Roof sagging limit in an early warning system for safe coal pillar extraction

Field studies have found that, generally, a roof fall takes place after certain amount of roof strata sagging. Recent applications of auto warning tell-tale (AWTT) instrument as an early warning system at different depillaring panels found it to be simple, economical and efficient during the coal pillar extraction (called depillaring). AWTT is able to watch the set value of roof sagging continuously in time. However, the application of AWTT experienced two practical issues: (i) fixing roof sagging warning value before the fall of overlying strata and (ii) deciding horizon above which its anchorage point of AWTT needs to be installed. Different field experiences noticed that an anchorage horizon at 10 m depth from the ceiling of a gallery works satisfactorily for goaf edge. But fixation of the roof sagging warning value, before the fall of overlying strata, is found to be complex. Generally, the warning value for a given site conditions is determined by extensive field monitoring and correlation of the roof sagging value at the goaf edge with corresponding roof falls. Field studies found that modulus of elasticity and thicknesses of immediate roof are important parameters in deciding the warning value of the roof sagging. These studies also indicated that the variation in size of the left-out ribs and goaf span influence the warning value. These observed facts about the warning value for an AWTT are studied in laboratory using a numerical modelling approach in FLAC(3D). The results obtained through developed simulated models are found to be in close agreement with those of the field studies. Attempts are also made to quantify the influence of above-mentioned parameters over the warning value through collated results of different studied models. The obtained results of field and laboratory investigations are presented and discussed in this paper for strata control in a depillaring panel.