Application And Research On "Cutting Block" Structure Of Overburden Strata In Shallow Buried Workface With Ultra-large Mining Height

The mining practice of shallow coal seam in Shendong mining area in recent 30 years shows that the rock pressure is more intense.Especially when the workface is under special mining conditions,such as passing through overburden concentrated coal pillar,surface valley and abandoned roadway,the roof accident is easy to occur.Domestic and foreign scholars have carried out a lot of research work on the overburden structure and movement characteristics of shallow buried workfaces,and have obtained many useful conclusions,which have guided production practices.Mining height has been increasing in recent years,the first shallow buried workface with mining height of 6.3m in China achieved success in 2007.Since then,fully mechanized mining technology with mining height above 6m is gradually popularized and applied.In 2018,maximum mining height working face in China was put into trial operation in Shangwan Coal Mine.The working resistance of hydraulic support increased from 10800 kN when mining height was 6.3m to 26000 kN,and the support strength reached 1.83 MPa.The rock pressure increases greatly with the growing of mining height.Whether simply increasing the working resistance of support can fundamentally solve the problem of roof control in shallow seam remains to be further studied.Therefore,taking the maximum mining height workface(Shangwan 12401workface)as the background,the characteristics of rock pressure and overburden structure are studied,and the real-time prediction model of rock pressure is developed,which provides the basis for the estimation of hydraulic support load and the early warning of rock pressure.The main conclusions are as follows:(1)The rock pressure law of workfaces with designed mining heights of 7 m,8m and 8.8 m has been measured and analyzed.Compared with the buried depth and nearly one kilometer depth,the hydraulic support load and dynamic load in the shallow buried workface are larger,the weighting step is relatively short,and the rock pressure shows significantly enhancement.The rock pressure is more intense and the roof accidents are easy to occur when the workface is under special conditions,such as passing through overburden concentrated coal pillar,surface valley,abandoned roadway,and the final mining area.The load of support increases exponentially with the mining height and parabola with the buried depth.When the buried depth is 160 ~250m,the support load reaches the peak value.The main reason is that the shallow buried structure has particularity.(2)The overlying of ultra-large mining height working face is characterized by overall shearing and periodically breaking.The cutting block structure model is proposed and the stability is greatly influenced by cutting angle and block extrusion pressure,which are showing as the block will sliding to instability when cutting angle is greater than 64 ° and extrusion pressure is less than 2.62 MPa.Root of roof accidents in Shendong mining area is considered that the cutting block structure is prone to sliding and instability.(3)Based on the cutting block structure,the formula for estimating the load of support is put forward,and the calculation result in 12401 working face is 25721 kN.Compared with the traditional method of 8 times mining height,the calculation resultsbased on cutting block structure model is closer to the field measurement results.(4)The mechanism of large area roof cutting and collapse of hydraulic supports caused by overburden concentrated coal pillar is analyzed by using cutting block structure model.It is considered that overburden nearly vertically cutting down is the main cause of the collapse accident.The design principle of less than 64° borehole elevation angle is put forward when blasting measures are taken to presplit the overlying concentrated coal pillar and roof.The mechanism of large area roof cutting and collapse of hydraulic supports caused by abandoned roadway is analyzed by using cutting block structure model.The application conditions and reasonable "post-weighting mining " opportunity are put forward.(5)Based on cutting block structure model,the study shows that there are three kinds of function relations between the load and time of the support in ultra-large mining height workface: exponential,logarithmic and linear.The regression prediction model of support load in the mining cycle is established.Data mining method is used to build the final resistance template curve,and the model of matching prediction of the final resistance template curve is developed.The preliminary application in 12401 workface of Shangwan Coal Mine has achieved good results,and the prediction of support load,final resistance and the step distance of the roof pressure has been realized.The prediction accuracy is more than 86%.