Site Measurement and Analysis on Failure Height of Overburden Strata Under Thick Loose Alluvium and Thin Bedrock

The thickness of the overburden strata above the first coal mining face in Zhaogu No.1 Mine was over 518 m and the thickness of the bedrock was less than 80 m.In order to have the height of the overburden strata falling zone and water conducted crack zone after coal mining operation completed and to rationally set up the top limit for the mining operation of the coal mining face,the surface borehole observation method was applied to observe the failure height of the overburden strata before and after the coal mining operation of the coal mining face.A study and analysis was conducted on the borehole design,borehole drilling operations and height distinguishing method of the two zones.The observation results showed that the development height of the falling zone in the first coal mining face was 13.1 m,the falling and mining ratio was 3.85,the development height of the water conductive crack zone was higher than 29.2 m and the crack and mining ratio was larger than 8.59.The results could be basically meet the similar simulation test value and the numerical simulation value and could be mutually confirmed.