scholarly journals Influence of Unloading Conditions of Confining Pressure on the Compressive Strength and Permeability of Deep Mudstone

2021 ◽  
Vol 2021 ◽  
pp. 1-10
Author(s):  
Tianyu Xin ◽  
Yashengnan Sun ◽  
Junguang Wang ◽  
Weiji Sun
Keyword(s):  
Compressive Strength ◽  
Brittle Failure ◽  
Axial Stress ◽  
Triaxial Compression ◽  
Volumetric Strain ◽  
Ductile Failure ◽  
Confining Pressure ◽  
Initial Stage ◽  
Strain Change ◽  
Deep Rock

To investigate the compressive strength and permeability of deep mudstone under stress disturbance, a triaxial rheometer is used to conduct seepage experiments on mudstone specimens with different buried depths under triaxial compression and unloading conditions. The experimental results show that the compressive strength of mudstone specimen with a depth of 1000 m is much lower than that of specimen with a depth of 200 m, and the compressive strength of mudstone increases with the increase in confining pressure. Under constant axial pressure and unloading of the confining pressure, the mudstone with a depth of 200 m exhibits brittle failure, and the strain fluctuates in a pointwise manner with the increase in axial stress. In this case, the mudstone with a depth of 1000 m exhibits a transition from brittle failure to ductile failure, and the strain fluctuates linearly with the axial stress. Further, when the volumetric strain change reaches 0.01, it shows an oblique “Z” fluctuation. During the initial stage of unloading of confining pressure, the permeabilities of both the mudstone specimens (with depths of 200 and 1000 m) decrease gradually. As the confining pressure is unloaded, the permeability of mudstone with a depth of 1000 m increases. Until the specimen is completely destroyed, the permeability of mudstone increases rapidly. Overall, this study can serve as a useful reference for analyzing the engineering disasters associated with deep rock mass, tunnel ventilation, and gas storage.

scholarly journals Triaxial Compressive Failure Characteristics and Constitutive Model Study of Jurassic-Cretaceous Weakly Cemented Sandstone

2020 ◽  
Vol 2020 ◽  
pp. 1-12
Author(s):  
Jinlong Cai ◽  
Wei Zou
Keyword(s):  
Constitutive Model ◽  
Residual Strength ◽  
Triaxial Compression ◽  
Volumetric Strain ◽  
Ductile Failure ◽  
Mathematical Expression ◽  
Confining Pressure ◽  
Mining Area ◽  
Peak Strength ◽  
Correction Coefficient

A conventional triaxial compression test of Jurassic-Cretaceous typical weakly consolidated sandstone from a mining area in Ordos, China, was conducted using an MTS816 tester. Results showed that, before the peak, the rock had a distinct yield stage. When the specimen reached its peak strength, the strength decreased rapidly and showed an obvious brittle failure. When the confining pressure was increased to 15 MPa, the decrease of strength was slow and the rock tended toward ductile failure. With the increase of confining pressure, the cyclic strain initially increased slightly, whereas the volumetric strain increased greatly and the rock sample was in a compression state. When the load reached a critical value, the curve was reversely bent, resulting in volume expansion, whereas the peak strength, residual strength, and elastic modulus increased with confining pressure, and Poisson’s ratio decreased with the confining pressure. In the model based on macroscopic failure rock, the expression of the relationship between fracture angle and confining pressure provided a solid theoretical basis for the direction and failure mode of the macroscopic crack. Based on the rock strength theory and Weibull random distribution assumption of rock element strength, the damage variable correction coefficient was introduced when the residual strength was considered. Then, the mathematical expression of the 3D damage statistical constitutive model was established. Finally, the theoretical curve of the established constitutive model was compared with the triaxial test curve, which showed a high degree of coincidence.

scholarly journals Deformation Failure and Gas Seepage of Raw Coal in Alternate Loading and Unloading by Stages

2021 ◽  
Author(s):  
Bang-an Zhang ◽  
Yang yushun
Keyword(s):  
Axial Stress ◽  
Axial Strain ◽  
Radial Strain ◽  
Confining Pressure ◽  
Change Rate ◽  
Initial Stage ◽  
Strain Change ◽  
Loading And Unloading ◽  
Unit Stress ◽  
Unloading Confining Pressure

Abstract In this paper, the cyclic loading and unloading confining pressure tests of raw coal samples were carried out by using the "Triaxial seepage test device of thermal fluid solid of coal and rock" developed by Chongqing University. The conclusions are as follows: (1) The axial strain change rate ε1´, the radial strain change rate ε3´ and the permeability change rate k´ under unit stress state are used to represent the sensitivity of axial stress and confining pressure to deformation and permeability characteristics of samples under unit stress state. (2) At the initial stage of unloading the confining pressure, the confining pressure has a greater influence on the permeability of the sample. At the initial stage of loading confining pressure, the confining pressure has a greater influence on the radial strain of the specimen. During the subsequent loading and unloading process, the confining pressure of loading and unloading has a greater influence on the permeability of the sample, and a smaller influence on the axial strain. The loading axial stress has a greater influence on the axial strain of the sample, and a smaller influence on the permeability of the sample. (3) When the axial stress is constant, the increase range of sample permeability increases with the increase of unloading confining pressure range, and the decreasing range of sample permeability increases with the increase of loading confining pressure range, and the increase range of sample permeability under unloading confining pressure is higher than that under increasing confining pressure. (4) In the process of loading axial stress and loading confining pressure, the permeability of samples decreases nonlinearly with the increase of principal stress difference, while the permeability of samples increases nonlinearly with the decrease of principal stress difference in the process of unloading confining pressure.

scholarly journals Investigation of the Effect of Confining Pressure on the Mechanics-Permeability Behavior of Mudstone under Triaxial Compression

Geofluids ◽  
2019 ◽  
Vol 2019 ◽  
pp. 1-14
Author(s):  
Lu Shi ◽  
Zhijiao Zeng ◽  
Zhiming Fang ◽  
Xiaochun Li
Keyword(s):  
Turning Point ◽  
Triaxial Compression ◽  
Volumetric Strain ◽  
Ductile Failure ◽  
Confining Pressure ◽  
Small Strain ◽  
Permeability Evolution ◽  
The Maximum Principle ◽  
Confining Pressures ◽  
Onset Of Dilatancy

Injecting CO2 into a reservoir disturbs the geostress field, which leads to variations in the permeability of caprock and affects its sealing performance. In this paper, the evolution characteristics of the permeability of Yingcheng mudstone were experimentally studied during deviatoric compression under different confining pressures. As the confining pressure increased, the strength of the mudstone increased bilinearly, the angle between the fault and the maximum principle stress increased, and the fault became flatter. During compression, the permeability of mudstone first decreased and then increased and the turning point of the permeability was between the onset of dilatancy and the turning point of volumetric strain; when the fault formed, the permeability increased sharply and the fault-induced increment was reduced exponentially with increasing confining pressure. In addition, the mudstone transformed to the ductile failure mode when the effective confining pressure was greater than 35 MPa, which means that the permeability did not jump within a small strain. Finally, a practical strain-based model of permeability evolution that separately considers compaction and dilatancy was proposed, and the predicted permeability values were in good agreement with the experimental results. This study revealed the effect of confining pressure on permeability evolution during compression and can help evaluate the sealing ability of mudstone caprock.

Mechanical behaviour of powders using a medium pressure flexible boundary cubical triaxial tester

2003 ◽  
Vol 217 (3) ◽  
pp. 233-241 ◽  
Author(s):  
F Li ◽  
V M Puri
Keyword(s):  
Shear Modulus ◽  
Mechanical Behaviour ◽  
Triaxial Compression ◽  
Three Dimensional ◽  
Volumetric Strain ◽  
Confining Pressure ◽  
Alumina Powder ◽  
Medium Pressure ◽  
Mean Pressure ◽  
Confining Pressures

A medium pressure (<21 MPa) flexible boundary cubical triaxial tester was designed to measure the true three-dimensional response of powders. In this study, compression behaviour and strength of a microcrystalline cellulose powder (Avicel® PH102), a spray-dried alumina powder (A16SG), and a fluid-bed-granulated silicon nitride based powder (KY3500) were measured. To characterize the mechanical behaviour, three types of triaxial stress paths, that is, the hydrostatic triaxial compression (HTC), the conventional triaxial compression (CTC), and the constant mean pressure triaxial compression (CMPTC) tests were performed. The HTC test measured the volumetric response of the test powders under isostatic pressure from 0 to 13.79MPa, during which the three powders underwent a maximum volumetric strain of 40.8 per cent for Avicel® PH102, 30.5 per cent for A16SG, and 33.0 per cent for KY3500. The bulk modulus values increased 6.4-fold from 57 to 367MPa for Avicel® PH102, 3.7-fold from 174 to 637 MPa for A16SG, and 8.1-fold from 74 to 597MPa for KY3500, when the isotropic stress increased from 0.69 to 13.79 MPa. The CTC and CMPTC tests measured the shear response of the three powders. From 0.035 to 3.45MPa confining pressure, the shear modulus increased 28.7-fold from 1.6 to 45.9MPa for Avicel® PH102, 35-fold from 1.7 to 60.5MPa for A16SG, and 28.5-fold from 1.5 to 42.8MPa for KY3500. In addition, the failure stresses of the three powders increased from 0.129 to 4.41 MPa for Avicel® PH102, 0.082 to 3.62 MPa for A16SG, and 0.090 to 4.66MPa for KY3500, respectively, when consolidation pressure increased from 0.035 to 3.45MPa. In addition, the shear modulus and failure stress values determined from the CTC test at 2.07, 2.76, and 3.45MPa confining pressures are consistently greater than those from the CMPTC test at the same constant mean pressures. This observation demonstrates the influence of stress paths on material properties. The CTT is a useful tool for characterizing the three-dimensional response of powders and powder mixtures.

Experimental investigation of the effect of compliant pores on reservoir rocks under hydrostatic and triaxial compression stress states

2019 ◽  
Vol 56 (7) ◽  
pp. 983-991
Author(s):  
Hua Yu ◽  
Kam Ng ◽  
Dario Grana ◽  
John Kaszuba ◽  
Vladimir Alvarado ◽  
...  
Keyword(s):  
Pore Pressure ◽  
Triaxial Compression ◽  
Matrix Material ◽  
Volumetric Strain ◽  
Confining Pressure ◽  
Differential Pressure ◽  
Sandstone Sample ◽  
Stress States ◽  
Rock Springs ◽  
Stage 1

The presence of compliant pores in rocks is important for understanding the stress–strain behaviors under different stress conditions. This paper describes findings on the effect of compliant pores on the mechanical behavior of a reservoir sandstone under hydrostatic and triaxial compression. Laboratory experiments were conducted at reservoir temperature on Weber Sandstone samples from the Rock Springs Uplift, Wyoming. Each experiment was conducted at three sequential stages: (stage 1) increase in the confining pressure while maintaining the pore pressure, (stage 2) increase in the pore pressure while maintaining the confining pressure, and (stage 3) application of the deviatoric load to failure. The nonlinear pore pressure – volumetric strain relationship governed by compliant pores under low confining pressure changes to a linear behavior governed by stiff pores under higher confining pressure. The estimated compressibilities of the matrix material in sandstone samples are close to the typical compressibility of quartz. Because of the change in pore structures during stage 1 and stage 2 loadings, the estimated bulk compressibilities of the sandstone sample under the lowest confining pressure decrease with increasing differential pressure. The increase in crack initiation stress is limited with increasing differential pressure because of similar total crack length governed by initial compliant porosity in sandstone samples.

scholarly journals Mechanical Behavior of Cemented Sand Reinforced with Different Polymer Fibers

2019 ◽  
Vol 2019 ◽  
pp. 1-10 ◽  
Author(s):  
Xiangfeng Lv ◽  
Xiaohui Yang ◽  
Hongyuan Zhou ◽  
Shuo Zhang
Keyword(s):  
Compressive Strength ◽  
Mechanical Behavior ◽  
Brittle Failure ◽  
Unconfined Compressive Strength ◽  
Polypropylene Fiber ◽  
Ductile Failure ◽  
Polymer Fibers ◽  
Chemical Compositions ◽  
Peak Strain ◽  
Cemented Sand

In this study, the specimens of cemented sand were prepared by reinforcing it separately with different contents (0.5%, 1.0%, 1.5%, and 2.0%) of three different polymer fibers (polyamide, polyester, and polypropylene) prepared as filaments of different lengths (6, 9, and 12 mm). Then, these specimens were tested, and the improvement effects of the three fibers on the engineering-mechanical behavior of the cemented sand were analyzed and compared. The different microstructures and chemical compositions of the fiber-reinforced cemented sand specimens were investigated using electron microscopy and X-ray diffraction. Compression tests were performed to obtain the stress-strain curves of the specimens. Comparative analysis was performed on the variation patterns of the mechanical parameters (such as unconfined compressive strength and peak strain) of the specimens. Quantitative analysis was performed on the effect of fiber content and fiber filament length on the failure mode of the specimens. It was shown that the inclusion of fibers led to a change from brittle failure to ductile failure. The macro- and microexperimental results revealed that polypropylene fiber had the best improvement effect on the mechanical behavior of the cemented sand, followed by polyester fiber and polyamide fiber. In particular, the cemented sand specimen reinforced with 1.5% polypropylene fiber prepared as 9 mm length filaments had a brittleness index of 0.0578, exhibited ductile failure (in contrast to the brittle failure of the nonreinforced cemented sand), and yielded the highest unconfined compressive strength and shear strength among the specimens.

Effect of particle size and strain conditions on the strength of crushed basalt

1983 ◽  
Vol 20 (4) ◽  
pp. 706-717 ◽  
Author(s):  
Mosaid Al-Hussaini
Keyword(s):  
Particle Size ◽  
Stress Level ◽  
Level Density ◽  
Triaxial Compression ◽  
Volumetric Strain ◽  
Confining Pressure ◽  
Strain Diagram ◽  
Compression Tests ◽  
Cohesionless Soils ◽  
Lower Shear

This paper describes the results of an investigation carried out to study the influence of particle size, stress level, density, method of consolidation, and strain conditions on the strength and compressibility of crushed basalt. All specimens were prepared at medium or high density, consolidated isotropically or under K0 consolidation, and sheared under effective confining pressure ranging from 443 to 2297 kPa. The material used in the test program had a straight line grain size distribution with maximum particle size ranges from 0.63 to 76.2 mm and minimum particle size equivalent to No. 30 U.S. standard sieve size:The tests indicated that an increase in the particle size (i.e., gradation) increases the strength and decreases the axial and the volumetric strain at failure. The study indicated further that the crushed basalt under triaxial compression has a lower shear strength than when sheared under plane strain conditions. Particles crushed were significantly influenced by the stress level and gradation and to a lesser degree by density of material. Keywords: cohesionless soils, density (mass/volume), shear tests, stress–strain diagram, compression tests, rockfill dams.

Research Based on Relationship between Micro-Structural Damage and Intensity Attenuation Laws of Water-Bearing Limestone

2012 ◽  
Vol 166-169 ◽  
pp. 684-689
Author(s):  
Shu Xin Liu ◽  
Chang Wu Liu ◽  
Yaming Kang ◽  
Duo Yang
Keyword(s):  
Compressive Strength ◽  
Structural Damage ◽  
Damage Mechanics ◽  
Triaxial Compression ◽  
Strength Criterion ◽  
Confining Pressure ◽  
Complex Stress ◽  
Complex Stress State ◽  
Natural State ◽  
Quantitative Expression

Rock strength is closely related to composition and internal structure of water-bearing rock. By using scanning electron microscopy, This paper analyses limestone composition and micro-structural damage mechanisms in the water physical and chemical effects. Besides, By combining with triaxial compression test according to natural state and saturated state of rock samples, and by applying mohr-coulomb strength criterion based on the basic concepts of damage mechanics, The paper obtains the quantitative expression of micro-structural damage by using statistical methods. Finally, The paper establishes the relationship between compressive strength and micro-structural damage and discusses evolution laws of micro-structural damage in limestone under complex stress state, The results show that degree of micro-structural damage increase as the water affection, further more, The correlations between micro structural damage and the compressive strength of water-bearing limestone shows a nonlinear relationship according to different confining pressure.

scholarly journals A Strain-Based Percolation Model and Triaxial Tests to Investigate the Evolution of Permeability and Critical Dilatancy Behavior of Coal

Processes ◽  
2018 ◽  
Vol 6 (8) ◽  
pp. 127 ◽  
Author(s):  
Dongjie Xue ◽  
Jie Zhou ◽  
Yintong Liu ◽  
Sishuai Zhang
Keyword(s):  
Axial Strain ◽  
Triaxial Compression ◽  
Sudden Change ◽  
Volumetric Strain ◽  
Confining Pressure ◽  
Percolation Model ◽  
Triaxial Tests ◽  
Transition Behavior ◽  
Orthogonal Experiments ◽  
Stress Dependent

Modeling the coupled evolution of strain and CH4 seepage under conventional triaxial compression is the key to understanding enhanced permeability in coal. An abrupt transition of gas-stress coupled behavior at the dilatancy boundary is studied by the strain-based percolation model. Based on orthogonal experiments of triaxial stress with CH4 seepage, a complete stress-strain relationship and the corresponding evolution of volumetric strain and permeability are obtained. At the dilatant boundary of volumetric strain, modeling of stress-dependent permeability is ineffective when considering the effective deviatoric stress influenced by confining pressure and pore pressure. The computed tomography (CT) analysis shows that coal can be a continuous medium of pore-based structure before the dilatant boundary, but a discontinuous medium of fracture-based structure. The multiscale pore structure geometry dominates the mechanical behavior transition and the sudden change in CH4 seepage. By the volume-covering method proposed, the linear relationship between the fractal dimension and porosity indicates that the multiscale network can be a fractal percolation structure. A percolation model of connectivity by the axial strain-permeability relationship is proposed to explain the transition behavior of volumetric strain and CH4 seepage. The volumetric strain on permeability is illustrated by axial strain controlling the trend of transition behavior and radical strain controlling the shift of behavior. A good correlation between the theoretical and experimental results shows that the strain-based percolation model is effective in describing the transition behavior of CH4 seepage in coal.

Prefabricated Structures Surface Marble Rheological Tests and Constitutive Model Analysis

2014 ◽  
Vol 501-504 ◽  
pp. 419-425
Author(s):  
Qing Xu ◽  
Jiang Da He ◽  
Hong Qiang Xie ◽  
Ming Li Xiao ◽  
Jian Feng Liu
Keyword(s):  
Mechanical Properties ◽  
Rheological Properties ◽  
Flow Rate ◽  
Axial Stress ◽  
Triaxial Compression ◽  
Confining Pressure ◽  
Rheological Parameters ◽  
Structural Plane ◽  
High Confining Pressure ◽  
Confining Pressures

The mechanical properties of intact rock and rock containing structural plane are very different. From the diversion tunnel of Jinping deep rock site to retrieve the complete block of marble, after a high confining pressure triaxial compression simulation tectonic movements, the formation of structural plane, it represents the mechanical properties of the original rock. On the surface of the marble structure containing triaxial compression creep tests, the results showed: at low confining pressure, the weak marble surface as micro-damage accumulation, the emergence of non-uniform partial destruction, while at high confining pressure, creep curve better continuity and integrity; different confining pressures, marble initial rheology and stability both appear rheological phase, accelerated phase rheological obvious; different confining pressures, the same stage of the axial stress steady flow rate compared with the confining pressure increases, the axial steady state flow rate becomes smaller; marble under test showed the rheological properties, the use of Nishihara model can better demonstrate the rheological properties and determine the rheological parameters for other practical engineering reference.

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