Investigation into yield pillar behavior and design considerations

Adopting yield pillars has been considered an effective way of alleviating ground control problems and increasing production. The purpose of this research was to study the behavior of yield pillars and to develop the design criteria.

After a literature review, two 2-D finite element models were developed, each following a different non-linear approach. The first model adopted the successive iteration technique incorporated with the Mohr-Coulomb yield criterion. The second followed the elastic—plastic approach, implementing a generalized Von Mises yield criterion. Extensive underground monitoring was conducted and the finite element models were compared with the field data, both yielding promising results.

Three different longwall entry layouts were investigated. The yield-stable-yield pillar system was considered to be the best design. A parametric analysis was also performed. The triaxial factor and Poisson's ratio were found to be the most important material properties affecting pillar yielding.

The progressive failure hypothesis for pillar design was critically examined. The analysis suggested that the formulation defining the stress distribution in the yield zone under this hypothesis may be satisfied only in extreme cases and, therefore, the actual distribution can be different. An improved equation, describing the stress distribution in the yield zone, was derived by statistically analyzing the results of finite element simulations. The latter equation fitted the observed field data better than did the original equation, and it was further developed for estimation of yield zone width.

Consideration was also given to yield pillar design. Three possible yield pillar sizes were proposed in this paper. The maximum yield pillar size was considered to be twice the width of the yield zone. Based on the pressure arch concept, the minimum yield pillar size was determined by accepting that yield pillars were only supporting the rock strata under this pressure arch. A suggested yield pillar size was obtained by selecting a size which would force the peak stress at the center of the yield pillar to equal the average tributary stress. The case studies conducted in this research indicated that the predicted yield pillar sizes were reasonably accurate.