Increase Of Confining Pressure Research Articles

A parametric study on behavior of plastic concrete material in earth dams

In the current research, the mechanical behavior of plastic concrete in cut-off walls of embankment dams has been investigated. To do so, several triaxial compression tests have been carried out on the specimens with various mix designs under different confining pressures. It has been observed that the compressive strength of such a material increases dramatically with the confining pressures applied on the samples. Moreover, the material behavior is substantially dependent on the confining pressure values; so that, as the confining pressure increases, the material behavior intends to be more ductile. Furthermore, a parametric study has been done to investigate the influence of mix design on these materials. The results showed that the strength and elasticity modulus of plastic concrete increase with the increase of cement factor and decrease of bentonite content. However, the rate of increase is apparently dependent on the mix design changes.

Performance of rock crack stress thresholds determination criteria and investigating strength and confining pressure effects

Identifying crack stress thresholds of rocks under different loading conditions, e.g. long-term monotonic or cyclic loads, are crucial in the design of rock excavations and underground openings. The evaluation of the performance of damage thresholds, i.e. crack closure, crack initiation and crack damage stresses, for a variety of rocks, is crucial to assess the most suitable method to capture the stages of rock strength degradation for rock having different strengths subjected to different confining pressures expected to in the field. In this study, several uniaxial and triaxial tests on different rock specimens (Uniaxial compressive strengths from 5 to 278 MPa) were performed to investigate the application and performance of different criteria to determine damage threshold values, using their stress–strain response. It was found that the volumetric strain response of the rock is the most promising parameter to identify crack stress thresholds. For the rocks having different strength values and subjected to different confining pressures, on average, ratios of crack closure, crack initiation and crack damage to the peak strength are found to be 0.21, 0.33 and 0.80 respectively. Effect of rock strength on crack closure and crack initiation stress thresholds was found to be negligible. Crack damage threshold, however, increases with a rock strength increase. This was mainly attributed to the brittle failure behaviour of stronger rocks when compared to weaker rocks. In triaxial compressive testing, crack closure partially occurs during rock consolidation and therefore, crack closure stress decreases with confining pressure increase. Furthermore, a limiting effect on the generation and further growing of lateral cracks in the rocks did lead to an increase in the stress to induced crack damage with an increase in the confining pressure. On the other hand, the effect of confining pressure on the crack initiation stress was observed to be different in different rocks.

Analysis of triaxial compression deformation and strength characteristics of limestone after high temperature

Understanding the mechanical behavior of rock under conditions of high temperature and pressure is critical when it comes to implementing underground coal gasification, deep disposal of highly radioactive nuclear waste, development and utilization of deep underground space, development and utilization of geothermal resources, etc. To understand the effect of confining pressure on limestone at high temperature, uniaxial and triaxial compression testing was conducted on limestone under high temperature (20–800 °C) using the MTS 815 rock mechanics testing system. Using this system, the change in the strength and deformation parameters of the limestone at varying temperature and confining pressure conditions were studied. The results show that the triaxial compression stress-strain curve of limestone at high temperatures is divided into five stages: compaction stage, elastic stage, plastic deformation stage, post-peak failure stage, and residual stage. In addition, the rock brittleness and ductility decrease as the temperature increases. The peak stress and residual strength of the limestone at high temperatures increase as the confining pressure increases, and the strength of the rock specimens decreases as the temperature increases. The internal friction angle (φ) first increases and then decreases as the temperature increases, but the relationship of the cohesive force (c) to temperature is opposite that of φ, indicating that the shear strength of limestone at high temperatures is determined by both c and φ. The elastic modulus of limestone at high temperatures increases with the increase in confining pressure and decreases as the temperature increases, and the peak strain of limestone at high temperatures increases as the confining pressure and temperature increase. The effect of temperature on the failure of limestone is not obvious, and the failure of rock specimens during uniaxial compression testing was mostly through axial splitting, while the failure of rock specimens under triaxial compression testing was mostly through shear failure. As the confining pressure increases, the fracture type of the rock specimens gradually changes from brittle tensile fracture to shear fracture, and the instability type of the rock specimen changed from the sudden instability type to the progressive failure type. Our research results are especially useful in areas where engineering is performed on rocks under high temperature.

EXPERIMENTAL RESEARCH ON DEVELOPMENT/DIMINISHING OF ANISOTROPY AND ITS EFFECT ON MECHANICAL BEHAVIOR OF CLAY

Anisotropy refers to the directional dependence of material properties. The knowledge of development/diminishing of anisotropy are very important, to understand the true behavior of naturally deposited soil. The anisotropy of clays and silty clay intimately connected with their structure, which depends on the environmental conditions during which the soil is deposited as well as the stress changes subsequent to deposition. In this paper, triaxial tests were carried out using the vertical and the horizontal extraction specimen of the reconstituted clay and silty clay, for accumulating experimental facts of development of anisotropy during the preliminary consolidation process and the influence of the anisotropy on the shear behavior. Preconsolidation pressure of 200kPa applied to induced initial anisotropy. Undrained shear triaxial test was performed with different isotropic stresses on clay (50 to 1800kPa) and silty clay (50,300 and 600kPa), and undrained shearing was carried out under constant axial strain rate of 0.0056(mm/min). Vertical sample shows larger peak strength as compared to horizontal, because of the development of anisotropy on the compression side. As the confining pressure increases, the difference between peak strength of vertical and horizontal becomes smaller and smaller which indicate that the anisotropy diminished and intensity ratio decreases. By comparing clay and silty clay, silty clay materials lose their anisotropy at lower confining pressure (600kPa) as compared to clay materials (1800kPa). Therefore, the grain sizes have significant effect on the developing and diminishing of anisotropy. Another important fact observed was, critical state index is decreasing and becomes constant as confining pressure increases.

Nonlinear Creep Behavior and Viscoelastic‐Plastic Constitutive Model of Rock‐Concrete Composite Mass

The joint force deformation of rock‐concrete composite structures is different from that of simple rock specimens or concrete specimens, such as the tunnel surrounding rock‐lining concrete, dam foundations, and concrete. In order to study the creep mechanical properties of rock‐concrete composite structures under long‐duration load, the TFD‐2000 microcomputer servo triaxial creep testing machine is used to carry out step loading creep tests on rock‐concrete composite specimens (hereinafter referred to as composite specimens) under different confining pressures (including the confining pressures σ3 = 0 MPa). The creep test results show that, under the same confining pressure, when axial deviatoric stress is applied step‐by‐step according to 10%, 20%, 30%, 40%, 50%, and so forth of the UCS (σ3 = 0 MPa) and TCS (triaxial compressive strength) of the composite specimens, the failure stress that the specimen can bear is closely related to the confining pressure. When the confining pressures are 0 MPa, 7 MPa, 15 MPa, and 22 MPa, respectively, the failure stresses that the composite specimens can bear are 60% (corresponds to 0 MPa), 50%, 30%, and 20% of the TCS under the current confining pressures, respectively. Under the same confining pressure, the initial creep rate of the composite specimen on each step shows a U‐shaped change trend. Meanwhile, the instantaneous creep rate and failure creep rate of the specimen increase as the confining pressure increases. When the failure creep rate is excluded, the initial creep rate of other stepped loads at the same confining pressure level decreases step‐by‐step. The improved Nishihara model can better describe the whole creep process of rock‐concrete composite specimens, especially in the accelerating creep step. The testing data and research results in this paper can serve as references for further research on mechanical properties of rock‐concrete composite structures.

Influencing Factor Analysis on the Anomalously Low‐Friction Effect in the Block Rock Mass

According to the instability failure of the deep rock mass, a superposition block model of anomalously low‐friction effect was established. The numerical results were compared with the previous experiment, which verifies the feasibility and effectiveness of the simulation. A vertical impact and confining pressure were applied to the superimposed block model, and a horizontal static force was applied to the working block (the third block). This study aimed to determine the influence rules of vertical impact energy, confining pressure, and block lithology on the horizontal displacement of the working block and normal force on the contact surface. The results show that, with the increase of the vertical impact energy, the horizontal residual displacement of the working block increases linearly, and the horizontal displacement amplitude increases by the exponential function. The minimum normal force on the contact surface decreases linearly. As the confining pressure increases, the horizontal residual displacement of the working block decreases logarithmically, and the horizontal displacement amplitude decreases linearly. The minimum normal force on the contact surface increases linearly. The horizontal residual displacement and displacement amplitude of the working block in the coal‐rock combination are 1.51 times and 1.63 times of the rock mass, and the minimum normal force of the former is 0.84 times of the latter. Coal‐rock combination is more prone to the anomalously low‐friction effect than the rock mass.

Dynamic response and failure mechanism of hydrostatically pressurized rocks subjected to high loading rate impacting

In the deep underground, rocks are likely to subject to hydrostatic confining pressure by in-situ stresses and dynamic loading from earthquakes, blasts and impacts. Understanding the dynamic behaviors of rocks under coupled hydrostatic confining pressure and dynamic loading is thus crucial for disaster prevention and mitigation of deep underground engineering. Via the modified split Hopkinson pressure bar (SHPB) system, dynamic tests are conducted on hydrostatically pressurized sandstone specimens, with four confinements varying from 7 to 28 MPa and loading rates in the range of 1000–5000 GPa/s. Our results show that the dynamic strength of hydrostatically pressurized rocks is proportional to the loading rates and hydrostatic confinements, while the loading rate dependence of dynamic strength decreases as the hydrostatic confining pressure increases. The dynamic Young's modulus of rocks increases with increasing confinement, and it is independent of dynamic loading rate. Rocks are more fragmented under lower hydrostatic confinement or higher dynamic loading rate. Post-mortem examination of recovered specimens reveals that the failure pattern of hydrostatically pressurized rocks changes from the tensile-dominated failure to the shear-dominated failure with increasing confinement. Energy analysis indicates that more energy is needed to fracture the rocks under higher confinement or loading rate, while the energy utilization ratio of hydrostatically pressurized rocks reaches the maximum at certain intermediate loading rate.

Constitutive Model for Brittle Granular Materials Considering Competition between Breakage and Dilation

A constitutive model is presented for brittle granular materials based on a recent reformulation of the breakage mechanics theory. Compared with previous breakage mechanics-based models, the proposed model is improved to capture strain softening towards the critical state following the peak stress observed in dense specimens under shearing, and simultaneous evolution of breakage and dilation. Considering the competition between dilation and particle breakage allows the model to capture breakage-induced reduction in dilatancy and peak strength as confining pressure increases. The influence of the model parameters on the overall material response is described through a detailed calibration procedure based on a benchmark experimental dataset. Comparison of the results of drained triaxial compression experiments on two sands with the predictions of the model indicates that the enriched model successfully captures strain softening in dilatant specimens, the shearing-driven evolution of stress-strain behavior towards the critical state at different confinement levels, the transition from dilatant to compactive volumetric response, and the evolution of particle grading due to distributed breakage events. The proposed framework is capable of qualitatively reproducing the experimentally observed stress-dilatancy-breakage relationship in brittle granular materials in the low pressure regime.

Acoustic emission characteristics of pre-cracked specimens under biaxial compression

AbstractThis paper presents an experimental study in which molded gypsum pre-cracked specimens with two types of flaw (single and X-shaped cross flaws) were tested under biaxial compression. Results show that acoustic emission (AE) energy rate curves can be divided into three different periods during uniaxial compression, which correspond to crack closure, linear elastic and unstable failure stages of specimens. However, only two periods are observed during biaxial compression, which correspond to linear elastic deformation and unstable failure. Furthermore, two proposed AE parameters, namely the AE energy ratio and AE energy release ratio, and a classical AE parameter b-value were used to analyze the influence of biaxial compression loading on AE energy, respectively. With increasing confining pressure, the AE energy release ratio, as well as AE b-value decreases gradually while AE energy ratio increases. Moreover, maximum and average AE energy rate values decrease as confining pressure increases. Data presented herein is useful to study the AE characteristics of pre-cracked specimens under biaxial compression.

Numerical simulation of induced cutting in deep coal.

Deep coal cutting is a hot research topic at present. In this paper, the cutting technology of three-drum shearer was proposed based on previous studies. Besides, the influence of confining pressure on coal cutting performance was studied by using the discrete element method, and the induction effect of central cutting on coal cutting performance was discussed. Moreover, coal cutting with different boundary conditions was simulated with the aid of PFC2D software. The results show that as the confining pressure increases, the model dominated by tensile failure does not change, but the crack gradually develops from the vertical direction to the free surface of coal. The cutting debris first increases and then decreases; so does the cutting force. Under the effect of central cutting, the crack tends to develop towards the free surface of coal more, and both the peak cutting force and the specific energy consumption increase with the increase of confining pressure. Induced by central cutting, with the increase of confining pressure, the reduction value of peak cutting force increases first and then decreases while the reduction value of cutting specific energy consumption increases.

Dynamic Mode II Fracture Toughness of Rocks Subjected to Confining Pressure

The dynamic shear failure of rocks with confinements is important in underground rock engineering and it is thus necessary to quantify the dynamic Mode II fracture toughness KIIC of rocks under confining conditions. A punch-through shear (PTS) method is introduced to measure the KIIC of rocks under different confining pressures. The dynamic PTS specimen is a cylinder with two 10-mm-deep circular notches at two ends. A modified split Hopkinson pressure bar system with a Hoek cell is employed to apply the dynamic loading and confining pressure to the rock specimen. The equation to calculate the dynamic KIIC value is determined for the new dynamic PTS specimen. The validity and applicability of the dynamic PTS method with confinement are demonstrated via the measurement of the dynamic KIIC of Fangshan marble (FM) under five confining pressures. The PTS specimens are sheared into a short bar and a hollow cylinder and are soft recovered for post-mortem examination. The dynamic shear fracture pattern of the recovered FM specimen is examined through X-ray micro-CT technique. The results demonstrate that the rock PTS specimen with confinement fails with the shear dominant fracture and the values of KIIC of FM at a certain confining pressure increase as the loading rate. Another observation is that the values of KIIC of FM at a given loading rate rise with the lateral confinement.

Experimental investigations into stress sensitivity in three Chinese coal samples by NMR

ABSTRACT The coal reservoirs have low mechanical strength and strong stress sensitivity. The study of stress sensitivity is of great significance to the efficient development of coalbed methane (CBM). In order to reveal the stress and strain characteristics of coal reservoirs, the low-field nuclear magnetic resonance (NMR) experiments with variable confining pressures were carried out on three coal samples from China. Based on the T2 spectrum of coal samples under different confining pressures, the pore distribution, connectivity, and porosity variation rules of the coal samples were analyzed. Results show that the peaks of mesopore, macropore-fissure in the coal samples are well developed; however, the micropore peaks are poorly developed. There are differences in the stress deformation of pores and fractures in coals. With increasing confining pressures from 0 to 16 MPa, the peak of fractures decrease most obvious, and the peak of mesopore and macropore-fissure are relatively obvious, while the peak of the small pores is not significantly reduced. With the confining pressure increases, the total NMR porosity of coal decreases as the rule of negative exponential function, which is mainly controlled by changes of the porosity of seepage pore. The effect of adsorption pore on porosity changes is not obvious.

The Experimental Investigation of Longmaxi Shale Dynamic Parameters under Water-Based Mud Soaking

Shale damage investigation is important in shale gas development. This paper is concerned with the experimental identification of ultrasonic wave velocities and damage mechanic parameters of Longmaxi shale under water-based mud soaking and confining pressure loading. The wave velocities increased with increasing confining pressure, while wave velocities decreased with increasing soaking time. The anisotropy of Young’s modulus decreases when confining pressure increases. As soaking time increases, the anisotropy coefficient increases. As soaking time and confining pressure rise, the damage parameters also show complex changes. The results are beneficial for shale gas development.

Study on dynamic stress-strain response law of frozen saline soil

The subgrade saline soil in a certain area of western Qinghai province was studied, based on the British GDS dynamic triaxial test system, the dynamic triaxial test under laboratory test was carried out to study variation dynamic stress-strain and dynamic shear modulus under the different temperature, frequency and confining pressure. The test results show that the dynamic stress-strain relationship is significantly affected by temperature change under cyclic dynamic load, followed by frequency and confining pressure. Under the same strain amplitude, the lower the temperature, the larger the dynamic shear modulus. When frequency and confining pressure increase, dynamic shear modulus also increases. Whereas, the influence of confining pressure on the dynamic shear modulus is more obvious, and the backbone curve development trend is steeper. Both maximum dynamic shear modulus and final shear stress amplitude increase with the frequency increases when temperature and confining pressure remain unchanged. Meantime, the maximum dynamic shear modulus curve is a convex shape, and the final shear stress amplitude curve is a convex shape at positive temperature,while which a concave shape at negative temperature. At constant frequency and confining pressure, the maximum dynamic shear modulus and final shear stress amplitude increase with the decrease of temperature. Curves of change trend are slower above 0°C, and steeper below 0°C.

Study on the Development and Distribution Rule of Tensile Shear Fracture Propagation Angle of Fractured Rock Mass Under Seepage Pressure

In order to study the development and distribution rule of tensile shear fracture propagation angle of fractured rock mass under seepage pressure, the shear-fracture model of single-fractured rock mass under seepage-stress is established, and the functional relationship between fracture expansion angle and fracture angle, confining pressure and seepage pressure is derived. Then based on RFPA-Flow software, the simulation verification of the crack expansion angle expansion rule is carried out. The simulation results verified the feasibility of the fracture propagation rule obtained by theoretical analysis to some extent. (1) In the case of single-fractured rock mass under seepage pressure, the fracture expansion angle increases with the increase of the seepage pressure in the fracture at a certain fracture angle and confining pressure; (2) In the case of a certain seepage pressure and a fracture angle, as the confining pressure increases, the fracture propagation angle also increases; (3) In the case of a certain seepage pressure and confining pressure, as the fracture angle increases, the fracture propagation angle decreases; (4) The rock fracture expansion does not extend completely along the fracture propagation angle due to the influence of confining pressure, seepage pressure and fracture angle. Generally, fractures will first develop along the direction of the fracture propagation angle, but ultimately along the direction of the maximum principal stress to form a fracture. The research results have some complementary and verifying significance for the research of rock fracture damage mechanism.

Test and analysis of mechanics and permeation properties of raw coal

AbstractIn this paper, we investigated the mechanics and permeation properties of the raw coal sample during loading and unloading process, and come to some conclusions: the permeability of the sample increases nonlinearly during the unloading confining pressure process. The larger the gas pressure is, the greater the increase range of the permeability will be. The larger the confining pressure unloading amplitude is, the greater the increase range of the permeability will be. In the loading process, the permeability and the axial strain of the specimen change in a “V” shape. As the gas pressure increases, the peak strength of the specimen gradually decreases, and fitted the gas pressure and the peak strength are accordance with the linear relationship. As the confining pressure increase, the peak strength of the sample gradually increases, and fitted the confining pressure and the peak strength are accordance with the exponential function. The internal friction angle of the coal sample was 43.22°, and the cohesion was 3.77 MPa. The relationship between the fitted gas pressure and internal friction angle, gas pressure and cohesion is in line with linear functions. Maintain an axial stress constant, unloaded confining pressure to a constant state of target value, the permeability of specimen shows a constant pressure boosting flow zone, the relationship between permeability and deviatoric stress is accordance with the ExpDec1 function during the process of unloading confining pressure and prepeak loading axial stress.

Experimental and numerical study on mechanical and cracking behaviors of flawed granite under triaxial compression

The mechanical and cracking behaviors of rock mass are strongly affected by natural discontinuities and confining pressure. Uniaxial and triaxial compressive experiments were conducted on intact and single- and double-flawed granite specimens using MTS 815 electrohydraulic servo-controlled triaxial testing system to understand further the influences of confining pressure and fracture on rock strength and failure mode. Confining pressures of 5 MPa and 10 MPa were designed in this study. The effects of the confining pressure and the configuration of pre-existing flaws on the mechanical and cracking behaviors of flawed granite specimens were studied numerically using PFC. Experimental study results show that under the same confining pressure, the stress–strain curve of the intact granite specimen drops rapidly at the peak stress point, whereas the deformation with different numbers of flawed granite shows a stepwise softening phenomenon, which occurs when the confining pressure is relatively small. However, such phenomenon becomes progressively unobservable as the confining pressure increases. With the increase in confining pressure, the mechanical property of the granite gradually turns from brittle to ductile. Consequently, the cracking behaviors become progressive. Numerical simulation results show that the tensile fracture of flawed granite specimens is often observed under a low confining pressure, whereas the tensile–shear mixture fracture and the shear fracture of flawed granite specimens are often observed under moderate and high confining pressures, respectively. Under a constant confining pressure, the peak stress of flawed specimens decreases with the increase in the number of prefabricated flaws and the stress concentration becomes more serious, thereby decreasing the number and development degree of the micro-cracks outside the stress concentration region of the specimens.

Effects of axial static stress and confining pressure on the dynamic compressive behaviours of granite

This study aims to investigate experimentally the effects of axial static stress and confining pressure on the dynamic compressive behaviours of granite. A new index of confining pressure increase factor (CPIF) was introduced to evaluate the confining pressure-induced strength enhancement, a modified definition was proposed to determine the dynamic deformation modulus, and the roles of energy storage limit (ESL) were highlighted to explain the energy absorption and release process. The results showed that when the axial static stress was less than a threshold value that was equivalent to the rock’s elastic limit, it coupled with the confining pressure and elicited positive effects on the strength enhancement of the rock. However, once the threshold value was exceeded, the axial static stress weakened the rock while the confining pressure began to strengthen the rock in a more significant way. Moreover, the dynamic deformation modulus increased as the static stresses increased (including axial static stress and confining pressure) when the rock remained in the linear elasticity under the static loading, while it declined when the rock suffered from damage before the impact. Furthermore, the results revealed that the ESL tended to play critical roles in the process of energy absorption and release in different situations.

Mechanical and acoustic behaviors of brine-saturated sandstone at elevated temperature

The mechanical behavior of rock is essential to estimate the capacity and long-term stability of CO2 storage in deep saline aquifers. As the depth of reservoir increases, the pressure and temperature that applied on the rock increase. To answer the question of how the confining pressure and temperature influence the mechanical behavior of reservoir rock, triaxial compression experiments were carried out on brine-saturated sandstone at elevated temperature. The triaxial compressive strength of brine-saturated sandstone was observed to decrease with increasing testing temperature, and the temperature weakening effect in strength enhanced with the increase of confining pressure. Sandstone specimens showed single fracture failures under triaxial compression. Three typical regions around the main fracture were identified: fracture band, damaged zone and undamaged zone. A function was proposed to describe the evolution of acoustic emission count under loading. Finally, the mechanism of elevated temperature causing the reduction of strength of brine-saturated sandstone was discussed.

Experimental Investigation on Failure Modes and Mechanical Properties of Rock‐Like Specimens with a Grout‐Infilled Flaw under Triaxial Compression

Flaws existing in rock mass are one of the main factors resulting in the instability of rock mass. Epoxy resin is often used to reinforce fractured rock mass. However, few researches focused on mechanical properties of the specimens with a resin‐infilled flaw under triaxial compression. Therefore, in this research, epoxy resin was selected as the grouting material, and triaxial compression tests were conducted on the rock‐like specimens with a grout‐infilled flaw having different geometries. This study draws some new conclusions. The high confining pressure suppresses the generation of tensile cracks, and the failure mode changes from tensile‐shear failure to shear failure as the confining pressure increases. Grouting with epoxy resin leads to the improvement of peak strengths of the specimens under triaxial compression. The reinforcement effect of epoxy resin is better for the specimens having a large flaw length and those under a relatively low confining pressure. Grouting with epoxy resin reduces the internal friction angle of the samples but improves their cohesion. This research may provide some useful insights for understanding the mechanical behaviors of grouted rock masses.