The particle size of coal particles is an important factor affecting the physical and mechanical properties of coal. In this study, uniaxial and triaxial compression tests were conducted to investigate the effects of coal particle size on the physical and mechanical properties of briquettes and their impact mechanism using a rock mechanics test-150B servo system (RMT-150B). The results showed that the uniaxial compression strength, elastic modulus and deformation modulus of briquettes increase when particle size is decreased. The deformation characteristics of the briquettes directly prepared by raw tectonic coal were similar to those of coal specimens with a particle size of 0.18–0.25 mm. The cohesion and strength of coal specimens increased when particle size was decreased, and the plastic deformation capacity decreased when particle size was decreased, showing a strong correlation. The f briquette directly prepared by the raw tectonic coal had a strength between that of coal specimens with a particle size of 2–6 mm and those with a particle size of 0.18–0.25 mm. The mechanical properties of briquettes mainly depend on the meshing force between the coal particles. The smaller the particles, the greater the mechanical meshing force. The “floating particles”, generated in the voids between coal particles during the preparation process, are a significant factor affecting the plasticity characteristics. The research results may be used as a basic reference in the study of the mechanical properties of tectonic coal, gas migration and coal and gas outburst mechanisms.

A series of coal and gas outburst tests were conducted on coal seams in north China to determine the important order of gas pressure, in situ stress, and coal strength during coal and gas outbursts. And the typical phenomena of coal and gas outbursts were investigated. In addition, improved outburst energy equations were built to study the coal energy evolution process during coal and gas outbursts. The results show that the coal strength has the strongest influence on coal and gas outbursts, followed by the gas pressure and the in situ stress. The weights of pulverized coal with a particle size of less than 0.28 mm are consistent with the changing trend of the total weights of the pulverized coal particles in the corresponding outburst interval. Furthermore, the results suggest that the gas pressure monitored by the sensors close to the outburst hole begins to drop first and lasts for the longest time. The outburst coal presents obvious fracture and pulverization damage characteristics, and the pulverization damage features of the coal near the outburst hole are more obvious. In addition, the improved outburst energy equation was established, and the rationality of the improved outburst energy equation was verified by using the outburst orthogonal simulation experimental data and the on-site outburst accident cases. The results of this experiment have important guiding significance for preventing and controlling the occurrence of coal and gas outbursts and ensuring safe and efficient mining of coal mines.

Raw coal and briquette coal samples have similar deformation characteristics. Addition of binders added into briquette coal could change the coal property. To better capture the characteristics of briquette coal samples in comparison to raw coal samples, we performed triaxial compression tests on raw coal and briquette coal samples with 7% content of four different binders. The experiment results show that the MD group (7% rosin) briquette coal has strong similarities to raw coal samples in strength, deformation, and acoustic emission (AE) features. We find that although four different binders (water, cement, rosin, and coal tar) are added into the briquette coal samples, the failure characteristic has high consistence. Briquette coal samples always show plastic failure, but raw coal always shows brittle failure. The increase in raw and briquette coal samples’ peak strength is associated with an increase in the confining pressure constant. However, as the confining pressure constant increases, the raw and briquette coal samples’ residual strength gradually reaches close to the peak strength. After the peak strength is reached, briquette coal samples always show a stronger strain and the raw coal samples have a weaker strain characteristic. AE events have a peak value on compression and elastic stage of briquette coal samples. AE events do not show a positive correlation relationship with the material strength of the briquette coal samples. Our study highlights that briquette coal samples with 7% rosin have more similarity in strength, deformation, and AE characteristic with raw coal samples.

With the increasing frequency and intensity of coal and gas outburst disasters under deep mining conditions, studying the outburst mechanism of occurrence has great significance for outbursts prevention and control. The evolution law of coal and gas outbursts under different gas pressure is proposed. The outbursts law is analyzed utilizing the self-developed simulation experiment system of coal and gas outbursts, and the simulation experiment is carried out under the gas pressure of 0.45, 0.8, and 1.5 MPa. In the experiment, the gas pressure drops curves, the relative intensity change, the interval distribution of coal powder, and the evolution of outburst hole and the migration rate of coal powder are analyzed. The results indicate that (1) the gas pressure detected by the No. 4 gas pressure sensor starts to drop first; (2) the gas pressure is positively proportional to the relative outburst intensity. When the gas pressure increases from 0.45 to 0.80 MPa and then to 1.5 MPa, the farthest outburst distance of coal powder increases from 10 to 15 m and then to 21 m, and the corresponding relative outburst intensity increases from 22.94% to 35.74% and then to 45.73%, respectively. (3) The average proportion of coal particles size less than 0.28 mm and larger than 1 mm under each corresponding outburst interval is 40.75% and 22.53%, respectively. Experimental results show that the gas pressure plays an essential role in the secondary crushing and pulverization of coal samples during the outburst process. (4) The throwing velocity of the pulverized coal is increased with the gas pressure near the outburst hole. When the gas pressure is 0.8 MPa, the throwing velocity of pulverized coal reaches the maximum value of 32.40 m/s. (5) The dimensional characteristics and the location initiation of the outburst hole are obtained. The results showed that the outburst process of coal is mainly in two failure forms: pulverization and spallation. The research results provide a theoretical basis and test data support for the prevention and control of coal and gas outburst disasters.

In order to effectively control the large deformation of coal pillar and roadway during entry retained along gob side with small coal pillar, taking 1311 and 1312 working faces of a coal mine in Shanxi Province as engineering background, the structural characteristics of overlying rock are studied, and the stress source of entry retained along gob side with small coal pillar and the principle of roof cutting and pressure relief are analyzed, then the roof cutting and pressure relief technology of shaped charge blasting with combined and grouped of deep and shallow holes is put forward. According to the geological conditions of 1311 working face, through the calculation of caving zone and crack zone height of the working face, the analysis of roof breaking and instability conditions and the calculation of crack zone blasting radius, the key parameters of roof cutting by deep and shallow hole combined blasting are determined. Through the numerical simulation and analysis of roof cutting and pressure relief in different key strata, the variation law of vertical stress acting on small coal pillar is obtained, as follows: no roof cutting > only cut layer 2 > only cut layer 1 > cut layer 3 + partial layer 2 > cut layer 1 ~ 3; Based on these, a cooperative control scheme of “roof cutting and pressure relief by shaped charge blasting with combined and grouped of deep and shallow holes” + “anchor-cable combined support” + “coal pillar strengthened” is put forward. The field industrial test results show that the control effect of small coal pillar and roadway surrounding rock is good.