Dynamic Evolution Of Bed Separation In Coal Seam Mining And Assessment Of Bed-separation Water Hazards

In this article,two coal mines in the north and south ends of the Ordos basin are selected as the research objects,and the bed-separation water hazard in the thick Cretaceous sandstone is the core of this research.The characteristics of Cretaceous bed-separation water hazard,the establishment and application of the computational model for bed-separation dynamic evolution analysis,the risk assessment of flooding from Cretaceous bed separations,the classification of bed-separation water hazard patterns and the prevention and control technology of bed-separation water hazards have been fully studied.Finally,a system theory of bed-separation water hazards in the thick Cretaceous sandstone was formed,and the main achievements are as follows.(1)On the characteristics of Cretaceous bed-separation water hazards: The Cretaceous strata in the study area contain only the Luohe formation,with an average thickness of about 350 m,and the bed separation in the bottom of the Cretaceous sandstone has good water filling conditions.It has been found that there is a water-inrush channel closed zone(WICCZ)within the distribution range of the predicted flowing-water fracture zone(FWFZ).The WICCZ is located in the area between the first mudstone section at the top of the caving zone and the first mudstone section at the bottom of the Luohe formation.Based on the fact of the existence of WICCZ in the Jurassic strata,the bed separations that can accumulate water have been redefined,and 10 bed-separation types that can accumulate water are proposed.The sedimentary characteristics of the bed-separation water hazards in Cretaceous can be summed up from top to bottom as "Bed-separation zone containing water +WICCZ + Water-inrush channel unclosed zone ".The water resistance of the WICCZ is the main cause of the water accumulation in the overlying bed-separations,and the penetration of the WICCZ is the direct cause of the Cretaceous bed-separation water influx into the mining face.(2)On the dynamic evolution analysis of bed separation: According to the fact that the pressure balance arch does exist in the overburden strata during the coal mining process,combining with some existing research results,the concept of triangle bed-separation area(TBSA)was put forward,and the scope of the TBSA was also given.Given the shortcomings in the traditional method,an improved method of determining the location of the separated strata was proposed: the stepwise comparison combination method(SCCM).By stepwise comparison and combination,the actual combined state of the rock strata could be determined,and the location of any separated strata was more accurate.A ladder-shaped composite beam calculation model(LCBCM)that could be used to analyze the dynamic evolution of bed separations was established by combining the TBSA with SCCM.The application results showed that the LCBCM can be applied to analyze the dynamic evolution of bed separations during mining and accurately predict the footage of occurring water inrush.(3)On the evaluation of bed-separation water hazards: It is considered that the four main factors related to the risk of flooding from bed separations in the Luohe formation are the thickness of mining,the average thickness of the rock strata in Luohe formation,the proportion of medium sandstone and coarse sandstone in Luohe formation,and the closed potential of fractures within WICCZ.The weights of these four factors are calculated by using the entropy weight method.The weight calculation results showed that the mining thickness plays a decisive role in the classification of water inrush form of bed-separation water hazards.Through the analysis of the differences between the existing water inrush cases,two bed-separation water hazard patterns of leakage and inrush are put forward.The pattern with the risk index less than 0.5 is the leakage type,and the pattern with the risk index greater than or equal to 0.5 is the inrush type.Through this study,the understanding of the Cretaceous bed-separation water hazards has been deepened,and a calculation model for dynamic evolution analysis of bed separation has been created,and the indexes of evaluation and classification of bed-separation water hazards are also provided.These research results are of great theoretical significance for enriching the related theories of bed-separation water hazards,and have great practical value for many coal mines whose safety may be threatened by such hazards.