Structure form and evolution characteristics of collapsed roof in goaf

It is of great significance to reveal the structure form of collapsed roof in gob for the upward mining of multiple coal seams with close distance, mining under aquifer and gas distribution in abandoned gob. Based on the detection results of the collapsed roof structure form in goaf after No.12-1 coal seam mining in Kailuan coal group, the characteristics and the formation mechanism of collapsed roof structure form in goaf were studied with theoretical analysis of material mechanics ground pressure and UDEC numerical simulation. A concept of "upright and inverted triangle" collapsed roof structure form was proposed and the dynamic evolution characteristics of collapsed roof structure form was described. The results were as follows:① the detection results of ground penetrating radar showed that the roof stratification was obvious. The upright triangular zone and inverted triangular zone of the collapsed roof appeared alternately in the goaf with the characteristics of orderly stacking of lower collapsing rock blocks and disorderly stacking of upper collapsing rock blocks in the upright triangular area, and disorderly stacking of lower collapsing blocks and orderly stacking of upper collapsing blocks in inverted triangular area. ② The reason for the collapsed roof structure form in goaf was the periodic weighting of roof strata and secondary breaking of key block of roof strata in goaf. When the ultimate bending strength of the key block of roof strata in goaf was less than the maximum bending moment, the secondary breaking of the key block of roof strata occurred at different positions. The re-breaking position of key rock blocks of main roof and overlying strata was different. Because of the difference of the re-breaking position of different key rock blocks of roof strata, the collapsed roof was divided into two parts, which was the upright triangular area and the inverted triangular area. ③ The concept of "upright and inverted triangle" collapsed roof structure form was proposed. When the stratification of roof strata is obvious, the key rock blocks of main roof and overlying strata re-break under the action of maximum bending moment after the periodic weighting of panel face. The periodic weighting and re-breaking positions present the collapsed roof in the gob as an upright triangular zone and an inverted triangular zone, forming the "upright and inverted triangle" collapsed roof structure form. ④ The "upright and inverted triangle" collapsed roof structure form was also obviously founded by the UDEC software, which is approximately consistent with field detection and theoretical analysis.