revealed that the axial strain of the DCM was positively correlated with the deviatoric stress and lateral strain. The slopes of deviatoric stress-axial strain curves decreased with the increase of axial strain and so did the slopes of the axial strain-volumetric strain curves. The strength of the DCM increased with the increase of the confining pressure and the degree of compaction. In addition, the axial strain induced by dilatancy was also positively correlated with the degree of compaction and the confining pressure. Furthermore, under triaxial loading conditions, the relationship between the stress and strain of the DCM can be expressed by a logarithmic function; based on this, a nonlinear constitutive model with ten material parameters was derived. In addition, the results of numerical tests using the model showed similar stress-strain characteristics of the DCM comparing with the triaxial tests. Hence, it indicated that the nonlinear constitutive model based on the logarithmic function can reflect the nonlinear stress-strain characteristics of the DCM.

Experimental Study of Triaxial Test of Unsaturated Expansive Soil Shear Strength

Based on unsaturated triaxial testing system, with different water content, the strength characteristics of expansive soils of Nanning are studied. Under certain confining pressure, with low water content, stress-strain curve is strain-softening, whereas with high water content, is strain hardening; Under high confining pressure, with low water content stress-strain curve is strain-hardening type, whereas with low confining pressure the stress-strain curve with the increase of axial strain is a slight softening trend. The shear strength of soil has a large sensitivity with moisture content. As the moisture content increases, the expansive soil friction angle decreases linearly. With the increase of water content, firstly its cohesive force increases, it reaches maximum value at the optimum water content, then it decreases with the increase of water content.

Influences of initial static shear stress on the cyclic behaviour of over consolidated soft marine clay

In offshore engineering, soft marine clays which are overconsolidated due to surcharge pre-loading beneath embankments may be subjected to initial static shear stress. To experimentally investigate the influences of the initial static shear stress on the cyclic behaviour of overconsolidated soft marine clay, a series of triaxial tests are performed covering various values of the initial static shear stress ratio (SSR) and overconsolidated ratio (OCR). Several useful conclusions are obtained. First, the total axial strain increases as the SSR increases, whereas an increase in the OCR reduces the rate of increase in axial strain. Second, the area encompassed by the stress–strain loop decreases with an increasing OCR, whereas it increases with an increasing SSR. Third, a threshold SSR is proposed. Two quantitative relationships between the permanent axial strain and SSR are then established based on the proposed threshold and verified against the experimental results.

Macroscopic and microscopic study on the compression bearing characteristics and deformation failure mechanism of gangue with different particle sizes

To study the mechanical behavior of gangue filling materials in depth, through the physical compression testing of red-sandstone gangue of different sizes and numerical simulation of grain flow, the bearing characteristics and deformation and failure rules of gangue compacting were comprehensively compared and analyzed from macroscopic and microscopic perspectives. The results show that under the same vertical load, the settlement decreases monotonously with the decrease of grain size, and the filling effect is significantly enhanced. Structure of force chain between particles evolves from the “polygon” structure to the “triangle” structure, and small particle forms a stable structure earlier than large particle. With the gradual increase in axial strain, the main propagation direction of the force chain is the vertical direction. The distribution law of the tangential contact force of large gangue in the later stage of loading is basically the same as that of small gangue in the early stage

Mechanical properties of clay reinforced with Bermuda grass root under drying–wetting cycles

The mechanical properties of the clay slope gradually weaken after the drying–wetting (D–W) cycles, which increases the possibility of landslides. Therefore, the variation of mechanical properties of the root–soil composite during D–W cycles was studied in this paper. Through unconsolidated undrained triaxial test (UU test), the effect of D–W cycles on vegetation slope engineering under different confining pressures was studied. UU tests measured the shear strength parameters of the root–soil composite after each cycle. The results show that under different D–W cyclic numbers and confining pressures, with the increase of axial strain, the deviatoric stress increases sharply at first (elastic stage). Then grows slowly (elastopplastic stage) and finally decreases gradually (failure stage). After D–W cycles, the weakening trend of shear strength parameters (including cohesion and internal friction angle) of the root–soil composite is the same, while the weakening ratio of cohesion is higher than that of internal friction angle. Among them, the weakening range of cohesion and internal friction angle is 48.1% and 5.4%, respectively. Also, with the increase of D–W cyclic numbers, both of them decrease linearly.

Dynamic Characteristics of Saturated Soft Clay under Constant Confining Pressure Cyclic Loading

The research on the dynamic characteristics of soft clay under cyclic loading has high value. This paper takes Ningbo saturated soft clay as the research object and uses the GCTS true triaxial test system to analyze the effects of consolidated confining pressure and cyclic deviator stress on the axial cumulative strain. The experimental results found that when the number of vibrations and the cyclic stress ratio are the same, the axial cumulative plastic strain increases with the increase of the consolidation confining pressure. When the dynamic stress amplitude is the same, the axial cumulative strain decreases with the increase of the consolidation confining pressure. Under different confining pressures, as the number of vibrations increases, the cumulative axial strain increases. The development law of axial cumulative strain under different cyclic stress ratios is inconsistent, and there is a critical cyclic stress ratio. The experimental results provide a certain reference for engineering seismic design.

A Novel Damage-Based Permeability Model for Coal in the Compaction and Fracturing Process Under Different Temperature Conditions

Before a coal mining operation, it is necessary to carry out coal-bed methane (CBM) pre-extraction to prevent coal and gas explosions, causing accidents. However, gas extraction and coal mining will lead to coal damage, which results in a change in the gas migration law. Especially, under deep mining conditions, the gas migration mechanism is more complicated owing to a high ground temperature. Coal permeability is the most important constituent that determines gas flow properties. Therefore, the coal permeability evolutionary law related to damage-induced conditions under different temperatures should be further researched. In this paper, a series of triaxial seepage experiments during the whole stress-induced process, and in the changing of the effective stress process were carried out. The results have shown that during the whole stress–strain process, with an increase in axial strain, coal permeability gradually decreases to a minimum value at first, then increases sharply, and finally keeps nearly constant. A higher temperature resulted in a lower elastic modulus, peak strain, and peak strength, but it caused higher thermal damage. When the coal fractures, coal permeability increases with the increase in temperature. During a change in the effective stress process, higher temperatures resulted in higher permeability. Under higher effective stress, the impact of temperature on permeability was not significant. Based on the above results, a novel damage-based permeability model was developed to describe the permeability evolutionary law caused by damage-induced conditions under compaction, and in a fracturing situation. In the proposed model, an exponential function has been used for the combination between permeability and damage variable. The damage variable is composed of thermal damage and mechanical damage. In addition, the damage variable has been modified by introducing a modified function of the initial damage. Finally, the proposed model has been applied to fit two sets of experimental data available. The fitting results showed that the proposed permeability model could well reflect the permeability behaviors of damage-induced coal at different temperatures.

Investigations of the effects of particle morphology on granular material behaviors using a multi-sphere approach

This article studies the influences of particle morphology on the behaviors of granular materials at both macroscopic and microscopic levels based on the discrete element method (DEM). A set of numerical tests under drained triaxial compression was performed by controlling two morphological descriptors, i.e. ratio of the smallest to the largest pebble diameter, ξ , and the maximum pebble–pebble intersection angle, β . These descriptors are vital in generating particle geometry and surface textures. It was found that the stress responses of all assemblies exhibited similar behavior and showed post-peak strain-softening. The normalized stress ratio and volumetric strains flatten off and tended to reach a steady value after an axial strain of 40%. While the friction angles at peak state varied with different morphological descriptors, the friction angles at critical state showed no significant variation. Moreover, evolution of the average coordination numbers showed a dramatic exponential decay until an axial strain of about 15% after which it stabilized and was unaffected by further increase of axial strain. In addition, stress ratio q / p and strong fabric parameter ϕ d s / ϕ m s were found to follow an approximately linear relationship for each assembly. These findings emphasized the significance of the influences of particle morphology on the macroscopic and microscopic responses of granular materials.