Static And Dynamic Mechanical Properties And Failure Characteristics Of Combined Coal And Rock Under High Stress

The roadway instability caused by gas explosion and coal-rock dynamic disaster is an obstacle to the underground rescue work.Roadway surrounding rock is a coal-rock combined structure composed of coal seam and roof and floor rock strata.In the past,the damage mechanism of this combined structure under dynamic loading has not been studied deeply.In this paper,the mechanical properties and failure characteristics of combined coal and rock were studied by using MTS servo press and Hopkinson pressure bar system(SHPB).The structural influences of combination type,coal-rock ratio and angle,and the external influences of impact velocity,confining pressure and impact direction were considered.The main findings are as follows:(1)Compared with rock,the static stress-strain curve of coal has obvious compaction stage with longer duration.In terms of destruction,the coal is mainly pressure shear failure,the rock is split failure.Different from the static stress-strain curve,the dynamic stress-strain curve has no obvious compaction stage,and the strain rebound occurs after unloading.(2)The static compressive strength of combined coal and rock is approximately equal to the average strength of the composite pure coal and rock.The larger the proportion of coal is,the longer the compaction stage becomes.In addition,the static compressive strength,elastic modulus and yield strain are reduced.Moreover,when the strength of coal and rock varies greatly,the failure degree of rock on both sides of the interface is greatly different,which shows that the crushing degree of coal is much higher than that of rock.In addition,when the bonding angle of coal and rock is less than 45°,the failure form of combined coal and rock is axial splitting failure,when the bonding angle is 45°,shear and splitting failure coexist,and when the bonding angle is 60° and 90°,the failure form is mainly shear failure along the interface.In contrast,the static loading direction has little effect on the static mechanical parameters of combined coal and rock.(3)Based on Hopkinson pressure bar test(SHPB),the effects of impact direction,combination angle,coal-rock ratio,impact velocity and confining pressure on the dynamic failure of combined coal-rock are studied.With the increase of impact velocity,dynamic compressive strength,elastic modulus and ultimate strain all increase linearly.When the impact velocity is large,the dynamic stress-strain curves of the combined samples show the characteristics of double wave peaks,and the second peak is more sensitive to the impact velocity.Confining pressure can significantly improve the dynamic compressive strength and deformation resistance of combined samples.At the same time,confining pressure suppresses the longitudinal crack and promotes the development of transverse crack.In addition,the higher the proportion of coal in combined coal and rock is,the lower the dynamic compressive strength and elastic modulus becomes,and the higher the limit strain is.With the increase of the combination angle,the dynamic compressive strength showed a trend of decreasing first and then increasing,and the dynamic elastic modulus gradually increased.The compressive strength of 45° was the smallest and that of 90° was the largest.To study the mechanical properties of combined coal and rock and its failure mode is the key to reveal the failure mechanism of roadway surrounding rock under impact loading.It also has important practical significance to guide the anti-explosion design of roadway.There are 70 figures,19 tables and 83 references in this paper.