Research On Occurrence Conditions And Mechanism Of Rock And Gas Outburst

Recently, rock and gas outburst always happened in coal mines at home and abroad when driving into the deep roadway. The rock and gas outburst includes rock outburst and rock outburst accompanied with gas outburst, which does not occur only in rock excavation, but also in tunnels and other underground engineering construction environment. However, researches about the occurrence conditions and mechanism of rock and gas outburst were not enough to exactly understand the sources and transformation processes of energy, the relationship and interaction mechanism between the blast and the outburst, the start and stop of the outburst.In this paper, geological conditions of Yongchuan coal mine, physical properties of the outburst sandstones, injury, strength and damage of the outburst sandstone deformed under load, gas expansion energy in the process of rock and gas outburst, general principles and causes of the rock and gas outburst and rock burst, energy, critical conditions and mechanism of the rock and gas outburst were studied. The main results are as follows:Rock and gas outburst of Yongchuan coal mine happened in the T3xj4 sandstone stratum, which is 43 m below the main coal seam-Dalong coal seam, and is the sedimentary facies of the delta plain. The T3xj4 sandstone contains more Si and Al elements. Quartz, mica and feldspar are the main mineral composition of T3xj4 sandstone. The T3xj4 sandstone has low permeability and high specific surface area, and the diameter of the inside pore is bigger than that of the methane molecule. In the microstructure, not developed cracks and joints, poor connectivity, and lump and dense structure appear, and the black inclusion within the sandstone stratum is kind of complex mineral. Methane is the main composition of the gas absorbed in the T3xj4 sandstone stratum. There are also few ethane, propane and isobutene. The T3xj4 sandstone has adsorptive capacity for gas. One-third of coal in Dalong coal seam is coke, which has a high specific surface area of 280 m2/g. From SEM images, cracks has been developed and crosslinked.The T3xj4 sandstone had low intensity and obvious yielding, and the process of fatigue failure under cyclic loading and unloading fitted static complete stress-strain curve. The type of acoustic emission of T3xj4 sandstone was similar to that of MOGI-I. All the signals in the Kaiser point, yield point and peak point of the stress-strain curve were significant, and there were Kaiser and Felicity effects. Under the confining pressure, the fracture ratio showed a good linear relationship with the secant modulus, axial maximum strain and strain energy of the fractured rock sample, and the full sandstone sample was further destructed to fractured sandstone sample. Both fractured T₃xj⁴ sandstone sample and full sandstone sample followed the same creep law. All the stages in the creeping process could be described with the modified Nishihara model. Sandy shale in the apical plate had high brittleness, loud crack, an acoustic emission similar to dense unstable-IV, and significant signals in Kaiser point and peak point.Gas expands doing work law in coal and T₃xj⁴ sandstone under different conditions were systematically studied with the independently made device. The results showed that under low gas pressure, the protrusive length of piston, which reflected the ability of doing work, had an exponent relationship with the pressure of gas in the coal, while showed a linear relationship with the pressure of gas in the sandstone. After computing the polytropic exponent of gas expanding, the gas expansion process is considered to be an isothermal process.Both rock and gas outburst and rock burst belong to the rock dynamic phenomena. And both of them happen in high stress zone, and have an relationship with construction disturbance. Generally, rock and gas outburst was caused by blasting operation, accompanied with rock fragments and gas emission. The energy source of the outburst was the elastic strain energy and ravity potential energy of rock mass, and gas expansion energy. Outburst cavern was irregular, which means the pyriform and bag, and the cavern extend to interior of rock mass. Rock burst occurred spontaneously, and mostly in the rocks with high strength and hard brittle. Shallow blasting pits formed after rock burst. After computing the fracture zone in the rock lane of Yongchuan coal mine, and comparing with that under ground stress field which is mainly composed of horizontal structure stress, it was indicated that the plastic deformation zone, caused by the vertical ground stress, distributed symmetrically on the double shoulders of the roadway, while the distribution of the plastic deformation zone caused by the horizontal ground stress was different. Comparing the rock fracture zone with the actual outburst cavern, it was shown that the outburst cavern was evoluted and developed from the partial damage zone caused by high ground stress, the external loading stress could not be changed by the explosive stress, and rock and gas outburst could be induced by blasting.Elastic deformation energy, gas expansion energy, gravitational potential energy, and energy conversion of the three energies of fracture zone in rock and gas outburst were discussed. According to energy balance principle, critical condition of elastic modulus and initial velocity of thrown out of roc were obtained. The maximum Poisson's ratio of rock should be obtained with cyclic loading test after post peak. Theoretically, the start time for rock and gas outburst can be estimated with the strain-time curve of rock creep failure under plane stress condition. The mechanism of rock and gas outburst was also proposed. The outburst is a kind of mineral dynamic phenomenon, which is caused by the deformation zone under the combined action of gas and blasting and other factors. The deformation zone is formed in the rock with physical-mechanical properties under high ground stress. The energy for rock and gas outburst results from elastic deformation energy , gas expansion energy and gravitational potential energy. When the potential of elastic deformation energy of rock mass is sufficiently large, rock mass will be destroyed, and outburst begins. If gradient of in-situ stress and gas pressure is sufficiently large, outburst will be developed and expand. Rock and gas outburst will stop once the depletion of energy and hinder channel of outburst, and gradient of in-situ stress and gas pressure is low.