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During the multiseam mining process, the overlying strata were damaged due to different mining disturbances, where a large number of fractures were produced which were the main seepage paths of gas and water, easy to cause gas or water inrush accidents. Thus, it is necessary to explore the development characteristics of the fractured zone in the overlying strata from multiseam mining. In this paper, laboratory triaxial compression tests were firstly conducted on the rock specimens subjected to different mining disturbances. In these tests, the specimen was loaded to different stress before a regular test to simulate the different damage effects on the rock strata caused by different mining disturbances. And the corresponding values of m and s could be obtained through a calculation based on the test results. Subsequently, the numerical simulation was conducted to simulate the field situation, in which the m and s values were used in the numerical model combined with the Hoek‐Brown–modified criterion, and the results show that the overburden failure height was increased with the mining disturbances from 48 to 120 m, and then, the effects of mining height and interval thickness on the development height of fracture zone were studied by numerical simulation. Finally, the leakage of roof drilling at different stages was tested and calculated in the field. The results showed that the loss of leakage drilling can directly reflect the development degree of roof cracks, and the detection results were close to the development height of the numerical simulation fracture zone. It is verified that the simulation method used in this paper is feasible and can be used as a reference for similar research.

When faced with multi‐seam mining, some coal pillars are left to support the overlying strata when mining the lower seam, where stress concentration usually occurs. When the working face in the lower seam advances to the area influenced by these coal pillars, high ground pressure behavior occurs. Furthermore, the instability of coal pillars under the participation of the roof may induce rock burst, which threaten the safe production of coal mine. To study the roof strata movement and coal pillar instability in multi‐seam mining, a coal mine in Datong was taken as an example. Firstly, the bursting liability of coal and rock stratum was tested. The test results show that the 7#, 8#, and 11# coal seams in the mining area all have strong bursting liability. And the roof was dominated by siltstone and firestone, which also have strong bursting liability. Then, based on the geological conditions of the coal mine, we investigated roof movement and instability of the coal pillar during multi‐seam mining through physical simulation. The simulation results indicate that multi‐seam mining aggravates the roof damage, which leads to the increase in the fracture range of overlying strata, and that the failure height increases from 46 to 121 m. In addition, the detailed distribution of stress on residual coal pillars and its influence on the lower working face were studied through numerical simulation. The results show that, when the working face 8707 of the 8# coal seam advances to a horizontal distance of 20 m from the coal pillar, it enters the influence zone. As the working face advances beneath the coal pillar, the stress reached to 24 MPa. Moreover, when the working face passes by the coal pillar, the coal pillar is cutoff, thus affecting the working face.

Influenced by various mining activities, fractures in rock masses have different densities, set numbers and lengths, which induce different mechanical properties and failure modes of rock masses. Therefore, precisely expressing the failure criterion of the fractured rock influenced by coal mining is significant for the support design, safety assessment and disaster prevention of underground mining engineering subjected to multiple mining activities. By adopting PFC2D particle flow simulation software, this study investigated the propagation and fractal evolution laws of the micro cracks occurring in two typical kinds of rocks under uniaxial compressive condition. Furthermore, it calculated compressive strengths of the rocks with different confining pressures and box-counting dimensions. Moreover, the quantitative relation between the box-counting dimension of the rocks and the empirical parameters m and s in Hoek-Brown strength criterion was established. Results showed that with the increase of the strain, the box-counting dimension of the rocks first increased slowly at the beginning and then exhibited an exponential increase approximately. In the case of small strains of same value, the box-counting dimensions of hard rocks were smaller than those of weak rocks, while the former increased rapidly and were larger than the latter under large strain. The results also presented that there was a negative correlation between the parameters m and s in Hoek-Brown strength criterion and the box-counting dimension of the rocks suffering from variable mining activities. In other words, as the box-counting dimensions increased, the parameters m and s decreased linearly, and their relationship could be described using first order polynomial function.

Numerical simulations are conducted on the scheme of energetic proton-bunch driven plasma wakefield acceleration and the possibility of the selfinjection scenario in proton-bunch driven plasma wakefield acceleration by using a density transition of plasma with the help of 2D3V PIC code. The simulations show that enhanced selfinjection of electrons in the scenario is realized successfully. It is found that the number of trapped particles increases as the amplitude of density transition increases．