Multi-Scale Degradation Mechanism Of Disposal Paste In Coal Mine Collapse Pit Under Compound Erosion

The action of the mining stress underneath the active coal mine collapse pits that have been processed by loose bodies will cause secondary disasters.Aiming at this problem,a new method for disposal of collapsing pit paste in active coal mines is proposed.After the paste is backfilled to the coal mine collapse pit,it will be degraded by freeze-thaw,corrosion and other factors.The degradation mechanism is still unclear.This paper adopts a research method combining laboratory experiment-numerical simulationtheoretical derivation.Firstly,the optimal mix ratio of coal gangue gypsum material is obtained by using response surface method and multi-objective algorithm.Its formation mechanism is through the combination of SEM-EDS.The basic mechanical parameters of the disposal paste are obtained through conventional triaxial compression test and splitting test.The Mat DEM matrix discrete element is used to simulate the whole process of coal mining under different active coal mine collapse pit disposal schemes.Carry out freeze-thaw,corrosion and composite corrosion tests on the paste specimens with the optimal mix ratio.Test its mass loss rate,relative dynamic elastic modulus,and uniaxial compressive strength loss rate from the macro scale,and use the SEM-EDS combination method from the micro scale.Exploring the multi-scale deterioration mechanism of paste disposal in coal mine collapse pit.Based on the appearance of the compaction stage caused by erosion,a constitutive model of the damage of the paste under erosion is established.Research indicates:(1)The optimal mixing ratio of collapse pit disposal paste: the water-to-binder ratio is 4.0,the aggregate-to-binder ratio is 16.91,and the gangue rate is 60%.When the paste is cured with the optimal mix ratio for 1 day,the cement hydrates to form C-S-H gel to cement the coal gangue aggregate into a whole,3-14 d cement hydration generates a large amount of Aft to form a skeleton structure,14-28 d at the same time as the cement hydrates Coal gangue is used as an auxiliary cementing material to generate a large amount of flocculent C-S-H gel to fill the Aft framework,and finally form a relatively dense "skeleton-gel" network structure.(2)The discrete element simulation results of different active coal mine collapse pit disposal methods show that: When disposing of the bulk body collapse pit,the load-bearing capacity will be completely lost under the mining stress,and eventually a "sinking funnel" will be formed,causing serious damage to the surface.When the paste is used to dispose of the collapse pit,the disposing body will act as an "artificial low-strength rock beam" and will not collapse under mining stress;Filling coal mining-paste disposal of collapse pits can effectively control surface subsidence.The final average surface settlement is 0.72 m,which is 12.27 m compared to the final average surface settlement of coal mining with granular collapse pit and caving method,and the settlement reduction rate is 94.13%;(3)Through the freeze-thaw cycle test,dry-wet cycle test and compound erosion test of paste materials,the degradation mechanism of paste under the action of freeze-thaw corrosion and compound erosion was explored.On the macro scale,the degradation laws of paste quality,relative dynamic elastic modulus,and uniaxial compressive strength are obtained.On the microscopic scale,obtain the evolution law of the microstructure of the paste and the type of chemical products inside the material;(4)The accumulation of damage leads to a prolonged compaction phase of the stress-strain curve of the disposal paste.Taking the compaction stage as the critical point,considering the coupling effect of environmental erosion and load.It is assumed that the defect closure process under the load of the compaction stage and the destruction process of the paste micro-element body in the post-compaction stage meet the Weibull distribution.A segmented statistical damage constitutive model with the compaction point as the critical point is established,which can more completely describe the stress-strain curve of the disposal paste after erosion in a complex environment.