| Interlayer spacing between shallow-burial coal seams No.2 and No.3 in Laomupo Mine ranges from 6 m to 12 m, and there is no methane occurrence in these two coal seams. In this mine, pillarless gob-side entry retaining under repeated coal mining influence is adopted for the sake of solving mining faces replacement problems in close interlayer seams. Correlated researches on gob-side entry retaining in single coal seam have fully developed, nonetheless, study on surrounding rock support in pillarless gob-side entry retaining, suffers repeated mining-induced disturbance due to close interlayer coal seams spacing, is rarely discussed.(1) By utilizing elastic-plastic theory, fracture depth in floor on account of extraction of overlying coal seam was deduced based on roof caving characteristics and comparative stress distribution, and it also been testified with the aid of numerical simulation and RQD index in field test. Single layer of immediate roof in underlying entry was clarified based on calculation on yield ratio and practical geology in Laomupo Mine, it thus provided preliminary basis for the design of bolt length in underlying No.3 coal seam.(2) Distribution law for mining-induced pressure in underlying coal seam is explained by numerical simulation, which provides reliable theoretical basis for study on surrounding rock controlling in pillarless gob-side entry retaining.(3) In view of roof characteristics behind the mining face under the circumstance of close distance coal seams, calculation basis for roof load in retained entry buried in underlying coal seam is theoretically analyzed, and fracture instability characteristics of lateral roof are also probed.(4) The application conditions of pillarless gob-side entry retaining were studied by generalization and summarization of successful cases in home and abroad, the load design basis of entry-side roof-cutting support, on the basis of roof-cutting mechanical model for pillarless gob-side entry retaining, was probed by combining “preset load”, “preset deformation”, “lateral gangue pressure”, and “floor specific pressure”. The numerical simulation presented the fracture modes(under different roof-cutting loads, 400 k N/m、600k N/m、800k N/m,1000 k N/m) of immediate roof in retained entry, and the impacts of different modes on stress distribution around retained entry was also discussed. In order to clarify a reasonable density for auxiliary individual props, supporting pressure and deformation in roof and coal rib, with the aid of simulation software, were analyzed as the prop arrangement were 1 row, 2 rows, and 3 rows, respectively.(5) Aimed at the problems that the roof of underlying seam may suffer mining-induced fracture of overlying coal seam and some security hidden troubles concerning roof fracture and caving inclination, pre-tightening moment and advisable cable length of bolt was defined by field tries, this study also proposed power roof-cutting props for gob-rib support in retained entry, grouting technology combined deep holes and shallow holes, air leakage prevention separation mode, and gob sealing elastic material by spraying etc. Hence, a set of technical system was built for pillarless gob-side entry retaining under repeated mining influence and multiple coal seams geology, this technology cancelled traditional techniques with man-made backfilling wall set along gob rib in entry, and moreover, it guarantees the successes of gob-side entry retaining.The successful implementation of pillarless gob-side entry retaining under multiple coal seams and repeated mining-induced influence provides a novelty path for gob-side entry retaining under repeated mining influence, the technical and managerial experience accumulated by field practices has certain valuable promotional value. |
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