Yielding coal pillars - Field measurements and analysis of design methods

Yielding pillars are gaining increasing acceptance as a technique for improving ground control in coal mines. The field studies described in this paper provide insights into the failure mechanics of yielding pillars that may prove helpful in improving yielding pillar design. Analysis includes the derivation of expressions for the stress distribution within a yielding pillar implied by several well-known empirical pillar design formulas. The field studies were conducted in three narrow longwall pillars at two mines. The pillars were instrumented with vibrating wire stress meters, and, in one case, a sonic extensometer. As the longwall faces passed the pillars, measurements were made of vertical stress changes, changes in confining stress, and horizontal pillar deformation. The measurements, in particular the vertical stress profile measured in one pillar as it yielded, are compared with the predictions of several pillar strength models. The models include the analytical one developed by Wilson as well as those derived from the empirical pillar strength formulas. The paper concludes that the stress gradients derived from the empirical formulas provide good fits to the measurements, and that the analytical models used for yield pillar design would benefit from more reliable determinations of in situ material properties.