Pillars are designed to ensure regional stability or local support in stopes and along drifts, or to yield
under a measure of control. In all cases, the strength of the material and the variations in strength
must be known both for the pillar and for the roof and floor. The stability in longwall faces depends
mainly on the interaction between the roof strata, face support, roadway support and dimensions of
pillars. The main aim of this paper is to apply rock mass classification systems to longwall pillar
design at Abu-Tartur mining area. The pillar load is estimated taking into account the physical and
mechanical properties of phosphate deposit and roof rock, panel width, mining height, depth below
surface.
Two methods from classification systems are used in calculation pillars stress and strength to pillars
design namely Geological Strength Index (GSI) and Rock Mass Rating (RMR) systems. GSI
values for immediate, main roof rocks and phosphate ores are determined from geological
conditions, as lithology, structure of the interlocking of rock blocks and the conditions of the
surfaces between these blocks. RMR value can be determined by correlation it with GSI system.
The pillar widths calculated by applying rock mass classifications (GSI& RMR) are 49m and
64m at a factor of safety 2 and panel width 100m with extraction ratios of 70 and 64 %
respectively.
The data used in calculations are collected from geological reports of the company and from
laboratory tests of phosphate ores and shale rocks in the roof.