NUMERICAL MODELING OF YIELDING CHAIN PILLARS IN LONGWALL MINES

The practice in thick seam long wall mining at depths of 500 m and more shows that considerable sloughing of chain pillars, sometimes accompanied by bumps, may occur during development stage. The ability of these pillars to continue functioning as roof support during development and longwalling stages has to be assured for a safe and economical mining operation. Although already being used, little is known quantitatively about the response of these “yielding” chain pillars to varying loading conditions that come about during different stages of longwall mining. The paper presents the results from case history reviews and numerical modeling studies performed with the objective of improving the understanding of yield pillar behavior. The case history review indicates that yield pillars with width:height ratios 3 to 6 performed satisfactorily. The numerical modeling studies using a three-dimensional finite difference code FLAC3D [1] allowed realistic simulations of complex mining geometries and loading conditions of longwall mining. The numerical model incorporates a strain-softening constitutive model for post-peak pillar behavior and a non-linearly elastic material behavior for simulating gob compaction. The differences in performance of pillars with strain softening material and traditional Mohr-Coulomb material are also highlighted. bearing capacity, onset and development of fracturing in pillars and potential for unstable failures if pillars are incrementally load beyond their capacity. The studies mostly assumed 2D geometries and often made assumptions with regard to loading conditions during various mining stages and behavior of pillars in the post-peak regime. The influence of gob behavior on pillar loading is almost always ignored in the analytical studies. Current numerical models can deal with most of the difficulties encountered in analytical methods by allowing incorporation of 3D geometries and experimentations with non-linear materials such as strain softening for coal seam and compacting caved material in the gob area. The paper starts with presenting the results from a review of yield pillar applications as practiced in four deep longwall mines. A 3D numerical model of longwall mining is then described and the results obtained from its use are presented. The strainsoftening constitutive behavior and non-linearly elastic gob compaction models used for the modeling studies are discussed. The differences in modeling of yielding pillars between using strain softening and traditional Mohr-Coulomb plasticity models are also highlighted.