Research On Strata Caving And Support Resistance Of Lower Coal Face In Ultra-Close Shallow Coal Seam Mining

The Jurassic coal field in northern Shaanxi is abundant,and part of the mine in the area has started the exploitation of the lower coal.The upper coal mining roof has cracked,the roof structure and stress environment has changed,during the mining of the ultra-close coal seam.The law and mechanism of roof pressure and control theory is to be studied.It is necessary to reveal the law of the overburden strata caving and determine the reasonable support resistance in the mining under the goaf,and it becomes a problem to be solved urgently in the safe and efficient mining of the coal seams in northern Shaanxi.This paper put the mining of 22104 working face in Huoluowan Coal Mine as the background,which is 5?7m to the upper coal seam.Based on the methods of physical similar simulation experiment,numerical calculation and analysis of roof structure theory,the collapse and pressure law of roof are studied of in the mining of lower coal seam.The model of the roof structure in the lower seam is established,The calculating formula of support resistance is put forward.By using physical similar simulation experiment,it shows that the first weighting step of the lower coal seam is 20m,the periodic weighting step is 5?7.5m.By using UDEC numerical simulation,it shows that the first weighting step is about 18m,the average periodic weighting step is about 7.5m;the maximal front abutment pressure is 10m in front of the coal wall,and the range is less than 40m,and the maximal stress is less than the average strength of coal which is 15.0MPa,and it has little effect to the stability of the coal wall.In the compaction stage of goaf,the caved roof of upper seam is activated and has downward movement with "step voussoir beam" structure and strong dynamic load,and roof pressure appears periodic "high pressure zone" and "low pressure zone",the average length of high pressure zone is 16m,and periodic step of high pressure zone is 25m.The main reason of the high pressure zone is the activation of the roof structure of the upper coal seam.The average load of the high pressure zone is 7676kN per frame,the average load of the low pressure zone is 5580kN per frame,and the high pressure zone is 1.37 times of the low pressure zone.The support load under section pillar is 1.07 times of high pressure zone.Based on the theory of " step voussoir beam",the structural model of "actived slanting voussoir beam" in the maining of lower coal seam is established,the support working resistance of the 22104 working face should be greater than 8634kN per frame in calculation.Through physical similar simulation experiment and selection of support and pressure measurement analysis of 12102 working face in Shigetai Coal Mine,it is determined that the hydraulic support working resistance of 22104 working face should be greater than 8800kN per frame,and it is verified through mining application in Huoluowan Coal Mine.