Progressive failure processes and mechanisms of disasters caused by interrelated failure of residual coal pillars and rock strata.

The increasing number of closed/abandoned mines being reused has become a concern for people. However, a large number of coal pillars were left behind in the closed/abandoned mines. Before effectively utilizing the underground space of mines (CO2 goaf storage, etc.), it is necessary to study the stability of the residual pillars and rock strata. However, limited laboratory experiments and numerical simulation tests have been carried out to understand the relationship between residual pillars and strata in the context of their failure process and mechanism. In this study, the progressive failure and movement of the pillars and strata during the multi-seam mining is simulated using a physical model and numerical simulation. In addition, the failure mechanism was analyzed. The results suggest that the failure process of the pillar is strongly related to the rock strata. The mining of the below seam will bring about not only the collapse of the above strata, but also penetrate the gob of the overlying seam, and then cause the roof of the overlying seam to further collapse. The damage zone acting on the pillar increases accordingly, and the pillars are also gradually damaged. When the pillar is completely destroyed, it will further cause the stable strata to fracture, collapse and become unstable, and a rock burst may be formed. It is also found that under the effect of the stress concentration of the pillar, the floor rock will be damaged to a certain depth. When the collapse height of the overlying strata overlaps with the failure depth of the floor due to the stress concentration, it is more likely to bring about the occurrence of rock burst. The mechanisms of progressive failure of the residual pillars and rock strata revealed by this study offers guidance for the control of disasters, and also provides a reference for the stability research during goaf utilization in the later stage.

Safety evaluation method of bottom coal thickness in thick coal seam roadway.

In recent years, the number of roadway floor rock burst accidents is increasing, which seriously restricts the safe production of the mine. Therefore, safety evaluation method of bottom coal thickness in thick coal seam roadway was studied. The research results shown that the stress concentration area of composite floor is distributed in coal seam or rock stratum with large elastic modulus. With the increase of floor rock strength, the stress of coal-rock composite floor increased gradually, but the displacement and energy decreased gradually. When floor rock strength was equal to bottom coal strength, the increase of floor stress and displacement with the change of bottom coal thickness was the smallest, which was 34.29% and 33.61% respectively. The elastic strain energy decreased from 14.58 to 9.85%. With the increase of bottom coal thickness, the stress and displacement of coal-rock composite floor increased first and then decreased, and the elastic strain energy decreased gradually. It puts forward the safety evaluation method of bottom coal thickness: floor failure depth → reasonable thickness of bottom coal → safety thickness of bottom coal. It can provide reference for design of roadway bottom coal retention and surrounding rock control in thick coal seam face.

Microstructure, Characterization and Mechanical Properties of Coal and Coal-like Materials.

Energy is the most basic driving force for world development and economic growth and the basis for human survival [...].

Mechanical Properties and Failure Mechanism of Anchored Bedding Rock Material under Impact Loading.

In view of the problem that anchored bedding rock material is prone to instability and failure under impact loading in the process of deep coal mining, and taking the lower roadway of a deep 2424 coal working face in the Suncun coal mine as the engineering background, a mechanical model of anchored bedding rock material was established, and the instability criterion of compression and shear failure of anchored bedding rock material was obtained. Then, the separated Hopkinson pressure bar was used to carry out an impact-loading test on the anchored bedding rock material, and the dynamic mechanical properties of the rock with different anchoring modes and bolt bedding angles were studied; the evolution law of the strain field of the anchored bedding rock material was also obtained. The results show the following: (1) The bolt support could effectively improve the dynamic load strength and dynamic elastic modulus of the rock material with anchorage bedding, the degree of improvement increased with the increase in the angle of the bolt bedding, and the full anchorage effect was much higher than the end anchorage effect was. (2) The bolt bedding angle and anchorage mode greatly influenced crack development and displacement characteristics. After an impact, the bedding rock material had obvious shear displacement along the bedding direction, and obvious macroscopic cracks were produced in the bedding plane. The research results offer theoretical guidance to and have reference significance for deep roadway anchorage support engineering.

Key Factors Identification and Risk Assessment for the Stability of Deep Surrounding Rock in Coal Roadway.

In order to evaluate the stability of deep surrounding rock, all of the affecting factors should be theoretically identified. However, some factors have slight impacts on the stability of deep surrounding rock compared with others. To conduct an effective risk assessment, key factors should be first extracted. The analytic hierarchy process (AHP) and grey relation analysis (GRA) methods are integrated to determine the key factors. First, the AHP method is applied to sort the factors by calculating the weights of them. Seven out of fifteen factors are extracted as the key factors, which account for 80% of the weights. Further, the GCA method is used to validate the effects of these key factors by analyzing the correlation between the performance of each factor and that of the reference. Considering the influence of these key factors and experts' judgements, the multilevel fuzzy comprehensive evaluation method is adopted to obtain the risk level of the deep surrounding rock stability. Finally, the risk assessment of the deep surrounding rock in the E-Zhuang coal mine of Chinese Xinwen Mining Area illustrates the operability of the proposed method.

Uniaxial Compression Behavior of Cement Mortar and Its Damage-Constitutive Model Based on Energy Theory.

The mechanical properties of mortar materials in construction are influenced both by their own proportions and external loads. The trend of the stress-strain curve in cracks compaction stage has great influence on the relationship between the strength and deformation of cement mortar. Uniaxial compression tests of mortar specimens with different cement-sand ratios and loading rates were carried out, and the stored and dissipated energies were calculated. Results indicated that the elastic modulus and strength of mortar specimens increase with the cement-sand ratio and loading rate. The energy dissipation shows good consistency with the damage evolution. When the loading rate is less than 1.0 mm/min, most of the constitutive energy at the peak point is stored in the specimen and it increase with cement-sand ratio. A simple representation method of axial stress in cracks compaction stage was proposed and an energy-based damage constitutive model-which can describe well the whole process of cement mortar under uniaxial compression-was developed and verified.

Damage evolution of bi-body model composed of weakly cemented soft rock and coal considering different interface effect.

Considering the structure effect of tunnel stability in western mining of China, three typical kinds of numerical model were respectively built as follows based on the strain softening constitutive model and linear elastic-perfectly plastic model for soft rock and interface: R-M, R-C(s)-M and R-C(w)-M. Calculation results revealed that the stress-strain relation and failure characteristics of the three models vary between each other. The combination model without interface or with a strong interface presented continuous failure, while weak interface exhibited 'cut off' effect. Thus, conceptual models of bi-material model and bi-body model were established. Then numerical experiments of tri-axial compression were carried out for the two models. The relationships between stress evolution, failure zone and deformation rate fluctuations as well as the displacement of interface were detailed analyzed. Results show that two breakaway points of deformation rate actually demonstrate the starting and penetration of the main rupture, respectively. It is distinguishable due to the large fluctuation. The bi-material model shows general continuous failure while bi-body model shows 'V' type shear zone in weak body and failure in strong body near the interface due to the interface effect. With the increasing of confining pressure, the 'cut off' effect of weak interface is not obvious. These conclusions lay the theoretical foundation for further development of constitutive model for soft rock-coal combination body.

Rockburst disaster prediction of isolated coal pillar by electromagnetic radiation based on frictional effect.

Based on the understanding that charges generated during coal cracking are due to coal particle friction, a microstructure model was developed by considering four different variation laws of friction coefficient. Firstly, the frictional energy release of coal sample during uniaxial compressive tests was investigated and discussed. Then electromagnetic radiation method was used to predict the potential rockburst disaster in isolated coal pillar mining face, Muchengjian Colliery. The results indicate that the friction coefficient of coal particles decreases linearly with the increase of axial loading force. In predicting the strain-type rockburst, the high stress state of coal must be closely monitored. Field monitoring shows that electromagnetic radiation signal became abnormal before the occurrence of rockburst during isolated coal pillar mining. Furthermore, rockburst tends to occur at the early and ending stages of isolated coal pillar extraction. Mine-site investigation shows the occurrence zone of rockburst is consistent with the prediction, proving the reliability of the electromagnetic radiation method to predict strain-type rockburst disaster.