Study On Mechanical Properties And Failure Law Of Roof Fissured Sandstone In Fully Mechanized Caving Mining

Affected by the geological structure and engineering disturbances,there are fissures of different sizes in the rock roof of the coal face.Under the action of fully mechanized caving mining,fissures extend and form a fissure network,which changes the fissure rock structure and mechanical properties,weakens the stability of the roof of coal seam,and poses a great threat to the safe and efficient mining of coal mine.Therefore,studying the mechanical properties and failure law of fissure rock under fully mechanized caving mining behavior is of great significance to roadway maintenance,gas prevention and coal mining safety.However,the deformation and failure of rock mass under fully mechanized caving mining stress is a dynamic and changing process,especially the study of deformation and failure of fissures rock is more complicated.In this paper,Supported by the project of National Natural Science Foundation of China(No.51974042)“Coupling evolution law of fracturing hard roof and mining-induced fractures and mechanism of controlling mine pressure”,based on rock mechanics and fracture mechanics,with the help of laboratory test and research means,the mechanical characteristics and failure law of fissure rock with different geometric parameters under the conventional triaxial loading and unloading stress paths are studied.The main conclusions of this paper are as follows:(1)The influence of stress path on the lateral plastic deformation of single fissure sandstone is significant.Under the conventional triaxial stress,the lateral plastic deformation of fissured sandstone sample is larger than that of intact sandstone sample,and the plastic characteristics are more obvious.Under the path of unloading stress,the lateral plastic deformation of fissured sandstone sample is smaller than that of intact sandstone sample,and the brittleness is more obvious.(2)The fissure geometry parameters affect the mechanical characteristics of sandstone specimens.Under the two stress paths,with the increase of fissure length,the peak strength of samples decreases monotonically;with the increase of fissure angle,the peak strength of sample under the conventional triaxial test presented a monotonically increasing trend,while the peak strength of sample under the loading and unloading stress path present a first decrease and then increase and the curve shows a "V" shape.Under the conventional triaxial stress path,the elastic and deformation modulus of the sample first increase and then decrease with the increase of fissure length and angle.In another stress path,the elastic and deformation modulus of the sample tended to decrease with the increase of fissure length,and first decrease and then increase with the increase of fissure angle.Meanwhile,poisson's ratio is closely related to lateral pressure coefficient and volume strain.(3)The mechanical response and crack evolution law of fissure sandstone during deformation are studied based on ultrasonic P wave velocity.Under the conventional triaxial stress path,the wave velocity fluctuates obviously near the stress drop.The crack evolution of the sample can be divided into four stages: crack compaction stage,crack development stage,crack stabilization stage and post-failure instability stage.Besides,the total crack width is basically unchanged near the peak strength,but the duration is relatively short.Under the loading and unloading stress path,the reduction pressure affects the compactness of the sample,resulting in the rapid decrease of the wave velocity growth rate.Near the yield stress,the sample compactness reaches the critical value and the wave velocity reaches a peak.The fluctuation of wave velocity near the peak strength of sandstone sample obviously reflects that the stress concentration at the initial fissure tip is more obvious,and the micro-cracks appear to cross and combine with each other,and the crack evolution can be divided into three stages: compaction stage,crack stabilization stage and crack stabilization development stage.(4)The failure modes of the single-fissure sandstone samples under the two stress paths are different.Under conventional triaxial compression,the failure forms of the single-fissure sandstone samples of different lengths are basically tension-shear composite failure,and the failure forms of the single-fissure sandstone samples of different angles include pure shear failure,tensile failure,tension-shear composite failure and X-shaped shear failure.In the process of loading and unloading test,with increasing fissure length,the failure modes are tensile-shear composite failure,X-shaped non-coplanar shear failure,and X-shaped coplanar shear failure,and the tensile cracks are replaced by secondary coplanar cracks.Fissure angle influences failure initiation.The failure modes include tensile-shear composite failure,single-shear failure,and X-shaped non-coplanar shear failure.