| With the depletion of shallow mineral resources,the exploitation of metal mineral resources is gradually shifting to underground.With the advantages of safety,green and high efficiency,the filling mining method is widely used in many mines.For many large goafs,mines often cannot fill the goaf with one filling,so they need to fill the goaf in several times,which leads to a layered structure of the cured backfill and destroys the integrity of the backfill,thus weakening the stability of the goaf as an underground support structure to a certain extent.Therefore,in this paper,we analyze the influence mechanism of macroscopic and mesoscopic defects structure in the process of backfill specimens loading by using CT scanning,DIC and other technical means,combined with numerical simulation,with the object of layered cemented tailings backfill(LCTB).The following research results were obtained:(1)By conducting uniaxial compression experiments on the backfill specimens,it was found that,with the appearance of macroscopic defect structures such as layers,the load-bearing capacity of the backfill specimens decreased,and the uniaxial compressive strength(UCS)of the 2-layered and 3-layered backfill specimens decreased by 5.57% and 11.82%,compared to the backfill specimens without layers,and the more number of layers,the smaller the UCS of backfill specimens.In the 2-layered specimens,the UCS decreased by 4.02% and 8.64% for the specimens with 5°and 10° inclined angles compared to the specimens with 0°.In the 3-layered specimens,the UCS decreased by 7.97% and 22.37% for the specimens with 5° and 10° inclined angles compared to the specimens with 0°.the larger the inclined angle of layers,the smaller the UCS of the backfill specimens.With the appearance of layers,the damage of the backfill is concentrated toward the layered contact surface(LCS).(2)Real-time CT scans of the compression process of the specimens were performed,and in the 2D slice images,it was found that the porosity of the sample tomographic sections at the upper LCS increased by 70.83% from 0 to 30% stress level,and decreased by 23.82% from 30% to 70% stress level,showing a trend from increase to decrease.In the sample fault sections at the lower LCS from 0 to 30% stress level,the porosity increased by 19.44%,and from 30% to 70% stress level,the porosity decreased by 8.47%,also showing a trend from increase to decrease.In the upper and lower LCS,the porosity of the sample slices were all below 1%,and the changes were small.The changes in the fine structure eventually affect the macroscopic damage generating regions of the specimens,with dramatic damage occurring in the middle layer and the LCS.(3)By 3D reconstruction of the CT scan images of the LCTB specimens,3D models of the specimens and pores were obtained to calculate the pore volume fraction at each stage.At the stress level of 0%,the pore fraction was 0.235%;at the stress level of 30%,the pore fraction was 0.352%,which increased by 49.8%;at the stress level of70%,the pore fraction was 0.289%,which decreased by 15.3%,and the volume size of the pores showed a trend from increasing to decreasing.Among the five regions of the upper middle and lower layers and the upper and lower LCS,the greatest degree of pore change is the middle layer,and the porosity rises 117.2% compared with that at 0%peak stress,which exceeds the overall porosity of the specimen by 79.5%,indicating the weakening effect of the upper and lower LCS on the middle layers in the LCTB,resulting in the stress concentration in the middle layer.In the 30%-70% stress level stage,all the regional porosity changes are less than 10%,and in the damage stage,the porosity of the upper and middle layer is larger and the damage is more serious.The main pore structure changes in the specimen at the compression stage and the damage at the post-compression stage are mainly concentrated in the middle layer and near two LCS.(4)The DIC test was conducted on the compression process of the specimen,and the surface changes of the specimen were analyzed.The main strain shifted from concentrating at the upper and lower ends of the specimen to concentrating at the LCS,while the axial and radial strain distribution of the specimen changed from the uniform distribution of the specimen without inclined LCS to the distribution trend with obvious inclined angle with the increase of the layer angle,indicating that the appearance of the delamination angle led to the uneven distribution of stress and the deformation of the specimen showed an oblique trend.(5)The specimen models with different layers all showed stress-strain curves close to the actual ones during the compression simulation,and with the appearance of delaminations,crack sprouting was more likely to occur at the LCS.In the specimens of different structures,tension cracks are dominant and shear cracks are supplementary.Also the strain energy of the specimens was inversely proportional to the increase in the number of layers.Comparison of the results of several experiments showed a high agreement,and comparison with the results of CT scanning experiments revealed that the main damage mode of the LCTB is dominated by the influence of the macrostructure,but fine defects also affect the development trend of damage such as cracks to some extent. |
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