Influence of loose top-coal on support and surrounding rock relationship in longwall top-coal caving mining

The support and surrounding rock relationship is affected by the loose top-coal in longwall top-coal caving mining (LTCC). The dynamic load tests under different particle types,particle sizes,particle packing thicknesses and impact forces were carried out by using the self-developed dynamic load impact test device,and revealed the influence of loose top-coal in LTCC on supports and surrounding rock relationship. It was found that the buffer effect is positively correlated with the particle packing density,that is to say,the buffer effect is more obvious with the larger particle packing density;when the packing thickness is less than a critical value,the impact effect is more obvious with the smaller packing thickness,on the contrary,the impact force gradually decreases then to be stable with the increment of packing thickness;though the effect of buffering dynamic load becomes greater with the increase of particle sizes,the buffer curve fluctuates obviously,which indicates that the impact process is severe. At the same time,the factors such as the condition of coal seam,the properties of loose top-coal and the structure performance of support should be con- sidered in selecting support type,so as to ensure the safe mining.