Research On Failure Response Characteristics And Instability Precursor Information Recognition Of Fissured Sandstone In The Water-Accumulated Residual Mining Area

Under the influence of multiple mining disturbances,primary fissures in coal seams will continue to develop,expand and extend,forming a larger fissured rock mass.For the residual mining area,the extraction stress environment and coal rock properties are more significantly changed,resulting in more extensive distribution of fissured rock in the residual coal extraction space.The stability of the fissured rock in the residual mining area is influenced not only by its own fissure geometry parameters,but also by the extensive water accumulation in the adjacent area.The stability of the fissured rock in the waterlogged residual mining area is the core guarantee of safe green and high recovery mining of residual coal resources,which inevitably requires scientific knowledge of the failure response characteristics of the fissured rock.In addition,the ability to identify precursors of instability in the fissured rock in the waterlogged residual mining area in advance allows for timely disaster instability prevention and control.In other words,there is a clear need and orientation to carry out research on the characteristics of failure response and identification of destabilization precursors information of fissured rock bodies in waterlogged residual mining areas,which is expected to provide theoretical guidance for the prevention and control of fissured rock in waterlogged residual mining areas,and to guarantee the safe green and high recovery mining of residual coal resources in adjacent areas.Therefore,the fissured sandstone is taken as the research object of this paper,and the research theme of"failure response characteristics and instability precursor information recognition of fissured sandstone in the water-accumulated residual mining area"is proposed.Based on the construction of a fissured sandstone progressive damage test system and the development of a multiparameter identification method for characteristic stresses,the influence of intrinsic fracture geometry parameters and variability in waterlogged environments on the failure response characteristics of the sandstone in the residual mining area is analysed.The linkage between multiparametric information and fracture characteristics is established to achieve multiparametric identification of precursors of fissured sandstone instability under water-rock action.The main research results are as follows:（1）Different sandstone soaking scenarios for the waterlogged residual mining area are designed:the water environment in which the fissured sandstone in the residual mining area is located can be modelled by deionised water,low mineralised water,high mineralised water and acidic water.An AE-DIC-force multiparameter monitoring test system for fissured sandstone damage is constructed.The correlation between multiparametric data is verified based on the AE-DIC-force information of fissured sandstone damage under water-rock action.A multiparameter identification method for the characteristic stresses of fissured sandstone in the waterlogged residual mining area is proposed,and the progressive damage process of fissured sandstone in the waterlogged residual mining area is accurately delineated.（2）The effects of prefabricated fissure angle,length and distances on the mechanical behaviour and fracture extension of sandstone are analysed.The results show that the change of fissure angle length significantly affect the peak strength,elastic modulus,fracture expansion process and damage mode of sandstone.The peak strength and modulus of elasticity of the sandstone show a trend of decreasing and then increasing as the fissure angle increases.The crack initiation position gradually moves towards the fissure tip,the degree of pre-peak crack development decreases.At the same time,the sandstone damage pattern shows a tendency to increase the degree of shear damage.With increasing fissure length,the peak strength and modulus of elasticity of the sandstone show an exponential decrease in approximation.Secondary cracks are gradually transformed from mixed cracks to shear cracks.At the same time,the damage pattern of sandstone shows a transformation of"tension dominated-local shear→mixed tension-shear damage→shear damage"as the angle increases.With increasing fissure length,the peak strength and modulus of elasticity of the sandstone show an approximate"upward convex"trend.The increase of the fissure distance has less influence on the fissure extension process and damage mode,but the fissure penetration is more lagging and closer to the peak strength moment.At the same time,the damage process of fissured sandstone under different conditions is analysed using PFC numerical simulation software,and the failure response characteristics of sandstone under different scenarios of prefabricated fissure angle,length and distances are verified in terms of both stress field distribution and crack development.（3）The influence of the variability of the waterlogged environment on the mechanical behavior,crack extension and damage mode of fissured sandstones is investigated.It was found that the strength and modulus of elasticity of fissured sandstones in a water-filled state are reduced compared to their natural state,and that their deterioration increases with increasing water absorption.Theσ_ci/σ_f of fissured sandstones in a water-filled state is significantly reduced,whileσ_cd/σ_f is weakly affected.Water-filled also changes the crack initiation mode of fissure sandstone from tension crack initiation in the natural state to shear crack initiation,and changes the damage mode from"mixed tension-shear damage"in the natural state to"shear damage"."Water environment soaking→drying"state,the increased variability of the microstructure within the fissured sandstone will not only induce an increase in strength and deformation dispersion,but also contribute to a significant deterioration ofσ_ci/σ_f and will only lead to a deterioration ofσ_cd/σ_f in acidic mine water environments.On this basis,the main controlling factors of water-rock action for fissured sandstone damage were identified:the strength and elastic modulus of fissured sandstone under water-rock action are mainly influenced by water-rock physical action,the peak strain andσ_ci/σ_f are governed by both water-rock chemical action and water-rock mechanical action,σ_cd/σ_f is only related to water-rock chemical action,and the fracture initiation mode is only influenced by water-rock mechanical action.（4）The deterioration of sandstone mechanical indicators under fissure-water interaction is defined and the deterioration characteristics,crack extension and damage patterns of sandstone under different fissure geometry parameters are analysed.It is found that fissure-water interaction significantly degrades the peak strength and elastic modulus of sandstones.The degree of hydrogenic degradation of sandstone varies greatly with different fissure geometry parameters,i.e.,there is a"competitive degradation"between fissure and water on the mechanical behavior of sandstone:The greater the degree of fissure deterioration,the smaller the degree of hydrogenic deterioration.When the damage mode changes from"tensile damage dominated→mixed tensile-shear damage→shear damage dominated",the mechanical behavior of the fissured sandstone is gradually degraded by the hydraulic deterioration.The mechanism for the transformation of the damage mode of water-degraded fissured sandstones is revealed:the stress intensity factor KII at the tip of the fissures is much greater than KI when the sandstones are finally damaged under water-filled conditions,which is the fundamental reason for the transformation of the damage mode of water-degraded fissured sandstones.（5）The multivariate precursor information of fissured sandstone failure in the natural state was investigated,and it was found that AE parameters or DIC information can be used to discern the stage of crack extension of fissured sandstone failure in the natural state,and to identify its instability state.A multivariate identification method for destabilization precursors of fissured sandstones in waterlogged residual mining areas is proposed,and it is found that the identification of damage precursor information of fissured sandstones in waterlogged residual mining areas requires comprehensive consideration of AE parameters and DIC strain standard deviation.A model for discriminating the instability of the fissured sandstone in the waterlogged residual mining area based on machine learning algorithms is proposed,and it is found that the established model for discriminating the instability is more reliable,and the DIC has a higher contribution to the reliability of discriminating the instability of the fissured sandstone in the water-filled state.