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In recent years, many studies have shown that it is meaningful to place rocks under stress paths corresponding to various loading and unloading conditions. However, the deformation evolution of rock under cyclic loading with consideration of the mechanical behavior and characteristics has rarely been studied under triaxial cyclic unloading and loading conditions. In practical engineering, particularly in underground or mining engineering, the stress increase in the rock mass in areas affected by mining is mainly caused by crack initiation and development when the rock is subjected to the effects of cyclic unloading and loading. In this study, variations in the stress–strain curves, irreversible strain, elastic modulus, and Poisson’s ratio are discussed and explained. The test results demonstrate that in comparison with a lower initial confining stress, increasing the initial confining pressure restrains the radial deformation of sandstone samples, and the degree of compaction of the sandstone samples rapidly increases in the failure stage. This results in the loss of the failure buffering process of the sandstone sample. Changes in the degree of compaction of the rock samples lead to obvious differences in the irreversible strain of the rock under different initial confining pressures and different limit unloading and loading cyclic confining stresses. The scanning electron microscopy and analysis results demonstrate that the macroscopic mechanical and microscopic physical properties of sandstone show different characteristics under different initial confining stresses.

Experimental research on the growth of internal flaws has rarely been reported due to the fact that it is difficult to cut internal flaws in specimens and cannot capture the initiation and propagation processes of internal flaws through direct observations. This paper proposed a method for creating internal flaws in specimens by utilizing the volatilization of camphor. A series of compression tests were performed on rock-like specimens including two embedded circular flaws, and CT techniques were used to investigate the internal damage behavior of flawed specimens. Experimental results illustrate that the strength and deformation properties of flawed specimens increase nonlinearly with the confining pressure as well as flaw inclination angle. Crack coalescence patterns and failure modes of flawed specimens depend on not only the confining pressure but also the flaw inclination angle. The crack coalescence pattern varies from wing crack coalescence to mixed tension-shear crack coalescence and then to the shear crack coalescence as the crack inclination angle increases. Confining pressure contributes to shear crack growth and has an inhibiting effect on the propagation of tension cracks. For specimens with the same flaw inclination angle, the failure mode changed from tension failure to mixed shear-tension failure or from mixed shear-tension failure to pure shear failure with the increase of confining pressure.

Hydraulic fracturing is a powerful technology, especially in stimulating fluid production from reservoirs. However, the problem of the intersection between hydraulic fractures and natural fractures is inevitable in engineering practice due to naturally fractured formations. This paper presents a new criterion for a toughness-dominated hydraulic fracture crossing a natural frictional interface through coupling the fluid flow and elastic deformation of the hydraulic fracture prior to intersecting with the natural frictional interface. The critical condition for the hydraulic fracture crossing the natural frictional interface is that the total superimposed stress does not satisfy the failure condition of the Mohr–Coulomb criterion. Simultaneously, the new criterion considers nonorthogonal intersection angles and six independent parameters relating to fluid flow (hydraulic fracture half-length, approaching distance and injection rate), rock mechanic properties (rock fracture toughness and Young’s modulus) and in situ stress. The prediction outcomes show good agreement with laboratory experiments as well as sufficient advantages compared with the analytical criteria of Blanton, extended Renshaw-Pollard and Llanos. Parameter sensitivity analysis is conducted using the control variable method. The parametric analysis results reveal that the influence sphere of different parameters is limited to a certain extent by the variations in the intersection angle except for Young’s modulus and the injection rate, which show slight effects on the intersection behaviors.

There is potential danger of conical shear failure surface in karst cave roof under pile foundation load. To understand the mechanical and failure characteristics of rocks under such conditions. In this study, an auxiliary device consisting of an indenter and a hollow support combined with a servo-controlled mechanical equipment was used to perform mixed compression-shear tests. The laboratory tests were studied numerically by finite difference methods. This study set up 4 groups of height variables 20, 30, 40, 50 mm and 3 groups of aperture variables 20, 30, 40 mm. AE (acoustic emission) tests are adopted to study the failure characteristics of specimen. The experimental results show that the peak load increases with the sample height while decreases with the aperture of support. This paper defines an apparent elastic modulus in terms of displacements and loads, which increases with height by 42.12, 51.7, and 49.00 KN/mm for apertures from 20 to 40 mm. The AE data show that the ratio of RA (ratio of rise time to maximum amplitude) to AF (average frequency of AE count) gradually increases as the support aperture increases, indicating that the aperture is approximately close to the radius of the indenter and that shear damage is approximately significant. The numerical simulation results show that the shear stress is the largest at the position where the upper surface of the specimen contacts with the indenter, and the displacement is the largest at the position where the lower surface of the specimen overlaps with the support hole. The extrusion phenomenon caused by dilatation is the main reason for the tensile failure detected during the experiment.

Understanding the spontaneous water imbibition mechanism in coal of varying ranks has substantial implications for hydraulic operations and the safe, efficient extraction of coalbed methane. During spontaneous imbibition, low-field nuclear magnetic resonance techniques were used to test the T 2 spectra and imaging (MRI) of coal samples, enabling the determination of water signal distribution in the samples at different time intervals. By combining this with the pore structure, we explored the water migration characteristics of the coal samples during spontaneous imbibition from a microscopic perspective. Additionally, we deeply investigated the relationship between the MRI pixel eigenvalues, the capillary absorption coefficients, and the degree of pore development. The results demonstrated a positive correlation between the capillary water absorption coefficient and the degree of pore development in the coal samples, while revealing a negative correlation with both pore diameter and tortuosity. In the process of spontaneous imbibition, micropores exert a dominant influence and achieve saturation first, followed by a gradual increase in the contribution of mesopores and macropores (including microfractures) to spontaneous imbibition. The MRI results demonstrate that water migration primarily occurs within the interior of the coal samples before extending towards the exterior. The pixel values obtained from MRI follow a normal distribution, with the mean value reflecting the extent of pore development and the entropy representing the randomness in pore distribution. The higher the pixel eigenvalue, the greater the pore content of the coal sample, indicating a more random distribution of pores and enhanced connectivity among various types of pores, which results in an increased capillary water absorption coefficient.

In order to investigate the influence of shear on contact characteristics and fluid flow evolution of rough rock fractures, a series of shear-flow tests were carried out by numerical experiments. Firstly, a sandstone specimen with a rough fracture was made in the laboratory, and the numerical model of the fracture was reconstructed in FLAC3D software. Experiments were conducted to investigate the depth of penetration of the fracture under different normal stress (1, 3, and 5 MPa) and shear displacement (2, 4, 6, 8, and 10 mm). By establishing the relationship between the shear direction and the apparent inclination of the fracture asperity, the effects on the asperity contact characteristics and seepage properties are explored. The results of the study indicated that the larger the normal stress the larger the surface contact section and the smaller the aperture, showing the opposite trend with increasing shear displacement. Positive apparent inclination distribution can effectively predict the location of shear damage during the shear process. Negative apparent dip implies that those regions increase permeability after shear, and decrease with increasing crack opening. Fracture seepage shows obvious nonlinear characteristics, where the nonlinear coefficients are 5–6 orders of magnitude larger than the linear coefficients. The critical Reynolds number Rec is used to distinguish the linear and nonlinear categories of fluid flow, and the calculated results show that the range of Rec is between 0.0081 and 0.11.3, and the Rec increases with shear displacement and decreases with normal stress.

The microwave-assisted rock breaking technology has been proven to be feasible, and has received considerable attention in the field of civil and mining engineering. A copper foil was used to wrap basalt to simulate rock excavation of practical application scenario in this paper. To this end, a multi-mode cavity with an operating frequency of 2.45 GHz was used to conduct microwave irradiation experiments on basalts with different irradiation times and different power. The thermal properties, AE characteristics, and damage evolution process of basalt were studied. The results show that the high heat generated by microwave leads to the development of cracks in the upper part of basalt. The higher the power level, the higher the degree of crack propagation in the sample, the lower the basalt strength, and the more active the AE activity. The fluctuation rule of the b value indicates that the basalt is dominated by small-scale microfractures before failure. High power levels or long irradiation time lead to more microwave-induced cracks participating in the failure process during loading. Compared with unheated basalt, microwave-heating basalt detects the characteristics of the precursor of failure in advance. The AE source location and the nephogram of the maximum principal stress of microwave-treated basalt reflected that the fracture path begins in the upper part of the rock. In addition, the combination of high power level and short irradiation time can achieve the purpose of energy saving.

Permeability estimation from pressure-pulse decay method is complicated by two facts: (1) the decay curve often deviates from the single-exponential behavior in the early time period and (2) possible existence of gas adsorption. Both the two factors cause significant permeability error in most of pressure-pulse decay methods. In this paper, we first present a thorough analysis of pressure-pulse propagation process to reveal the mechanism behind the early time and later time behaviors of pressure decay curve. Inspired by the findings from these analyses, a new scaled pressure is proposed which can: (1) be easily used to distinguish the early time and later time data and (2) make the decay curves of all cases into a single 1:1 straight line for later time. A new data-proceeding method, which calculates the apparent porosity and permeability using the same set of measured data, is then developed. The new method could not only remove the effects of the adsorption on the permeability estimation, but also identify the apparent porosity as well as proper adsorption model and parameters. The proposed method is verified by comparing with true values and calculated values through numerical simulations that cover variations in typical rock properties (porosity, permeability, slippage, and adsorption) and the experiment configurations. It is found that the new method is accurate and reliable for all test cases, whereas the Brace’s and Cui’s approaches may cause permeability error in some cases. Finally, the new method has been successfully applied to real data measured in pressure-pulse decay experiments involving different types of rocks and gases.

In this study, multilevel and conventional unloading triaxial compression tests under different confining pressures are separately carried out to systematically reveal the deformation, energy evolution, and fracture characteristics of sandstone samples. Results show that under the multilevel unloading condition, the increase of the initial confining pressure has a more obvious inhibitory effect on the radial strain of sandstone, and the samples can fully exhibit elastic deformation and partial plastic deformation, showing obvious plastic characteristics. The radial energy growth factor is more sensitive than the axial energy growth factor during the process of confining pressure unloading, and the larger the initial confining pressure, the earlier the period-doubling bifurcation region and chaotic region are reached. To better understand the deformation and failure process of rock during engineering excavation, it is necessary to establish a constitutive relation describing the mechanical properties of rock. The three-step failure mode also proves that there are tensile and shear fractures in sandstone samples, in which the effects of tensile stress and shear stress are more or less interdependent in the failure process. It can be seen that multilevel unloading makes the energy conversion more adequate and reduces the sudden release of energy when the rock fails, reducing the possibility of rockburst and making the excavation unloading process safer. This will deepen the understanding of rock failure behavior and contribute to the better application of energy characteristics to relevant engineering practices.

This work conducts staged confining pressure cyclic loading and unloading (SCPLU) tests on coal rock to explore its energy evolution and damage features under complex stress states. A novel damage variable based on dissipated energy and enhanced modulus of elasticity is proposed. Results show that under low confining pressure, coal rock failure is minimally affected by cyclic unloading, with few micro-cracks. However, under high confining pressure, micro-cracks increase with rising confining pressure and unloading cycles. After each stage of loading and unloading, stress redistribution in coal rock causes micro-crack propagation, more dissipated energy, increased irreversible plastic deformation, and worsened damage. The damage variable based on dissipated energy and the improved modulus of elasticity has a high fitting degree of 0.998 with the damage evolution equation based on the Weibull distribution, which is superior to the improved modulus of elasticity method alone. The failure process of coal rock can be divided into five stages: compaction segment, elastic segment, crack instability development segment, crack instability propagation segment, and residual strength segment. This classification effectively characterizes the failure process of coal rock and provides a reference for further revealing the damage evolution mechanism of coal rock under complex stress paths.