Strength of rectangular pillars in partial extraction

Abstract Sandstone specimens of rectangular cross-section were tested to obtain a relationship between the effective width of these specimens, average of the two widths w1 and w2, and their compressive strength. The resulting straight line regression is shown to be similar to the variation in strength of simple square prisms with width-height ratio. The strength increase in rectangular specimens is shown to be a combination of the influences of width-height ratio and size. The effect of size on strength was reduced by preparing the rectangular specimens in such a way that they were greater than the ‘critical size’, this being the size beyond which a fall in strength is slow. It is ultimately shown that the strength of a rectangular specimen w1 × w2 × h is the same as that of a square specimen (w1 + w2)/2 × (w1 + w2)/2 × h. This equivalence is further proved by in situ strength trials on rectangular and cubic specimens of coal. A strength formula for rectangular pillars is proposed at the end, based on these laboratory and in situ findings. The formula shows that higher extractions can be achieved by forming rectangular stooks or adopting some wide stall method wherever partial extraction is contemplated.