The effect of shear stresses on pillar strength

Abstract

In mining, the loading environment on pillars is highly complex with combinations of normal and shear stresses. It is also understood that with an active and working face, constant redistribution of stresses will occur to maintain stress equilibrium. A rock sample subjected to uniaxial loading conditions can fail in either indirect tension or shear. A pillar in the mining environment would react in much the same way. However, as “confinement” is created at the pillar and hanging wall or footwall contacts, shear failure is commonly observed in failed pillars. A number of theories and equations have been derived trying to correctly calculate pillar strength in an underground design. These early attempts argued that the strength is only governed by the width of the pillar in relation to the mining height as well as a certain pillar “strength” constant (K-value). Both the coal and hard rock industries have adopted this design methodology for determining pillar sizes, all with relatively good success. Numerical analysis has shown that complex stress interaction occur between irregular shape pillars when not superimposed, highlighting the fact that superimposing of pillars should be treated with caution. As the stress environment changes, so does the loading on the pillar with preliminary results showing that the strength of the pillar will also be affected in the presence of shear stress. The research presented in this document shows that loading conditions that may affect pillar strength should be considered and detailed and special treatment should be given during the design in cases where shear loading may exist.