Ground control principals for longwall top coal caving mining

Ground control condition is one of the most important issues in mechanized longwall mining. The Alpu lignite field in Turkey presents a challenging situation because of its thick, weak and clay content surrounding strata. The purpose of this study is to figure out the ground control condition of the study area by the following steps: classification of the geotechnical units, rock mass classification, cavability index, required shield capacity and floor bearing capacity. Geotechnical classification of the strata layers and rock mass classification was determined by the lithology of the boreholes and laboratory test analyses of them. Caving behavior of the roof strata was predicted by the polish scientists method. Then by applying the “US National Institute for Occupational Safety and Health (NIOSH)” roof rating system at possible roof strata, results were compared. Roof strata were classified as “immediately caving” strata in all production alternatives. Required shield capacity was estimated by detached block method. The caving height was calculated based on the bulking factor of the lignite and roof strata. Low strength floor strata act as a limitation to mining height increasing in all production alternative. Possible mining height was determined as five and six meters with the LTCC method. Required shield capacity in each production alternative was raised by increasing cutting height. To avoid failure during the production, supports should be advance with pressure by touching the roof and soon after the cutting. In addition, cutting height should be limited to the critical height.