| Years of scientific research and engineering practice has proved that the protective seam and pressure releasing gas drainage technology, which can prevent coal and gas outburst and reduce coal seam gas content, is the most effective and most economical regional control measures. Although there has been considerable mining pressure releasing gas drainage field test in recent decades, coal and rock mass stress distribution, crack evolution and interaction of pressure relief and gas flow is still not well known. It cannot provide sufficient theoretical basis and technical support for the efficient pressure relief gas drainage. This thesis, aiming at high gas and low permeability coal seam group conditions, by utilizing theory analysis, laboratory physical simulation, numerical analysis and site measurements, focused on protective coal mining in close distance seam group, and studied the mining fractured evolution, seepage features, coal rock body stress field, fractured field and gas seepage field during mining. Preliminary results of spatial-temporal changes law were obtained. Based on the research conclusion of the "triple fields", the optimized drilling and pumping parameters were calculated. Practices were carried out in Shaqu coal mine of Huajin coking coal Co. Ltd., and the drainage effect is proven to be significant and successful. The research of upper protective seam mining theory provides theoretical basis for determining the effective pressure relief regions and efficient gas drainage practice, which can be used in Liliu and other coal mine with similar geological conditions nearby. Main conclusions of this research are as follows:(1) By assuming the floor coal-rock mass as continuous medium, a mechanical model of floor coal-rock mass with non-uniform load distribution was built in upper protection seam mining. The Matlab software was employed for numerical calculation, drawing vertical stress distribution curve of different position, horizontal and shear stress distribution curve under different depth along the floor. By analyzing these curves, it can be concluded that, in the floor coal and rock mass, the vertical stress decreased with the increment of horizontal distance, so did the horizontal stress. The shear stress in the rock with 10 m beside the protective coal seam is the lowest, while with 20 m, the shear stress is the highest. With the distance increasing to 30 m, the shear stress reduced again.(2) In group coal seams, the distance between the protecting and protected coal seams is one of the key parameters that effect the mining stress field. By comparing the mining stress field of different distances, it can be concluded that with the increment of distance, the stress relief degree right above the working face will decrease gradually, so does the roof and floor displacement of the lower coal seam, while it has little effect on the roof movement.(3) Based on the theory of similarity and the geological conditions of Shaqu mine, the 2 dimensional strain simulation experiment platform was used to study the relationship between the mining stress distribution and the revolution of fracture in upper protective coal seam mining. The experiment can reveal the stress field distribution and fracture field revolution of the coal and rock mass in the floor and lower protected coal seam after mining: During the production of protective coal seam(the first mining influence), the weighting distance was big, the revolution of the fracture in the roof and the stress relief in the floor were not well developed. During the production of the protected coal seam(the second mining influence), the weighting distance decreased, and the fracture and stress relief were well developed, the degree of the roof fracture was high. The frequency of the fracture rose with the production process. With the effect of overlying mining influences, coal seam 3# and 4# has a shorter area of high stress relief, which is 105 m, while has a higher relief number, the biggest reduction could be 12 MPa, which is 1.5 times than the stress relief number of coal seam 2#. During the production of coal seam 3# and 4#, the stress relief degree in roof and floor is higher than the degree of coal seam 2#. All three coal seams has a quite high stress relief within a distance of L in the sidewall of the gob. For coal seam 2, the range of L is 30~40m, while it's 20~30m for coal seam 3# and 4#. It indicated that the production of the protective coal seam has a significant effect of stress relief for the protected coal seam.(4) By utilizing numerical modelling software FLAC3 D and UDEC, the stress distribution in floor rock mass and the law of fracture revolution was studied. The conclusion shows that: ? The stress relief depth and range in the floor of working face increased with the increment of the extraction area at first, then it will stay at a certain point. The production of coal seam 2 had a significant and huge stress relief influence on coal seam 3# and 4#. ? By analyzing the angles of the fractures in each zones, the fractures with big angles in the floor ahead of the working face are mainly vertical. When pumping in these fractures, the arrangement of the drainage holes should be horizontal or nearly horizontal with a little angle, in order to have a better drainage results. Whilst extracting in the fractures of relieved zones, most of the drainage holes should be vertical or with a big angle for a better drainage effect. When pumping from the tunnels in the floor strata, the best drainage holes are the ones in the relieved zones.(5) In the Matlab software, we analyzed the images from different boreholes, in different depths and in different times, in order to investigate the fractures revolution during the extraction of the working face. It is concluded that, with the extraction of the protective coal seam, the fractures were different in different zones. The level of fracture in the roof of the protected coal seam decreased with increment of the distance from the working face. With the same distance of 10 m from the working face, the drilling hole is whole without fracture, while with 15 m, the fractures developed a little, accompanied with some drop of coal particles. In the process of extraction in the protective coal seam working face 22201, the biggest observed depth of roof broken in working face 24208 ranged between 12 m and 14.8m. During the process of stress concentration and stress relief caused by the extraction of the protective coal seam, the fractures in the floor of the working face developed downward and ran through the whole roof of the protected coal seam.(6) Based on a limited gas concentration data, with bicubic interpolation method, the 3 dimensional concentration distribution of the trace gas was reconstructed. It is concluded that, the range of ran through fractures between coal seam 2 and 3 + 4 is 30m~40m behind the protective working face, while between 3+ 4 and 5 is around 50 m. The range of fractured zone is 20m~60m behind the protective working face.(7) According to the numerical simulation in chapter 4 of protective pressure relief depth as well as the development of plastic zone, combined with the coal seam and rock mass permeability coefficient formula under interior different stress environment and destruction state, the change of protected seam permeability values of the rock in different regions and the distribution characteristics of permeability in protected seam were obtained by using Fish languages. Based on the permeability distribution in the protected coal seam, it can be divided into 4 regions, which are, original permeability zone, reduced permeability zone, enhanced permeability zone 1 and enhanced permeability zone 2. The zone 2 has the biggest permeability among them. According to the permeability distribution features and years of coal mine gas treatment experience, the drilling holes were optimized: from the opening cut area to the internal of the working face, the spacing of the drilling holes are 6m×6m, 10m×10m, and 30m×30m, separately. During the extraction of the protected coal seam, the average concentration in the drainage hole was 38.2% with 16% enhanced, and the flow rate was 7.12m3/min with 4m3/min enhanced.(8) By investigating the effect of the protective coal seam mining in situ in Shaqu mine, gas pressure, gas content, borehole flow rate and effect of gas drainage pattern of lower protected coal seam were obtained. The gas pressure of the protected coal seam 3 + 4 decreased from 1.48 MPa to 0.4 MPa, with a 73% reduction. The flow rate in all drainage drilling holes has more than 10% enhancement after the stress relief, 10.75% in drilling hole 1, 11.21% in 2, 11.06% in 3, and a small decrease in drilling hole 5. The gas content reduced from 10.89m3/t to 8.55m3/t after the stress relief, and the gas pressure relief effect of the protected coal seam was examined as well. The net drainage volume in the conveyance roadway in as high as 70m3/min, with a concentration of more than 50%, which means a significant stress relief effect for the lower protected coal seam caused by the extraction of the upper protective coal seam.(9) Through mine pressure monitoring for the protected coal seam working face, the ground pressure behaviors regularity of the protected coal seam was obtained. The range of abutment pressure is approximately 30 m ahead of the working face, with a stress concentration factor of 1.81. The periodic weighting distance was 38~45m, the peak pressure of hydraulic support gauge during the periodic weighting is 43 MPa. Based on the mine pressure observation and gas drainage data analysis, it was verified that there are correlations between the abutment pressure and gas extraction concentration, and between periodic weighting and gas emission amount. Based on the conclusions of the fracture spatial-temporal evolution in the previous chapters, the reasonableness of fracture site placement is verified by analyzing the gas drainage data from the drilling field in the fractured zones. |
PDF Full Text Request