Experimental study on time series character of roof cutting in shallow working face

In order to study the time series character of roof cutting process in shallow working face, the load test with three-dimensional confining pressure of typical shallow seam in the laboratory were used to study the time effect of based parameters, and similar simulation experiments were used to analyze the multi-parameter time series character of roof cutting process.The paper concludes that surrounding rock stress increases, the roof rock of shallow seam is easy to change from “X” type fracture and strength yield failure to single shear failure, and has obvious time effect characteristics.The face advance speed of shallow seam affects the difference between vertical mining stress and horizontal stress, the main reason of shear failure of roof is the increase of stress difference due to the slow face advance speed.Mining-induced fractures in overburden rock first form, develop, and extend to the surface (that is, the overall cutting and subsidence of the strata) before the overall cutting and subsidence of bedrock layer in shallow seam.The cutting position is related to the working face’s advance speed.When face advance speed is accelerated, weighting interval increases, support stress decreases, and the overall cutting position of the bedrock layer basically occurs behind the support.Abutment pressure reflects the formation process of roof caving accident, and it reaches stress extremum when cutting, then the stress decreases.In terms of time series, roof strain can be successively divided into “three zones”:“initial strain stability zone-strain violent evolution zone-goaf strain stability zone”.When working face advances slowly, strain starting point leads, the cutting line of roof moves forward, and basic roof strain amplitude increases.Both the starting point and peak value of advanced support pressure lag behind the overburden strain, and the time series curve of support pressure and mining stress is similar, which can be mutually characterized.The results show that the design of joint monitoring and early warning values of support pressure, mining stress, surrounding rock strain and other indicators can be used to predict and prevent roof cutting accident in shallow working face.