Failure height of overlying strata in lower slice mining under thick alluvium and thin bedrock
In order to study the failure height of overlying strata after mining in the lower slice working face under thick alluvium and thin bedrock in Zhaogu No.1 Mine,theoretical analysis,numerical simulation,borehole flushing fluid leakage observation and borehole peeping method were used.According to the empirical formula calculation,the development height of falling zone is 6.93～27.72 m and the development height of water conducted zone is 22.17～71.73 m after lower slice mining.The numerical simulation results show that,when the mining height of the lower slice is 2.5 m and the cumulative mining height is 6 m,the maximum caving-height ratio is 2.61.When the cumulative mining thickness is less than 6 m and the bedrock thickness is no less than 30 m,and the clay layer thickness is greater than 5 m,it can be considered that the clay layer thickness can block the upward development of the water conducted zone.According to the field measurement,when the cumulative mining thickness is 5.8 m,the falling zone is 15.26 m,the caving-height ratio is 2.62,the water conducted zone is 55.59 m,and the fracture ratio is 9.58.The law of the failure height of the overlying strata after the lower slice mining can provide a theoretical basis for determining the thickness of safety coal(rock)pillar of waterproof and sand control.
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