Deformation Characteristics Of Rock Research Articles

Experimental study on the failure process of sandstone subjected to cyclic loading and unloading after high temperature treatment

To understand the influence of temperature on sandstone failure process, the cyclic loading and unloading tests under the action of seepage and stress were carried out on sandstone after different temperature (25–800 °C) treatments. Based on the experimental results, the stress-strain curve, Young's modulus and crack volume strain of rock were analysed during cyclic loading and unloading, and the energy calculation equation was established considering the confining pressure and seepage pressure. The main research results are as follows: (1) with the increase in temperature, the stress-strain curve of the rock sample after heat treatment shows obvious signs of hysteresis loop, showing the characteristic of a three-stage evolution, that is, “sparse→dense→sparse”. (2) the Young's modulus in the unloading process is clearly larger than that in the loading stage, but both increase first and then decrease with the increase in temperature, with 400 °C as the critical temperature. (3) temperature can affect the deformation characteristics of rock, which results in an evident correlation between the crack volume strain and the temperature during each cycle of loading and unloading. (4) based on the established energy calculation equation, the energy evolution characteristics in the process of rock deformation and failure are obtained. Compared with the sample treated at lower temperatures than 400 °C, the cumulative dissipation energy of the sample increases after higher temperature (above 400 °C) treatment. The results are expected to enhance the understanding of mechanical properties and failure process of sandstone after temperature treatment, and provide guidance in engineering stability analysis under multi-physical field coupling.

Investigation of microcracking behaviors in brittle rock using polygonal grain‐based distinct method

AbstractThe failure process in brittle rocks is an important topic in rock mechanics, whose good understanding assists in predicting the strength and deformation characteristics of rocks. Because it is difficult to directly observe microcracks in laboratory tests, a numerical model is a useful tool for investigating microcracking behaviors. However, the mechanism of microcrack evolution is still unclear at the grain scale considering the microscopic heterogeneities. This paper proposes a polygonal universal distinct element code grain‐based model to solve this problem. Compared with other grain‐based models, this model is different in that mineral grains are subdivided into polygon blocks. The grain size, composition, and bond types of the contacts are incorporated, and both inter‐ and intra‐grain cracks are mimicked. Subsequently, micro‐parameters of the blocks and contacts are carefully calibrated according to the laboratory results. Following this, the calibrated model is employed to study the microcracking behaviors of granite in unconfined compression, confined compression, and Brazilian splitting tests. Both the cumulative numbers and locations of microcracks in different grains are recorded, and the types of macroscopic fractures are identified. The modeling results show that tensile cracks dominate the generated microcracks in the low confined compression tests, and the macroscopic fracture pattern is axial splitting. Shear cracks are dominant in the high confined compression tests, and the macroscopic failure pattern is X‐type shear failure. The simulated results are consistent with those observed in the laboratory tests, and thus, may assist in improving rock acoustic emission monitoring and understanding the damage process of rock.

Geological Structure and Rock Deformation Characteristics and Their Geohazard-Controlling Mechanism in Yajiang Area, West Sichuan

Geological Structure and Rock Deformation Characteristics and Their Geohazard-Controlling Mechanism in Yajiang Area, West Sichuan

Optimum position of roadway in the middle residual coal seam between the upper and lower longwall gobs

ABSTRACT Residual coal seam mining is regarded as an effective way to alleviate the contradiction between limited coal resource reserves and unlimited consumption demanding. Previous studies have shown that the location of the roadway is one of the key factors affecting the safe mining of the residual coal seams. However, limited research on the roadway layout of the residual coal seam under the multiple mining-induced influence has been conducted, especially on the roadway layout of the middle residual coal seam (MRCS). To determine the optimum position of the roadway, a method to determine the optimum position of the MRCS is proposed based on the three factors of stress, stress variation rate and fracture characteristics. Then, the Jinhuagong Coal Mine is selected as the research mine. The stress distribution, stress variation rate, and fracture distribution of the surrounding rock of the MRCS were systematically analyzed by theoretical calculation, physical and numerical simulations. The research results show that the surrounding rock of the MRCS can be divided into five areas based on the new method, namely ISA, SCA, SRA, HSCRA, and FRA, respectively. Moreover, the roadway position of the MRCS should be avoided in SCA, HSCRA, and FRA areas. Finally, the optimum position of the roadway for the MRCS is determined through comparing the surrounding rock deformation characteristics of the roadway at different locations. The optimum position of the roadway in the MRCS is determined to be 20 m of the internal malposition of the boundary coal pillar. The research results can provide a useful reference for the same types of mines.

Study on support Design of 2220 roadway in Majiliang Coal Mine based on FLAC3D

In view of the serious deformation of roadway 2220 in Majiliang Coal Mine after excavation, the surrounding rock deformation characteristics and stress redistribution of the roadway are studied and analyzed by numerical simulation technology, and the support optimization is carried out. The research results show that the original support scheme can not effectively reduce the roadway deformation and the area of plastic zone and stress concentration area after roadway excavation. After the optimization of the support scheme, the shape variables of the roof and floor and the two sides are obviously reduced, and the plastic zone and stress concentration area are also greatly reduced, which significantly improves the overall stability of the roadway.

Temporal dominant frequency evolution characteristics during the fracture process of flawed red sandstone

To study the time-varying dominant frequency distribution characteristics during the cracking process of intact and flawed rocks, uniaxial compression tests are carried out on three types of red sandstone specimens. Both acoustic emission (AE) and photographic technologies are performed to monitor the real-time cracking process of tested specimens. The relationship between the AE rate curve, the axial stress-time curve and the cracking process is first analyzed, which can be employed to evaluate the microscopic and macroscopic deformation characteristics of rocks. The time-varying dominant frequency and amplitude distribution characteristics in different cracking stages of tested rocks are investigated. Four modes of AE signals, i.e., AE signals with low-frequency and high-amplitude (LF-HA), AE signals with low-frequency and low-amplitude (LF-LA), AE signals with intermediate-frequency and low-amplitude (IF-LA) and AE signals with high-frequency and low-amplitude (HF-LA), are found. The coupling application of four modes of AE signals extracted from unstable deformation stages can be viewed as early warning for unstable failure of rocks. The fracture mechanism of intact and flawed rocks is discussed based on the statistical characteristics of dominant frequency during the cracking process. Micro-shear cracking mainly dominants the ultimate failure of all of tested rocks.

Numerical simulation research on the gob-side coal pillar width and surrounding rock deformation characteristics

Taking the Gaohe Coal Mine working face gob-side in Shanxi as the research object, the FLAC3D numerical simulation software was used to construct a numerical calculation model to analyze the coal pillar width and the surrounding rock deformation characteristics of the working face gob-side in the mining face during the mining process. The width of the coal pillars along the gob-side was determined, and the change law of the surrounding rock stress and deformation of the roadway with the advancing face was obtained. The research shows that the reasonable coal pillar width along the gob-side in the working face of Gaohe coal mine shall not be less than 36m. With the advancement of the working face, the stress and deformation of the surrounding rock of the roadway gradually increase, and the deformation amount and deformation range of the coal gang are both greater than the corresponding value of the flexible form gang, and the vertical stress of the coal gang is less than the stress of the flexible form concrete gang. The difference in deformation is the main reason for the difference in stress. The results of the study provide a reference for the safety of the gob-side along the coal mine.

Deformation and mechanical properties of rock: effect of hydromechanical coupling under unloading conditions

Mechanical behavior and deformation characteristics of rock under unloading conditions are of particular significance for analyzing excavation-induced instability problems of underground rock masses. In this paper, triaxial compression tests with progressive unloading of confining pressure were conducted upon sandstone specimens under different combinations of initial confining pressures and pore pressures. It is shown from the tests that the magnitude of initial confining pressure and pore pressure influences significantly the rock strength, energy conversion, crack propagation, and dilatancy during unloading process. The strength under unloading conditions is in compliance with the linear Mogi-Coulomb criterion. The energy conversion method can be employed to determine the crack damage thresholds during the unloading process. The analysis on cracking process suggests that crack damage thresholds increase linearly with initial confining pressure, but decrease with pore pressure. Under unloading conditions, material yield and brittle failure occur more easily with the increase in pore pressure. Furthermore, given the initial confining pressure, a higher level of pore pressure can accelerate volumetric expansion in the specimens.

Damage Characteristics and Constitutive Model of Deep Rock under Frequent Impact Disturbances in the Process of Unloading High Static Stress

During the mining of deep mineral resources, the rock is in the complex mechanical environment of high crustal stress and blasting excavation unloading, and the improved SHPB test system is used to carry out frequent impact disturbance test of deep rock in the process of unloading high static stress. Firstly, the general characteristics of dynamic stress-strain curve envelope of the whole process trend can be divided into four stages: the stable development stage of micro cracks, the non-stable development stage of micro cracks, the fatigue damage stage, and the fatigue failure stage. Then, the damage variables of the rock are defined by continuous factors, strain equivalence principle and statistical damage theory, which are based on the whole deformation characteristics of rock during the test. And the derived damage variable equation of rock in the damage process is proved to be reasonable, and the damage constitutive model of rock under frequent disturbances in the process of unloading high static stress is established by the combined model method. Finally, the rationality of constitutive equation is verified by test data, which shows that the dynamic stress-strain curve envelope is in good consistency with the theoretical curve of constitutive equation.

Experimental study on deformation characteristics of rock and coal under stress-fluid interactions in coal mine underground water reservoir

ABSTRACT Deformation response of rock and coal under long-term stress-fluid interactions not only directly affects the storage capacity of coal mine underground water reservoir but also seriously threatens its stability. In this paper, sandstone, mudstone, and coal fragments collected from the goaf of panel 22616 proposed underground reservoir in Daliuta coal mine were prepared to conduct experiments and to measure rock and coal bulking and compaction properties under stress-fluid interactions. Results show that the bulking factor of sandstone, mudstone, and coal fragments at loose state in order is 1.587–1.828, 1.561–1.787, and 1.435–1.753, respectively. The bulking factor of rock and coal fragments increases with the size of lumps, and that of mixed lumps is between the values of different single lumps. In addition, the bulking factor of same lumpiness rock and coal fragments with different strength is evidently different: the bulking factor of coal is the smallest, followed by that of mudstone and sandstone, respectively. Compared with natural rock and coal fragments, the bulking factor range of saturated sandstone, mudstone, and coal is 0.382–0.53, 0.41–0.627, and 0.506–0.807 with an average increase of 9.45%, 16.67%, and 13.34%, respectively. Certainly, bulking factor and axial stress of rock and coal basically satisfy the logarithmic relationship, regardless of strength, lumpiness, and water-bearing state. Mineral compositions and mesostructure characteristics of rock and coal were observed, meanwhile, quality and chemical composition of mine water were examined to clarify the relationship between internal composition, structure, and external deformation under stress-fluid interactions. These provide a theoretical basis for storage capacity estimation and stability control of coal mine underground water reservoir.

Shear Failure of Bolted Joints considering Mesoscopic Deformation Characteristics of Rock

In rock engineering of the cold region, there are a lot of rock joints. The shear characteristics of joints play a decisive role in the stability of rock engineering in the cold area. In this paper, based on the numerical simulation method of particle flow, reasonable microscopic parameters are selected for the numerical simulation of the direct shear test of bolted joints. The results show that the shear stiffness and contact modulus are linearly and positively correlated. The greater the contact modulus, the greater the residual stress, the better the synergetic effect between rock and bolt, and the more developed the microcrack. The smaller the contact stiffness ratio, the greater the residual stress. The shear stiffness decreases with the increase in the contact stiffness ratio, and the larger the contact stiffness ratio, the slower the shear stiffness decreases, while the shear strength does not change with the contact stiffness ratio. The contact stiffness ratio has a weak effect on the number of cracks in the model. The shear stiffness increases with the increase in the parallel bond modulus, and the shear strength decreases with the increase in the parallel bond modulus. The binding stiffness is independent of the shear stiffness, and the peak shear stress decreases with the increase in the binding stiffness ratio. The greater the bond stiffness ratio, the greater the number of cracks.

Stability analysis of surrounding rock-supporting system of shallow buried soft rock tunnel based on time effect

In view of the lack of consideration of the time effect parameters of surrounding rock and supporting structure in the past research, this paper aims at the surrounding rock deformation characteristics of highway soft rock tunnel, and by introducing the long-term strength of rock and the creep coefficient of sprayed concrete, the surrounding rock and concrete mechanical parameters considering time effect is obtained. This paper uses ANSYS software to establish the finite element model, according to the application of anchors, steel arches and shotcrete support after the excavation without any support and excavation. The simulation results of surrounding rock-support stability after tunnel excavation are obtained. The effects of surrounding rock mechanical parameters and supporting parameters on tunnel deformation and surrounding rock stability under different time effects are studied. The research results can provide useful theoretical suggestions for the safe construction of subsequent soft rock tunnels.

Deformation Caused by Dynamic Load and Support Requirements in a Deep Gob-Side Entry Rock Mass

Gob-side entries in deep coal mines can be subjected to dynamic loading when driving along the goaf. The effects of this dynamic loading and improved ground support requirements are investigated in the Gaojiabao Coal Mine, China. Numerical modeling is first used to simulate the deformation characteristics of rock surrounding the gob-side entry under dynamic load with the original support design. Dynamic loading in this scenario causes continual deterioration in rock properties, and deformation accumulates as the number of dynamic loads increases. Therefore, the overall strength and rigidity of the bolt support design need improvement, especially in terms of using yielding bolts that assist in releasing unnecessary support loads. The deformation characteristics of the rock surrounding the gob-side entry are combined with the concept of coupled pressure support to provide an optimized bolt support design. The support effects of the optimized design are then also numerically simulated and compared with the original support design. Deformation in the optimized design is significantly reduced, and roadway control is improved. Finally, the optimized design is applied and measured in a gob-side entry excavation. The field test results show that the potential for loosening, deformation, and failure of the surrounding rock is effectively controlled.

The theory development and application of deep nonlinear dynamic rock mechanics

This paper systematically summarized the development of deep nonlinear rock dynamics under the action of strong dynamic loading like explosions. It also pointed out that the structural characteristics of rock mass should be considered when studying the dynamic events such as artificial earthquakes and rockburst. By analyzing the basic deformation characteristics of rock blocks under large-scale explosions, a progressive evolution model to analyze irreversible displacements of rock mass was constructed. The relationship between the sizes of activated rock blocks under explosions with different equivalent and their distance from the explosion centers was established. A calculation method to determine the safe distance of earthquakes induced by underground nuclear explosions was proposed according to the range of irreversible displacement. The validity of the theoretical methods was comprehensively verified by comparing with underground nuclear test data of the United States, Russia and France.

Stress-drop effect on brittleness evaluation of rock materials

Uniaxial or triaxial compression test of cylindrical rock specimens using rock mechanics testing machine is a basic experimental method to study the strength and deformation characteristics of rock and the development process of rock fracture. Extensive literature review has been conducted on this issue; meanwhile, experimental and numerical studies have been conducted on the stress-drop effect on the brittleness of rock materials. A plastic flow factor of λ is proposed to describe the stress-drop effect. Evaluation methods of the factor λ corresponding to the four yield criteria of rock mass are proposed. Those four yield criteria are Tresca criterion, von-Mises criterion, Mohr-Coulomb criterion and Drucker-Prager criterion. For simplicity purposes, an engineering approximation approach has been proposed to evaluate the stress-drop with a non-zero strain increment. Numerical simulation results validated the effectiveness of the plastic flow factors λ as well as the engineering approximation approach. Based on the results in this study, finite element code can be programmed for brittle materials with stress-drop, which has the potential to be readily incorporated in finite element codes.

Discussion on stability analysis and support technology of surrounding rock of gob-side entry retaining

In order to further analyze the stability and control mechanism of surrounding rock in the gob-side entry retaining, the slip instability coefficient k1 and compression failure coefficient k2 are proposed by the mechanical analysis of the established filling body mechanics model, and when both k1 and k2 are less than 1, the roadside filling body can keep stable. The mechanical model of main roof is analyzed by the static equilibrium method, and the formula for calculating the maximum roof subsidence is deduced. Based on the data of haulage gateway of No. 21201 coal face of Juncheng Coal Mine, using the formula to calculate the amount of roof subsidence at the junction point of filling body and the roof, and proposes two methods to reduce roadway floor heave by increasing the roadside filling width and transferring the surrounding rock stress to the gob. Finally, the corresponding surrounding rock control technology is proposed according to the surrounding rock deformation characteristics during roadway excavation, roadway retention and roadway reuse, and then carries out field monitoring and evaluation. The results show that after adopting the corresponding supporting technology, the surrounding rock of the gob-side entry retaining has been effectively controlled from the excavation to the reuse period with the coal face advancing, the roof-to-floor convergence of roadway solid coal side is less than 220 mm, the roof-to-floor convergence of filling body side is less than 280 mm, and the convergence on the both sides of the roadway is about 190 mm. At the same time, the calculated roof subsidence with the junction point of filling body and the roof is similar to the field monitoring results, which achieves the expected control effect, and the research contents provides a certain reference for the support technology of the gob-side entry retaining.

Influence of the Intermediate Principal Stress on Sandstone Failure

This paper reports the results of fracture testing of sandstone under constant minor principal stress (20 MPa) and various intermediate principal stresses. The results show that when the minor principal stress is constant, as the intermediate principal stress increases, the ratio of the octahedral shear stress (τoct) to the octahedral normal stress (σoct) decreases. The strength criterion of τoct/σoct = f(σ2) is obtained. This criterion reflects not only the hydrostatic stress and intermediate principal stress effects but also the Lode angle effect. This criterion reveals the reason why the rock strength increases and then decreases with increasing intermediate principal stress. The decreasing trend is fitted by linear, logarithmic and Boltzmann equations. The applicability of the three fitting equations for strength prediction and the π plane strength envelopes is analysed, and the results of the Boltzmann fitting equation are the best. The deformation characteristics of rock during the failure process are analysed. The changing process of the tangential deformation modulus of the rock is found to be divided into three stages during the loading process: an increasing stage, an initial decreasing stage and a rapidly decreasing stage. Based on an analysis of computed tomography (CT) images of the internal fractures of rock and photographs of the fracture surfaces, the internal fractures are found to be clear and smooth, and the shear stresses in the fracture surfaces are strengthened with increasing intermediate principal stress. The dominant shear stress in the process of failure is considered to cause these phenomena.

Effects of the Loading and Unloading Conditions on Crack Propagation in High Composite Slope of Deep Open‐Pit Mine

In the process of open‐pit excavation and stacking of dump along the side wall, the rock mass inside the composite slope undergoes complicated loading and unloading effect, which are manifested as triaxial stress → horizontal unloading → axial loading. In this study, a corresponding mechanical model of crack propagation in composite slope under loading and unloading effects was established. The stress‐strain curves and deformation characteristics of rock under different stress paths were studied. A series of different tests under unloading pressure condition were carried out to study the behavior of the rock fracture under different confining pressure paths. The results showed that under the same initial stress conditions, the deviator stress required for rock unloading failure was less than that of the conventional triaxial compression. The axial deformation of rock samples was gradually increased with the increase of unloading, and the unloading easily led to rock failure. By analyzing the crack propagation mechanism under loading and unloading stress field, the analytic relation of the limit height of the composite slope with the crack morphology as the variation was determined when crack propagation occurred near the slope surface. This study provides a technical note on local damage and cracks propagation of a fractured rock mass under the loading‐unloading effects in high composite slope.

Application of NMR to Test Sandstone Stress Sensitivity of the Dongfang X Gas Field, China

The stress sensitivity of reservoir rocks has captured considerable attention in the field of oil and gas development. In this paper, nuclear magnetic resonance (NMR) was employed to measure the T2 spectra of sandstone samples from Dongfang X gas field under different confining pressures. According to T2 values, the pores in sandstone samples were divided into macropores (right peak of T2 spectra) and micropores (left peak of T2 spectra), and the stress sensitivity of macropores and micropores were discussed separately. The results show that the increase of net effective stress will lead to the pore deformation of rock samples, and the pore deformation characteristics of rocks in different blocks under different net effective stress can be effectively analyzed by the NMR. As the net effective pressure increases, the compression amplitudes of rock samples in different blocks are almost equal, indicating that the reservoirs in the Dongfang X gas field have good homogeneity. In addition, the study rocks have a weak stress sensitivity, the general stress sensitivity of micropores is stronger than that of macropores. For the samples with high permeability (over 80mD), the compression amplitudes of macropores and micropores are relatively small and close, and the porosity losses are almost the same.

Experimental Study on Strength and Deformation Characteristics of Rock–Concrete Composite Specimens Under Compressive Condition

Uniaxial and conventional triaxial tests were conducted on low-strength sandstone, concrete, and integral rock–concrete composite specimens by using TFD-2000 computer servo-controlled triaxial rock rheology testing machine, and the deformation and strength characteristics of rock–concrete composite specimens were studied. According to the test results, the uniaxial compression curves of the rock–concrete composite specimens can be divided into five distinct characteristic stages with peak stress up to 30.56 MPa, and brittle–ductile failure happens to the specimens. Under triaxial compression, the rock–concrete composite specimens have ductile dilatation failure (mainly compression), and the curves have obvious failure load, showing a linear change. In addition, the volumetric dilatation failure stress and yield stress of the rock–concrete composite specimens are basically the same, and the turning point of dilatation gets higher with the increasing confining pressure. Confining pressure can make the rock–concrete composite specimens more resistant to deformation and failure. The test results provide new ideas and basis for studying deformation and failure of rock–concrete composite and establishing constitutive equations.