Study On Dynamic Response And Damage Effect Of Frozen Red Sandstone Under Gradient Impact

At present,during the construction of the freezing method vertical well in the western mining area,the Jurassic coal seam is often encountered with huge thick Cretaceous water-rich and weakly cemented soft rock,and the design of frozen wells for such strata lacks perfect theoretical basis and reliability.Engineering experience has caused frequent occurrence of water outbreaks due to unstable wall freezing caused by construction blasting,mechanical rock breaking and other gradient impact loads in actual projects,resulting in delays in construction period and significant economic losses.Therefore,it is of great significance to carry out experimental research on the dynamic characteristics and damage effects of freezing saturated red sandstone under the gradient impact.In this paper,the Cretaceous water-rich soft rock through which Wuju coal mine in Gansu province was traversed by frozen drilling was taken as the research object.The indoor SHPB gradient impact and nuclear magnetic resonance test were carried out by on-site sampling.At the same time,combined with theoretical analysis and numerical simulation technology for comprehensive analysis and research,the main research content and conclusion are as follows:(1)Based on the laboratory static test,mineral composition analysis and electron microscope scanning,the basic physical parameters,mineral composition and microstructure of the water rich weak cemented red sandstone in the western region are determined.It is concluded that the red sandstone in the western mining area is high porosity(porosity up to 25.8%)weak Cemented Sandstone,and the weakening of the static parameters is prominent in the water saturated state.(2)The ? 50mm Split Hopkinson pressure bar(SHPB)system was used to perform impact compression test on frozen red sandstone.Based on the test results,the stress-strain curve characteristics,peak strength and peak strain of frozen red sandstone under different gradient impacts were studied.And the relationship between dynamic elastic modulus and strain rate.It is found that the characteristics of the dynamic stress-strain curve of red sandstone under the coupling of gradient impact and low temperature can be divided into three stages:linear elastic rising section,longer yield stage and unloading failure section.As the freezing temperature decreases,the dynamic parameters of the positive gradient impact samples are slightly weaker than the negative gradient impact,indicating that the frozen red sandstone under the positive gradient impact resistance to deformation is weaker than the negative gradient impact.At the same time,under the impact of high strain rate,the positive and negative gradient impacts exist in the freezing instability weakening zone between-5??-10?.(3)Combined with the SHPB impact compression test and nuclear magnetic resonance test results,the cumulative damage evolution process of frozen red sandstone was analyzed,and the damage mechanism of frozen red sandstone under gradient dynamic load was revealed.It is concluded that the damage degree and extended evolution rate under positive gradient impact are higher than those of negative gradient impact,the inflection temperature of positive gradient impact appears at-10?,and the inflection temperature of negative gradient impact appears earlier than-5? at positive gradient;Under the impact of positive and negative gradients,before the inflection point temperature,the reduction of the freezing temperature plays a role in deteriorating the damage evolution of the sample,and after the inflection point temperature,it plays a strengthening role.(4)Identify the parameters of the HJC constitutive model of frozen red sandstone under gradient impact based on the results of the dynamic test,the numerical simulation software ANSYS/LS-DYNA was used to simulate the impact compression test data of different gradients,the propagation process of stress wave in the specimen and the damage evolution process are studied.The results show that HJC constitutive model can basically reflect the dynamic behavior and damage evolution of frozen red sandstone in SHPB impact test.