89 / 2023-06-14 17:14:45

Prediction method to calculate stress relief effects for upper protective layers mining of half coal-rock based on stratigraphic key strata structure

half coal-rock protective layer mining,key strata,coal seam floor stress distribution,stress relief effect,gas ex-traction

It is of great significance to study stress relief effects by upper protective layers mining of half coal-rock for determining the reasonable mining height and guiding the efficient extraction of gas. However, the existing researches have not clarified the influence laws of mining height and underlying key strata on the unloading effects of the coal rock mass for upper protective layers mining of half coal-rock, and there is a lack of prediction method to calculate the unloading effects for upper protective layer mining considering the decisive role of underlying key strata structures. Based on the key strata theory of strata control and Winkler elastic foundation theory, the key strata of different layers under the semi-coal-rock protective layer are simplified as multi-layer laminated plates, and the influence of the key strata structure under the semi-coal-rock protective layer on the pressure relief effect of coal and rock mass in the floor is considered. The prediction model of pressure relief effect of coal and rock mass in the floor of semi-coal-rock upper protective layer mining is established, and the prediction method of stress and deformation of coal and rock mass in the floor is constructed. The stress relief effects for upper protective layers mining of half coal-rock in Mengjin coal mine are predicted by using the new prediction method, and the predicted results are consistent with the field measured results. Using the new prediction method, it is found that the structure of the key stratum under the semi-coal rock protective layer has a significant effect on the pressure relief effect of coal and rock mass. The existence of the hard Dazhan sandstone key stratum between the semi-coal rock protective layer and the protected layer in Mengjin Coal Mine will hinder the pressure relief degree of the floor coal and rock mass. The greater the thickness of the Dazhan sandstone key stratum, the smaller the pressure relief degree of the protected layer of the II₁ coal seam, and the more obvious the effect of hindering the pressure relief of the floor coal and rock mass. When sandstone key stratum thickness reaches the maximum of 21.68m and mining height is II₂ coal seam average thickness 0.88m, the maximum expansion deformation of protected II₁ coal seam is only 1.8‰, which is lower than 3‰ required by the Rule for Prevention and Control of Coal and Gas Outburst. The maximum expansion deformation can meet the requirement by increasing mining height to more than 1.2m, and the maximum expansion deformation can be increased to 12.4‰ when the mining height reaches 2.0m. In order to achieve better stress relief protection effects and mining efficiency, the working face mining height is 2.0m for upper protective layers of half coal-rock in Mengjin Coal Mine. When gas extraction boreholes of the protected coal seam enters the stress relief range, the gas extraction concentration increases from 15% to about 93%. Through gas extraction, the maximum gas content of the protected coal seam is reduced from 13.04m³/t to 3.99m³/t, and the maximum gas pressure is reduced from 1.25MPa to 0.02MPa, which successfully guide the efficient gas extraction of the protected coal seam and the prevention and control of coal and gas outburst disasters in Mengjin coal mine.