The performance of coal pillars designed to the squat pillar formula

For over 20 years South African collieries have used the safety factor formula, developed by Salamon and Munro (1967) , to design coal pillar geometries with very satisfactory results. The maximum depth of bord and pillar workings included in the derivation of the current safety factor formula was 220 m, the size of collapsed pillars did not exceed 16 m and the width to height ratio of collapsed pillars was less than 4. However, with the expansion of the coal mining industry, collieries have been developed outside the traditional mining areas and beyond the empirical range used in the safety factor formula. An extended formula, Salamon (1982) called the squat pillar formula, has been proposed to account for the rapid increase in strength of pillars with high width to height ratio’s. This paper gives a brief background to the squat pillar formula and reports on an investigation into the performance of full-sized pillars designed to the squat pillar formula. The monitoring programme included the measurement of stress across the coal pillars by 55 CSIR doorstopper strain-cells. The depth to the floor of the seam exceeds 500 m and the squat pillar design reduced the pillar width from 60 m, under the current design, to 28 m.