| Fault zone of rock and gas outburst is a complex and destructively dynamical phenomenon, which would produce shock waves in the roadway and lead to injuries or death and damage of mine mechanical and electrical equipment. The influence of shock wave produced in gas outburst on the surrounding rock can cause rock mass particle swing and vibrating and local damage in the underground tunnel, and even can cause the overall collapse and geological hazards such as ground and mine earthquake, which threatens the safety production of mines seriously. In this paper, in order to further study the mechanism of fault zone of rock and gas and the dynamic effect of outburst shock wave, the stress state on the surface of cracks in rock pillars was analyzed from the perspective of elastic mechanics. Then, through the analysis of near-tip stress field of crack, coal and rock pore air pressure for crack initiation depending on fault zone of rock(coal) and gas outburst and crack with unstable propagation causing destructively dynamical phenomenon were found. The second, linear blast wave peak overpressure and speed equation were established according to the basic theory of fluctuations. Combined with 10.20 outburst accident case of the Daping coal mine, finite element program of LS-DYNA and LS-Pre Post were used to simulate the dynamic effect of linear shock wave on surrounding rock. The expressions of effective stress in the infinitesimal body was modified finally. The stress state for cracking tip was analyzed by fracture mechanics as well as the calculating model of safe thickness of rock pillars. According to coal and rock creep strain curve by using ABAQUS, the relation between the safety rock pillars and creep strain vs time was derived, which can provide certain theoretical support for tunnel safety construction. The research results are obtained as following.First, in the environment created by ground stress and natural force such as high pressure gas, the excavation can lead to effective stress of rock mass near tunnel face decreasing and permeability increasing. When the stress intensity factor at fault zone of coal and rock crack tip reaches fracture toughness of rock mass condition, and the pore air pressure reaches crack initiation condition, micro cracks will gradually expand and penetrate, instability and failure of rock mass occurs. Then high pressure gas moves in high-speed along the penetrating crack, which results in rock and gas outburst.Then, when the orifice gas flow rate and pressure exceeds critical state after gas outburst, shock wave will be formed in the goaf roadway. The angle between shock wave boundary and tunnel roof-floor is outburst spread a Angle. And the two triangle areas without shock wave direct action are called mine production blind spots. Shock wave is a kind of strong compression wave and the parameter equation of wave surface was deduced according to the theory of air dynamic. Overpressure is proportional to total energy and inversely proportional to propagation distance.In addition, in parts between the funnel shock wave transmitting boundary and tunnel roof-floor, the particle dynamic failure of rock mass is the most serious. The peak speed can be 78.015cm/s. Along the tunnel axis, when the shock wave overpressure peak of a certain point of surrounding rock decays to 0.02 MPa, or vibration velocity is less than 5 cm/s, the point is in safety ultimate limit state. The area from the limit point to the biggest failure zone is attenuation zone, where rock mass is still damaged. And the distance of attenuation zone is 280 m in this case.Besides, on the transverse section of tunnel, the erosive effect of shock waves on the sidewall is relatively light. Within 120 m of tunnel face, top speed and displacement of the roof node are two- four times higher than the corresponding values of side wall. And from 120 m away, top speed and displacement of the roof and side wall node gradually turned to be the same. When rock mass is relatively soft, its impact on the amplitude of particle is larger. When the elastic modulus of rock mass decreases to 0.05 GPa, the amplitude increases to 64.776 mm, roadway breakage effect also increases accordingly.At last, the stress state of crack tip under the combined effect of in-situ stress and gas pressure on rocks was analyzed by applying fracture mechanics theory and critical excavation thickness model is established by the stress intensity factor. The curve of coal and rock creep strain was simulated and relationship between the safety thickness of excavation, creep strain and time was derived by ABAQUS, which filled the blank in this field and be compatible with the actual project. |
PDF Full Text Request