Fissure Mechanism Induced By High Temperature And Cooling Approaches Of Concrete Filling Walls With Large Scale

A series of problems of filling walls caused by high temperature have alreadyrestricted the application of gob-side entry retaining in deep stress-releasing mining.To solve this problem, this paper studied the reasons and evolution law of hightemperature and failure mechanism of filling walls under coupling effect of boththermal stress and mine-induced stress with integrated methods of theoretical analysis,field measurement, laboratory tests and numerical simulation. Then on this basis,practicable cooling approaches were put forward. Main conclusions are as follows:(1) High temperature of filling walls has serious impacts on mining, not onlycan cause wall deformation including flaking and cracking which would influence thebearing capacity of filling walls, but also deteriorates working environment of mines,and creates hazards of residual coal self-ignition and gas explosion in gob.(2) The high temperature reasons of filling walls were described. The hightemperature inside the filling walls was caused by high geothermal induced by largeburied depth, heat released by high-power mining equipment, massive hydration heatproduced by filling materials and poor thermal conductivity of concrete, with the hightemperature degree more intense than ground mass concrete. Hydration heatdetermination experimental results show that the value of hydration heat releasedwithin3days of CHCT filling materials was measured up to75.2%of the total heat,much larger than30%of ordinary cement materials.(3) The evolution law of high temperature was studied through theoreticalanalysis, measurement and numerical simulation. The inner parts of filling walls reachthe highest temperature60.9℃on the5th day while the surface parts reach thehighest temperature40.4℃on the2.5th day. The maximum temperature differencebetween the inside and outside wall is22.0℃on the5th day and the temperaturedifference has been above20.0℃from the5th day to the7th day.(4) Coupling failure process and mechanism of filling walls were studied.Thermal stress exceeding the allowable stress leads to temperature cracks on thefilling walls surface. Filling walls are significantly affected behind working face bymining-induced stress. Under the coupling effect of thermal stress andmining-induced stress, thermal cracks extend into interior of filling walls andeventually develop into through cracks, resulting in coupling failure. Simulationshows that cracks occur on the5th day under the single effect of thermal stress. Withthe addition of mining-induced stress, cracks constantly extend. When mining-induced stress increases to16MPa, filling walls occur coupling failure. So themining-induced stress should be controlled not more than16MPa.(5) Filling walls cooling approaches were proposed, including lowering thetemperature of fresh concrete and synchronous filling wall cooling approach, whichcombines internal water transportation through embedded pipe and surface nozzlewater spraying. Lowering the temperature of fresh concrete should make full use ofmine air-conditioning system. Detailed principles and parameters design, and fieldapplication feasibility analysis were carried out for synchronous filling wall coolingapproach, indicating that the approach is feasible, both in the technical and economicaspects.