Basic Study On Technology Of Pre-blast Weakening Hard Top-coal In Fully Mechanized Caving Faces

When the fully mechanized top-coal caving is used in hard thick coal seam, the top-coal cannot be broken fully by ground pressure, and this will cause the coal recovery is lower and the violent ground behaviors threatened safety appear in working face. The key of fully mechanized caving safely and efficiently in hard thick seam is to control the top-coal breaking process effectively, the technology of the top-coal pre-blast weakening is a valid method to resolve this problem. The top-coal system of fully mechanized caving face in hard thick seam is a continuous and dynamic system on time and space, the pre-weakening is a important part composing the whole breaking process of the top-coal, and the essential to pre-weakening the hard top-coal is artificially and beforehand to make its whole strength lower. The hard top-coal is defined as a kind of the top-coal that its outflow and recovery exists obvious trouble when it did not be processed specially. The necessary conditions to put out the top-coal are presented, and a hard top-coal identifying method with the fuzzy appraisal as well a calculating method of the top-coal original damage including the macroscopic structure face strength influence is proposed out.The hard top-coal pre-weakening blast is an especial kind of the mine underground blast. The experiments indicate that the peak pressure of the blasting stress wave within the coal attenuates as the power function, and the loss of the wave energy in it exceeds that in the general rocks. According to the experiment and the site-testing, the methods to calculate the crushed area, the tensile failure area and the cracked area of the weakening blast in hard top-coal are put forward, and more, using the Weibull distribution of the rock blasting crack density and the equivalent strain principle, it is concluded that the attenuation law of the peak cubage strain of the blasting strain wave with the proportion distance is the power function and the damage degree function of the weakening blast is derived in hard top-coal.To change the characteristic of the whole structural mechanics of the top-coal is the main pre-blast weakening form, thereby the whole structural mechanics model and its breakage condition of the hard top-coal after pre-weakening blast in fully mechanized caving are set up. By the analysis of the whole rigidity change of the top-coal, the rationality that the blast hole must be arranged in row is proved and the statistical damage variable expression of it under the action of the abutment pressure is deduced.In the fully mechanized caving face of hard thick seam, the small structure in "cantilever" type is formed in the middle roof strata and the big structure in "voussoir" type is formed in the high roof strata generally, the overlying strata movement of the face is controlled by the two key strata in middle and high, the violent periodic weightings are arose by the composite effect of the two key strata movements, but the violent degree of the weighting behaviors rests with whole mechanics capability of the top-coal above the faces generally. It is the most remarkable characteristic of the ground behavior in hard seam fully mechanized caving face that there are the violent periodic weighting phenomena. The pre-blast weakening obviously influences the ground behavior of the face because of lowering the whole rigidity and strength of the top-coal, thereby the way that to use the beforehand weakening blast of the top-coal and the roof prevents the roof disaster accident is proposed.The site tests further testify the hard top-coal pre-weakening blasting method in fully mechanized caving is actual workable, show off the results from the hard top-coal pre-blast weakening theory accord with the actual situation in practice, and the good technological and economical benefits have be won.