| The grotto temple is a precious stone cultural relic,and about 80%of the grottoes in China are carved in loose and porous sandstone.Due to the influence of geological background and diagenetic environment,the microstructure characteristics of sandstone in grottoes in different regions vary greatly,leading to different cave diseases.This is due to the different microstructures of rocks,which can lead to different physical and mechanical properties and failure response laws of rocks.Therefore,this paper selects grotto sandstones with different microstructures from typical grotto areas in the south and north of China,carries out microstructure analysis based on mineralogy,and conducts uniaxial compression tests under real-time ultrasonic monitoring.Through comprehensive analysis of stress-strain curves and ultrasonic wave velocity changes,combined with scanning electron microscopy（SEM）image analysis and PFC2D numerical simulation results,this paper studies the response of mechanical characteristics of grotto sandstone to particle size,mineral composition,and cementation characteristics.Then the strength characteristic values of the grotto sandstone were determined.Finally,the impact mechanism of the microstructure of grotto sandstone on its progressive failure was clarified.The main conclusions obtained are as follows:（1）The microstructure significantly affects the mechanical properties of grotto sandstone.Real time ultrasonic monitoring uniaxial compression tests were conducted on sandstone with different microstructures in grottoes.The response of basic mechanical characteristics of grotto sandstone to average particle size,mineral composition,and cementation characteristics was analyzed.The results showed that the four mechanical properties of grotto sandstone,including uniaxial compressive strength,peak strain,elastic modulus,and P-wave velocity,were inversely proportional to the average particle size;Except for the peak strain,which is directly proportional to the quartz content,except for the North Grottoes,which are greatly affected by particle size;There is no significant relationship with the content of feldspar;The peak strain is positively correlated with the content of clay minerals;In addition,all four mechanical properties increase with the increase of bonding grade.（2）The microstructure directly controls the progressive failure process of grotto sandstone.Through ultrasonic wave velocity analysis,the initiation strength（84）and damage strength((8（9） of uniaxial compression tests of sandstone in four grottoes were determined.The influence of three microstructure characteristics,particle size,mineral composition,and cementation characteristics,on the（84）and((8（9） was analyzed.The（84）and((8（9） of sandstone in four grottoes and their normalized values decreased with the increase of their particle size;and increases with the increase of quartz content,except that the northern grottoes are more affected by particle size,and the response law to feldspar and clay minerals is not obvious;It increases with the increase of bonding level.（3）Different microstructures have different impact mechanisms on the damage and failure of grotto sandstone.Based on the indoor uniaxial compression test results of different microstructure cave sandstones,combined with SEM image analysis,a numerical model of cave sandstones with different particle sizes,mineral compositions,and bonding characteristics was constructed using the PFC2D discrete particle flow program and its parallel bonding model.The quantitative relationship between micro parameters and macro mechanical parameters was obtained,and the strength deformation parameters and failure characteristics of the sample after uniaxial compression failure were simulated,The results indicate that the numerical simulation test results are consistent with the indoor test results.In addition,mechanism analysis was conducted from the perspectives of micro crack distribution characteristics,micro displacement field,and micro stress field of the sample,which further clarified the damage and fracture mechanisms of different microstructure cave sandstone samples under uniaxial compression conditions... |
