The hard roof leads to large deformation or failure of the entry and poses risks to mining safety. Shaped charge blasting is applied to directionally fracture the hard roof and release mining pressure in this paper. The stress condition around the blasting borehole required to induce fracture propagation is calculated by theoretical calculation and numerical simulation. Evolution of blasting‐induced fractures between adjacent boreholes, main fracture in the shaped charge jet direction and secondary fractures in other directions is analyzed. The reliability of Livemore software of explicit dynamics analysis code (LS‐DYNA) in simulating shaped charge blasting is validated by experimental results. A numerical model is established to reveal the formation processes of the shaped charge jet, disclose the evolution of the equivalent stress and understand the development of the main fractures and secondary fractures between adjacent boreholes. The failure type of the rock mass and the fracture development zone are identified. The results indicate that blasting‐induced fractures mainly initiate and propagate in the shaped charge jet direction between adjacent boreholes. Shaped charge blasting leads to the directional propagation of fractures. The field monitoring results show that shaped charge blasting is able to realize the directional propagation of blasting‐induced fractures and release mining pressure.

Underground coal mining activities and ground movement are directly correlated, and coal mining-induced ground movement can cause damage to property and resources, thus its monitoring is essential for the safety and economics of a city. Fangezhuang coal mine is one of the largest coalfields in operation in Tangshan, China. The enormous amount of coal extraction has resulted in significant ground movement over the years. These phenomena have produced severe damages to the local infrastructure. This paper uses the finite difference method (FDM) 3D model and the stacking interferometric synthetic aperture radar (InSAR) method to monitor the ground movement in Fangezhuang coalfield during 2016. The FDM 3D model used calibrated Fangezhuang geological parameters and the satellite InSAR analysis involved the use of ascending C-band Sentinel-1A interferometric wide (IW) data for 2016. The results show that the most prominent subsidence signal occurs in mining panel 2553N and the area between panel 2553N and fault F0 with subsidence up to 57 cm. The subsidence observed for the FDM 3D model and stacking InSAR to monitor land deformation under the influence of fault are in close agreement and were verified using a two-sample t-test. It was observed that the maximum subsidence point shifted towards the fault location from the centre of the mining panel. The tectonic fault F0 was found to be reactivated by the coal mining and controls the spatial extent of the observed ground movement. The impact of dominant geological faults on local subsidence boundaries is investigated in details. It is concluded that ground movement in the study area was mainly induced by mining activities, with its spatial pattern being controlled by geological faults. These results highlight that the two methods are capable of measuring mining induced ground movement in fault dominated areas. The study will improve the understanding of subsidence control, and aid in developing preventive measures in Fangezhuang coalfield with fault reactivation.

Blasting has been widely used in coal mining as a method of reducing the integrity of rock mass. Its low controllability often leads to the unsatisfactory fragmentation effect of rock mass. The empty borehole effect has great significance in avoiding the excessive breakage of rock mass and guiding the directional propagation of blasting-induced fractures. Nevertheless, the rock mass damage type evolution induced by the empty borehole has been rarely studied. A tension–compression constitutive model of rock mass damage is established in this paper. The model is incorporated into the numerical modeling code LS-DYNA as a user-defined material model. Then, LS-DYNA is used to investigate the evolution mechanism of rock mass damage under the empty borehole effect. The damage types of rock mass caused by the empty borehole effect are also studied. The Fortran language is utilized to monitor the number variation of the tensile damaged elements and the compressive damaged elements in the rock mass around the empty borehole. The results indicate that existence of the empty borehole significantly enhances the tensile stress and the stress concentration factor in the rock mass nearby the empty borehole. Meanwhile, the rock mass nearby the empty borehole mainly damages in tension. Both the number of the tensile damaged elements and the tensile stresses in the elements increases as the empty borehole diameter increases. The number of the compressive damaged elements decreases with increasing empty borehole diameter.

The hanging of hard roof strata in the gob-side retaining entry in deep mines could lead to dynamic disasters. The pressure relief mechanism for the hard roof strata using shallow-hole blasting was investigated by a combined method of theoretical analysis, numerical simulation, and field test in the Sanhejian coal mine. The shallow-hole blasting model was established using LS-DYNA3D (Livemore software of explicit dynamics analysis code). The evolution and propagation law of effective stress induced by shallow-hole blasting was analyzed. The parameters related to shallow-hole blasting were determined via a combined method of numerical analysis and theoretical calculations. These were applied to the field tests. The effect of shallow-hole blasting on the strata control of deep gob-side retaining entry was studied by comparing two scenarios in the field: a deep gob-side retaining entry with and without shallow-hole blasting. It was found that the convergences of the entry decreased due to the shallow-hole blasting. This indicates that shallow-hole blasting is a good technique to maintain the stability of the gob-side retaining entry with a hard roof in deep mines.

Mechanical parameters of the rock are important for the design of geotechnical, mining engineering, and petroleum reservoir projects. Many researches have suggested that the mechanical variables of rock specimens, such as compressive strength and elastic modulus, do not have a single fixed value. Uncertainty in the basic mechanical variables of the rock material can significantly affect the structural performance and safety. In this study, a series of compression experiments with acoustic emission have been performed on rock specimens. The damage evolution characteristics of the rock in the process of loading were studied, and the macromechanical behaviors were obtained at the same time. Distribution characteristics of the strength and elastic modulus as random variables are illustrated, and the statistical damage model is presented by the authors to formulate analytical constitutive relations for deformation behavior. The comparisons between predicted results and experimental data show that the statistical damage constitutive model could well reproduce the deformation process of rock materials.

The existence of fracture flow has an undesirable effect on the creation of the frozen wall. Brine and liquid nitrogen combined freezing technology can ensure the safety of freezing engineering, reduce the construction period and save cost. Considering the permeability of the rock matrix, fluid exchange and Darcy–Stokes coupling effect between the rock matrix and fracture, a thermo-hydraulic model of the fractured porous rock mass under water seepage is herein established. The interfacial seepage field characteristics of fractured rock mass under different fluid flow models and interface conditions are compared. The numerical simulations of the initial brine freezing and liquid nitrogen reinforcement freezing are carried out. The results show that the overall permeability of fractured rock mass computed by free flow considering the Darcy–Stokes effect is greater than that computed by the Cubic law. The limit seepage velocity of the intact rock mass in brine freezing is 2.5 m/d, and that of fractured rock mass decreases to 1 m/d. The fracture aperture and groundwater seepage velocity are directly proportional to the closure time of the frozen wall. Liquid nitrogen freezing can seal water quickly and shorten the closure time of the frozen wall when the seepage velocity of the fractured rock mass is greater than the limit seepage velocity, and the rapid cooling of the upstream region plays an important role in the formation of the frozen wall in fractured rock mass.