In deep underground coal mining, engineering activities are performed within anisotropic in-situ stress fields due to engineering disturbances and tectonic stress. Many such activities involve the development of excavations in soft rock and anisotropic coal. Accordingly, studying the mechanical properties of soft rocks is important for the stability of deep underground excavations. In this study, the deformation, strength, and failure characteristics of soft sandstone and raw coal under two different true triaxial loading paths were investigated using a self-developed true triaxial test apparatus. The results indicated that the inelastic strain in the pre-peak stage of sandstone and coal gradually increased with increasing intermediate principal stress. Also, the strength-drop in the post-peak stage increased. The crack initiation stress, crack damage stress, and peak strength of sandstone and coal first increased and then decreased with increasing intermediate principal stress for a given σ3. Moreover, with increasing intermediate principal stress, the failure mode of sandstone and coal changed from shear to tensile shear, and from brittle to semi-brittle. The linear Mogi criteria as found to characterize the true triaxial strength of coal well, while the modified Lade criteria was more applicable to soft sandstone. Owning to the symmetry assumption, the linear Mogi criteria predicted low strength when the intermediate principal stress coefficient exceeded to 0.5. In addition, the peak strength curve of rock on the π plane and the influence of weak structures on the failure mode of anisotropic coal was discussed. Weak structures have an important influence on the failure mode of coal, which depends on the strength difference between the structural plane and the coal rock mass. The strength envelope on the π plane had a significant stress Lode angle effect, which gradually decreased as the mean stress increased.

With the increase of coal extracting depth, a considerable number of dynamic disasters display the co-occurrence and coupling effect of rockburst and coal–gas outburst, which is defined as the disturbed compound dynamic disaster. In this study, two loading modes were adopted to investigate disaster characteristics using true triaxial apparatus. The stress states of one freed and the other five stressed faces were introduced to simulate actual stress conditions. Two high-speed cameras were used to capture the disaster process. The mechanical and strength properties, failure modes, and ejection kinetic energy were analyzed. Results showed that the compound dynamic disaster mainly exhibited local grain ejection, fragment spalling, large-scale grain ejection, plate bending, and ultimately failure. The strength of the sample first increased and then decreased slowly with the increase in the intermediate principal stress. After failure, a V-shaped coal-burst pit was formed, which was approximately parallel to the intermediate principal stress and perpendicular to the free face. The grain ejection exhibited obvious characteristics of spatial sorting, and the grain size decreased with the distance from the free face. The kinetic energy showed little change with increase in the intermediate principal stress in the displacement loading mode; whereas, it first increased and then decreased in the stress loading mode. The pressurized gas promotes the development of coal cracks and fully fractures the coal rock. Under the combined actions of the elastic energy stored in coal mass and the internal energy of pressurized gas, compound dynamic disasters may occur. Gas extraction and coal seam elastic softening techniques can effectively reduce and prevent the occurrence probability of compound dynamic disasters.

In engineering, it is considerably important to understand the evolution process of sandstone deformation and permeability under complex stress conditions. Although numerous scholars have reported on sandstone permeability, many of their studies are only based on uniaxial or conventional triaxial stress conditions. In practical engineering, in situ stress evidently indicates three-dimensional anisotropy. In recent years, many scholars have performed experimental studies on rock permeability under true triaxial stress. There are, however, few investigations related to the influence of intermediate principal stress and intermediate principal stress coefficient on sandstone permeability; moreover, the mechanism is unclear. In this study, the permeability of sandstone under true triaxial stress conditions is measured using a true triaxial apparatus. The results show that the intermediate principal stress has significant effects on permeability and deformation of sandstone sample. Under different principal stress conditions, the permeability is observed to decrease with the increase of the principal stress. With increasing intermediate principal stress coefficient b (from 0 to 1), the major principal strain and the minor principal strain always compressed and finally decreased by 27.31 and 36.96%, while the intermediate principal strain finally changes from expansion to compression (from − 0.045 to 0.255%). Also the normalized permeability loss increased from 9.57 to 16.59%. In addition, by fitting the experimental data with several permeability models, it was found that a true triaxial permeability model can well describe the changes in sandstone permeability. Based on the fitting parameters, the influence of intermediate principal stress and its coefficient on the stress sensitivity of sandstone was analyzed.

The compound dynamic disaster is an abrupt and violent energy-releasing process under the combined action of gas and geological tectonic stress in deep underground coal mines. To investigate the occurrence mechanism of compound dynamic disasters, a novel experimental approach was designed and the one free-face true triaxial tests with different loading rates were conducted. Results shown that the compound dynamic disaster can be classified into several stages: initial quiet period, local particles and fragments ejecting, mid-term quiet period, external coal plate bending accompanied by gas-emitting, dynamic disaster occurring process, and the final re-stabilized period. The presence of the gas can significantly facilitate the development of cracks and enhance the kinetic energy, thus increasing the probability of the compound dynamic disasters. Under the one free-face true triaxial engineering stress conditions, the mechanical properties (e.g. deformation and strength) of coal samples are obviously rate-dependent, and the peak stress exhibits an approximate logarithmically increase trend with increasing loading rate. After failure, several tensile-shear fractures approximately parallel to the intermediate principal stress direction and perpendicular to the free face are generated. And an arc-shaped coal-burst pit on the free face of coal sample is observed, and the pit volume increases with the increase of the loading rate. Moreover, the particle ejection kinetic energy increases gradually as the loading rate increased, suggesting a higher loading rate can enhance the intensity of the disaster. The findings may provide guidance for the prevention and control of the compound dynamic disasters in deep underground coal mines.

Facing increasingly large and discontinuous structural evolution under the influence of extensive mining activities, some traditional geophysical technical methods of monitoring overburden deformation have become of restricted use. As an advanced sensing technology, the distributed optical fiber sensor has been introduced into rock deformation monitoring by virtue of its good adaptability and high precision. In this paper, an overburden stratum deformation monitoring experiment was performed within a physical model test to explore the deformation characteristics of mining-induced overburden stratum based on distributed optical fiber sensing. The results indicate that distributed optical fiber monitoring is effective, and the Brillouin frequency shift (BFS) variation distribution reflects every aspect of the space-time and movement laws of the overburden stratum. The comparison of measurements of the distributed optical fiber, fiber Bragg gratings, and dial gauges proved the feasibility and reliability and revealed the ability of optical fiber measurement to monitor large-scale macroscopic deformation, and capture subtle deformation along the entire overburden stratum. More importantly, the BFS variation distribution presents a three-step form, which explicitly identifies the zoning development of overburden stratum. In turn, a conceptual model for describing the overburden deformation process is provided; it illustrates the inversion mechanism of the BFS variation distribution in reaction to the typical overburden deformation structure. This study aims to provide a theoretical and methodological foundation for the application of distributed optical fiber sensing (DOFS) measurement in mining ground control research.

Underground mining or tunnelling activity is always associated with the composite geological formations. The mechanical properties of layered composite coal–rock subjected to true triaxial stress conditions are significantly different from those under conventional triaxial or uniaxial stress conditions. In this work, we conducted a series of true triaxial tests using the self-developed true-triaxial apparatus to investigate the mechanical response (e.g., deformation, strength, and failure characteristics) of the layered composite coal–rock (CCR). The results show that the uniaxial strength of CCR lies between the strength of pure sandstone and coal, and the direction of the bedding affects the overall strength of the samples. The true triaxial strength of both the pure rock and CCR increases first and then decreases with the increase of the intermediate principal stress. Moreover, for a given loading direction, as the thickness of the sandstone layer increased, the strength of the CCR increases. The deformation of the CCR shows more obvious plasticity than that of the pure sandstone due to the coordinated deformation of the coal and sandstone layers. In addition, a new true triaxial strength criterion expressed by the first and third equivalent principal stress invariants was proposed, which can well describe the strength characteristics of different coal rocks. The stress states, weak structural planes, and localized stress have a great influence on the failure modes of CCR. The local stress concentration near the contact surface promotes the development of the secondary failure fractures. These findings are of great significance in stability designing in deep underground engineering.

Coal and rock mass constitute a type of porous medium. This study investigated the influence of temperature impact on the mechanical properties and acoustic emission (AE) characteristics of coal. A mechanism analysis was performed from the perspective of microstructure. The results show that the temperature impact causes the development of pores and cracks in the coal, which reduces the strength of coal. The elastic modulus of coal generally decreases with increasing temperature gradient. AE parameters increase with the increase in the load and reach the maximum value at the peak stress. AE parameters and cumulative parameters decrease with increasing temperature gradient. Not only does temperature impact change the fracture structure of the coal surface, but also the internal fracture structure of the coal is significantly affected. After temperature impact, the cracks expand and new cracks are initiated, and the fracture volume of the coal increases. Temperature impact causes the volume and specific surface area of small pores and meso-pores in coal to increase, and promotes the opening of the necking pores within the coal. The impact causes macro-pores to penetrate through to form cracks, which increases the transport of coal gas and significantly improves the permeability of coal. The thermal stress generated by coal under temperature impact is greater than its tensile strength, which promotes the cracking of coal, along with the initiation, widening, extension, and expansion of crack networks, which significantly change the fracture structure of coal. The research results lay a certain theoretical and experimental foundation for further study of mechanical properties of coal affected by liquid nitrogen.

With the continuous demand for mineral resources, the construction design and stability of underground rock engineering have been a frontier research topic worldwide. Stability analysis of underground caverns, tunnels, boreholes, and cavities is of critical significance for underground engineering. To investigate the mechanical properties and failure mode of sandstone specimens with prefabricated cavities, experiments were conducted on sandstone samples with the borehole axis parallel to different principal stresses and for varying borehole depths and diameters using a novel true triaxial geophysical apparatus. Computed tomography (CT) was used to analyze the crack development around the borehole wall. The experimental results indicated that the direction of the borehole and the principal stress axis, as well as the borehole depth and size, significantly affected the strength and damage mode of the rock samples. For a given σ3, with increasing principal stress coefficient β, the strength of the samples with the borehole axis parallel to the different principal stresses first increased and then decreased, and the maximum strength appeared at β=0.5. The highest stability of the rock samples appeared when the borehole was parallel to σ1, and the lowest stability occurred when the borehole was parallel to σ2 or σ3. The strength of the sandstone sample decreased as the borehole diameter increased. Four types of typical damaged modes were observed in the CT scan results. Moreover, the intermediate principal stress had a significant effect on the damage-zone distribution around the borehole. This work can help to understand the damage area and failure characteristics of the borehole surrounding rock, also make a significant contribution to support the design of deep rock engineering.

Shale gas well deliverability and economics depend on extremely low permeability that is not only dependent on the rock bedding trend but also controlled by in situ stresses. The purpose of this study was to determine relative contributions of normal and tangential stresses with respect to the rock bedding plane on permeability evolution of shale. The study involved an analysis of the rock bedding structure, followed by triaxial testing of rock samples and theoretical modeling. Also simulated were the effects of stress-bedding and load cycling. The results showed shale permeability reduction during the stress loading process and its gradual recovery during the unloading process. Permeability change was more pronounced in response to normal stress but some effects of the tangential stresses were also observed. Moreover, a theoretical model was derived to describe permeability change with effective stress in the presence of normal and tangential stresses. The model was empirically matched with the experimental results. The assessment of relative contributions of normal and tangential stresses was quantified with the analysis of variance (ANOVA). The analysis revealed significance levels of normal stress, and two tangential stresses sigma(t1)and sigma(t2)on shale permeability as 81%, 5%, and 14%, respectively. An almost 20-percent contribution of tangential stress loading to permeability response indicates a need for the improvement in computing effective stress. Therefore, a new method was suggested to determine effective stress when predicting permeability evolution of shale.

煤与瓦斯突出、冲击地压等动力灾害严重威胁矿山安全高效生产.煤矿进入深部开采后,受原岩应力升高及地质赋存条件变化的影响,冲击与突出复合型动力灾害呈逐渐增多趋势,其发生原因、机理较单一动力灾害更为复杂.因此研究深部煤矿复合动力灾害致灾机理对于动力灾害的有效防治至关重要.本文基于自主研制的“多功能真三轴流固耦合试验系统”,进行了考虑气体影响的完整煤样和卸压孔煤样的5面加载、单面临空试验.结果 表明,复合动力灾害是煤岩在应变能和气体内能作用下非线性瞬发性破坏的动态过程.其发生过程具有明显的阶段性,主要经历颗粒弹射、碎片弹射、局部煤体破坏、煤体抛出失稳和重新平衡状态,煤样破坏后形成明显弧形阶梯状煤体抛出坑.中间主应力在一定范围内有增强煤样强度的特性,试样强度随中间主应力的增加而逐渐增加,试样破坏后形成平行于中间主应力方向的主断裂面.钻孔卸压措施可在一定程度上改善煤岩力学性质,软化煤岩结构,降低煤岩强度,增强其塑性变形特性,使集聚的弹性能量缓慢释放,降低动力灾害发生的可能性.对比试验结果表明,卸压孔平行于中间主应力时煤样产生的塑性区范围更大,塑性程度更强,钻孔后的试样破坏后形成明显的阶梯式层裂结构,与未钻孔和其他钻孔布置方式相比,致灾程度弱化,无明显动力显现特征,卸压效果更好.卸压钻孔主要通过优化能量释放结构,促进煤岩渐进式损伤,最终煤岩趋于静态缓慢式破坏.针对现场具体工程条件,提出了根据实际地应力、地质条件等布置卸压钻孔方位的技术方案和可行的治理措施.

The geological sequestration of CO2, underground coal mining, and coalbed methane production in deep coal reservoirs is executed under high levels of 3-D geo-stress, and it is accompanied by significant variations in both vertical and horizontal stresses. In addition, coal is a highly fractured porous medium characterized by complex natural fracture systems. This exterior and interior anisotropy complicates the replication of in situ conditions in the laboratory. In this study, we divided pre-existing fracture systems of cubic coal into three flow planes: a bedding plane, face cleat plane, and butt cleat plane. Gas flowed through cubic coal samples along each flow plane under differently designed true triaxial stress paths. We then further analyzed anisotropic mechanical and flow property responses of cubic coal after failure by model fitting, CT scan reconstruction, and fractal representation. The experimental results indicate that pre-existing flow planes play significant roles in the strength levels, failure modes, and permeability levels. Low strength levels, typical shear failure patterns, and low initial permeability levels were observed in the butt cleat plane direction. Anisotropic strength data can be effectively fit by applying a linear relationship between octahedral shear stress and mean effective normal stress. After coal failure, the peak permeability observed in the face and butt cleat plane directions also presents a strong linear relationship with the fractal dimension. An anisotropic conceptual failure process model was established for the description of internal fracture development during stress loading. Horizontal stress unloading decreased the strength and formed a more complex fracture system in cubic coal regardless of the different flow planes involved, producing the increments of associated peak permeability.

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.

How to safely and economically dispose mining tailings is a challenge to mine operators. This paper presents an alternative upstream method for tailings dam construction, termed as the template construction method (TCM), which has been successfully implemented at Zhelamuqing tailings impoundment since 2004. By the beginning of 2015, the tailings dam wall had reached 95 m in height for the 46 upstream raises, with the total height of the dam including the starter dyke being 128 m. The proposed TCM is relatively simple and cost-effective and provides a good way for constructing rapidly raising tailings dam based on this case.

The microstructure of coal has a significant influence on the permeability of the coal seam. To study the characteristics of microstructure changes in coal seam under temperature impact, we conducted temperature-impact experiments using a high–low temperature test system, and we studied the coal pores and fissure structure before and after the temperature impact using scanning electron microscopy, industrial micro-computed tomography, and mercury intrusion. Based on the digital image processing technology and thermal stress theory, we qualitatively and quantitatively analyzed the variation of crack width, specific surface area, and pore diameter, and deeply analyzed the failure mechanism of temperature impact on coal seam microstructure. The results showed that the temperature impact caused the macropores to interpenetrate and form macroscopic cracks in the coal sample, which resulted in a relatively small volume of macropores and increased the volume of mesopores and small pores. The maximum thermal stress generated during the temperature impact process was located in the tangential direction of the coal sample surface. The thermal stress generated by the temperature impact exceeded the tensile strength of the coal sample, which directly causes crack initiation, expansion, and mutual penetration. This study provided the technical support necessary for the efficient development of coalbed methane and the improvement of gas drainage rate in the coal seam.

Investigations of the directional permeability evolution of intact and fractured coal are conducted under different simulated geological conditions. The effects of fracture geometry, water adsorption and stress conditions on the permeability evolution of coal as a function of stress are systematically studied. The results indicate that permeability anisotropy is more pronounced in fractured coal than in intact coal. The permeability order, i.e., the kfa > kbu > kbe relationship, is maintained following the introduction of macrofractures into coal. The fracture compressibility in the butt cleat plane flow direction is higher than that in the other two flow directions for both intact and fractured coal. The presence of water in coal can reduce the permeability by up to one order of magnitude, and a more significant permeability decrease is observed in coal specimens containing rough macrofractures. Permeability hysteresis for both intact and fractured coal is somewhat dependent on the stress condition. The hysteresis effect of coal is more significant under triaxial stress conditions and less pronounced under true-triaxial stress conditions.

Shale permeability is considered the critical parameter for commercial gas production. In this work, we investigated the permeability evolution under true triaxial stress conditions of Longmaxi shale. The results showed that shale permeability decreased significantly with increasing intermediate principal stress σ2 and increased gradually with decreasing σ2. And permeability was larger when σ2 was perpendicular to the bedding than that when σ2 was parallel to the bedding. A theoretical model was derived to describe the permeability of shale with respect to effective stress and the results agreed well with the experiment. The matching results showed that the fracture was compressed more easily when σ2 was perpendicular to the bedding and the shale sample was generally compressed in the loading process while swelled during the unloading process. Strong hysteresis was observed for both permeability and strain results. It also warrants testing other gas shales to specifically determine this effect.

The permeability anisotropy of shales can be attributed largely to bedding planes. It is critical to study the bedding effect on permeability evolution for shales. We conducted an experiment on shale permeability anisotropy under true triaxial stress conditions. Even though the mean stress of shales was different, the permeability still exhibits obvious bedding dependent. In this study, we present a new permeability model considering bedding planes. The introduction of characteristic functions and parameters characterizes the transformation process from bedding to non-bedding during compression. Compared with the experimental data, we obtained a good fitting result. Owing to the increase in the resistance of the pressurized gas into the fracture system, Biot's coefficient α decreases with increasing mean stress. The porosity sensitivity exponent s parallel to the bedding planes are larger than those perpendicular to the bedding planes. In our opinion, owing to the smaller stiffness of the bedding plane, the relatively large amount of pore deformation is caused by the loading on the vertical bedding planes. Furthermore, we introduce η to characterize the effect of non-bedding and bedding on permeability. The permeability of shale or bedding is more sensitive at lower stress than at higher stress and the fluid flow through the non-bedding in shales cannot be ignored.

The greenhouse effect has attracted increasing attention globally. The geologic sequestration of CO2 is considered as an effective method for reducing the amount of CO2 in the atmosphere. Carbon-dioxide enhanced coalbed methane (CO2-ECBM) recovery is extremely attractive owing to its ability to store CO2 in coal seams while simultaneously enhancing the production of methane. However, the injection of CO2 can induce a deformation of the coal matrix, which has a significant effect on the fluid flow and stability of the coal seams. In this study, the sorption kinetics experiments indicated the high anisotropy and heterogeneity of local sorption deformation in coal samples and a unique kinetic behavior of sorption deformation which was rarely reported and analyzed in detail in previous studies. This kinetic behavior demonstrated a rapid increase of sorption strain followed by a slight or significant falloff until the equilibrium state reached. The characteristics of component distribution in coal samples were investigated using CT scanning and micro-lithotype and EDS analysis. An extensive analysis of the experiment results indicated the following: 1) Carbon dioxide dissolved during the first sorption process induced a rearrangement of the coal structure, leading to a drastic variation of deformation behavior between the first and subsequent sorption process. 2) The unique kinetic behavior of sorption deformation could be induced by either the loss of moisture in coal or the interaction of different coal components. The mechanism of these two factors induced coal shrinkage was different and had a distinct effect on the global behavior of sorption deformation. 3) The quantitative relationship between compressional and dilative components directly verified the significant compression of inertinite-rich regions by the slow swelling of vitrinite-rich regions. 4) The kinetic behavior of sorption deformation was affected by the combination of the macroscopic bedding structure and the heterogeneous component distribution in coal.