Grouting of bed separation spaces to control sliding of the high-located main key stratum during longwall mining

The gob-side entry 150 – 180 m behind the 14201 working face at Majialiang coal mine is severely deformed. We developed a new technique to control the behaviour of the high-located main key stratum (HMKS) to improve mine safety. The failure type and breaking span of the HMKS were determined based on key stratum theory and the voussoir beam model. By monitoring the deformation of the entry and surface subsidence, we found that the main cause of the large deformation of the gob-side entry was sliding of the HMKS. A new technique is proposed to add grout between the separated beds during mining. Physical simulations indicated that this technique is efficient in controlling sliding of the HMKS and avoiding strong dynamic loading, with the peak abutment stress reduced by 59%. Grouting stations were set up at a spacing of 150 m behind the advancing work face based on the principles and key parameters of the technique and the geological conditions at Majialiang coal mine. Before the HMKS began to slide, we injected high-water content materials with a water to cement ratio of 1.5:1 into the bed separation space to prevent breaking of the key strata.