Stress Relief and Permeability Enhancement with Hydraulic Fracturing in Overlying Key Strata of Deep and Soft Coal Seams.

The technical problems of high in situ stress, high gas pressure, high gas content, and low coal seam permeability are widespread in deep soft coal seam excavation, which leads to frequent occurrences of dynamic phenomena, such as coal cannons and blowout holes. Based on the high-pressure hydraulic fracturing technology and process, this study puts forward a new technology of gas drainage in deep and soft coal seams by fracturing the overlying key strata to cut off the stress transmission path among coal and rock strata. According to the theories of key layer and masonry beams, the distribution locations of the main and subkey strata are determined, and based on uniaxial compression and Brazilian splitting experiments, the mechanical parameters of key stratum were tested. Combined with the results of numerical simulation and field test, initial pressure and fracturing radius of hydraulic fracturing technology for overlying key strata were determined, the stress relief effect and permeability variation law of coal seam after hydraulic fracturing in the main and subkey strata were analyzed, and then technical schemes for simultaneous fracturing of the key layer were designed. Field application results showed that the stress concentration phenomenon in soft coal seam excavation had been alleviated, and the stress relief effect of coal seam and the permeability were increased obviously. The volume and concentration of gas drainage were increased by 10 and 11%, respectively, the gas amount by 1.22 times; the frequency of dynamic phenomena such as coal cannons decreased by 95%, the gas concentration in return air flow by 20% during mining processes. This paper provided an innovative technical idea and process for gas control in deep and soft coal seam excavation, which could effectively solve the common and difficult problems about frequent occurrences of excessive gas concentration and dynamic phenomena.

Frequency response characteristics and failure model of single-layered thin plate rock mass under dynamic loading.

In underground engineering, disturbance of dynamic load can change layered rock mass stress state and induce accidents. Traditional elastic mechanics can't effectively solve the complex deformation problem. However, Hamiltonian mechanics system can overcome this problem. Dual variables are introduced in symplectic space to solve the deflection equations of single-layered thin plate rock mass. Comparing vibration parameters, it's found the 1st, 5th and 6th order are effective vibration modes. The resonance characteristics of thin plate are obtained with three dynamic loads. It's found the thin plate is most likely to resonate and damage due to the smallest resonance frequency interval and the largest vibration amplitude by impact wave and rectangular wave respectively. Then, the vibration mode of multi-layered rock mass is analyzed through Multiple Reference Impact Testing. The failure of fine sandstone is caused by the resonance of effective vibration modes by hammer excitation. Finally, the failure mechanism of thin plate is obtained by the failure theory and LS-DYNA. It's found the four sides and corners suffer tensile shear failure and shear failure respectively. When tensile failure occurs in central, the main crack and secondary crack propagate along long axis and short axis to form "O-" failure mode.