Mechanical Properties And Damage Mechanism Of Cemented Backfill Body With Different Sizes

The size effect is an important property of rock materials,so this factor is considered in practical engineering design and theoretical research on rock constitutive relations.In underground backfill mining,cemented tailings backfill body(CTBB)is often used as a substitute for artificial ore pillars and plays a key supporting and bearing role,but the size effect of CTBB has not attracted widespread attention.This paper relies on the key project of Jiangxi Provincial Key R&D Program "Mine Underground Mining Disaster Early Warning,Key Technologies and Demonstration of Emergency Rescue"(No.20212BBG71009),and the Jiangxi Postdoctoral Research Project "Infrasonic Prediction of Destruction of Tailings Cemented Backfill in Deep Backfilling of Gannan Tungsten Mine" Exploratory Research on Methods and Mechanisms”(No.2020KY39),Young Talents Support Program of Jiangxi University of Science and Technology “Study on Source Characteristics and Energy Mechanisms of Acoustic Emissions of Rock Creep under High Temperature”(No.:JXUSTQJYX2019005),four different sizes of cubic backfills(40mm,70.7mm,100 mm and150mm)was studied,combined with laboratory tests and theoretical analysis,the CTBB under uniaxial compression and direct shear conditions was studied.At the same time,combined with ABAQUS finite element software,the stability of the excavation process of the approach stope under the roof of the backfill body with different sizes was compared and analyzed.The main findings are as follows:(1)There is obvious size effect of the backfill specimen under uniaxial compression.With the increase of size,the uniaxial compressive strength of the sample shows a monotonous downward trend,and the strength tends to be stable when the size reaches 100 mm.the elastic modulus and wave velocity of the small-sized sample are obviously larger than those of the large-sized sample;with the increase of the size,the failure mode of the sample The transition from shear failure to tensile failure gradually increases.During the failure process,the macroscopic crack penetration degree of the sample increases gradually,and the number and scale of slump zones increase significantly.(2)In the direct shear test,the shear stress of the specimen with a size of 70.7 mm has a more severe overall upward trend,and is accompanied by an obvious stress drop phenomenon after the peak,and the brittle failure characteristics are more significant.The shear expansion is basically the same.With the increase of size,the cohesive force of the sample decreases sequentially.Compared with the sample with the size of 70.7 mm,the cohesive force of the sample with the size of 100 mm and 150 mm decreases by 28% and 50%,respectively.The shear failure surface of the backfill body is formed by two obvious failure strips.With the increase of the size,the small broken strips on the surface of the backfill body increase,accompanied by more caving areas and broken bodies,and the greater the normal stress,the greater the friction phenomenon on the surface of the sample,the dilatation effect is obvious.(3)The principal strain cloud map of the backfill body under force failure process was obtained by processing the digital image correlation technology.It was found that the surface cracks of the small-sized specimens evolved rapidly and violently under uniaxial compression,and there was no obvious slump area,while the large-sized specimens showed an overall appearance.Plastic failure,cracks gradually expand and penetrate.(4)With the increase of size,the lag time of the peak acoustic emission ringing count of the backfill body relative to the stress peak is longer,which further indicates that the failure process of the large-sized sample is slower,and the acoustic emission activity at the stress peak is lower.The acoustic emission peak energy and average energy of the backfill body gradually decrease with the increase of the size,and the energy accumulation capacity of the sample is continuously weakened;the abrupt change point of the decrease of the acoustic emission r value and the generation of the lowest value of the average frequency centroid can be regarded as the precursor of the macroscopic damage of the backfill body.(5)The energy evolution law of backfill bodies of different sizes is reflected in the accumulation of elastic strain energy before the peak stress,and the proportion of dissipated energy after the peak stress rises continuously and rapidly exceeds the elastic strain energy.With the increase of the size of the specimen,the energy indexes of the backfill body at the peak stress all showed a nonlinear decreasing trend,indicating that with the increase of the size of the specimen,the energy storage limit and the bearing capacity of the backfill body continued to decrease.(6)The dissipated energy characteristics of the backfill body can better reflect the four stages of damage evolution inside the backfill body,the initial damage stage and the damage stable development stage.In the damage and damage stage,the corresponding curves of the samples with the size of 40 mm and 70.7mm and the samples with the size of 100 mm and150mm showed two different growth trends.The smaller the size is,the more severe the damage is.(7)The larger the size of the backfill specimen,the greater the variation of the elastic energy consumption ratio K value of the specimen.The overall elastic energy consumption ratio K value curve first increases and then decreases,and lasts to a lower value,and then turns to increase when it is close to the peak stress.The change law of the elastic energy consumption ratio K value curve can be used as a precursor to the critical failure of the backfill body.(8)The excavation process of the approach stope with different mesh sizes was simulated by ABAQUS finite element software.The results show that the stability of the excavation process of the down approach stope of the three grid sizes is similar,but due to the different mechanical parameters corresponding to the grid elements of each size,the specific values of the model stability parameters are different.The numerical model with a mesh size of 70.7 mm has the smallest displacement change at the access roof;with the increase of the distance between the backfill area and the roof,the backfill area gradually changes from tension to compression,and the model with smaller mesh size corresponds to the largest backfill area.The larger the principal stress,the compressive stress is applied to the corresponding meshes of different sizes in the two sides of the approach,and the three are not distinguished very much.The depth of the plastic zone of the numerical model increases with the increase of the mesh size.It can be seen that the existence of the size effect of the backfill body changes the stability of the grid models of different sizes.Therefore,when designing the mine optimization scheme,the mesh size of the numerical model of the different optimization schemes should remain unchanged.