Research on development of water flowing fractures in soft overlying strata for coal mining under sea

Mining safety under sea is direct influenced by the height of water flowing zone in soft overlying strata in the Longkou mining area.Based on the analysis of engineering geological characteristics of soft rock in the Longkou mining area,the damage and deformation mechanism of soft overlying strata due to coal mining under sea were investigated by finite difference numerical simulation,scaled model test and measurements.The results indicate that soft strata still has a certain capacity for resisting water intrusion because of its low permeability and continuity after shear failure.The vertical profile of scaled model tests show that there are three areas of overlying strata after mining,which consists of shear failure area,tensile-shear failure area and tension failure area from top to bottom.The height of water flowing fractured zone increases with the coal seam excavation and eventually reaches to 48.2 m to 55.0 m when a 6 m thick seam has been totally mined out.An equation to calculate the height of water flowing fractured zone was derived according to these model and in-situ statistics.