NFOLD is based on the displacement discontinuity stress analysis method whereby the ore body is modeled as a planar feature with the thickness of the ore body located within an infi nite elastic material (the host rock). The method has its greatest application in determination of stresses and displacements asso- ciated with tabular ore bodies, such as the Missouri lead belt.
The elements characterizing the mining horizon may be either elastic (in which failure of the pillar is suppressed), material. with postfailure or the The strengths assigned to pillars depend primarily o the strength of the rock and the geometry the pillars. In the Missouri lead belt models, is initiated in a freestanding pillar, the pillar is assumed to follow a falling stress-strain to complete failure, with little or no residual strength. This is consistent with observations pillars that have actually The effect of trapping a pillar with backfill on the postfailure response of the pillar is one of the points of discussion of the following sections. Pillar strengths for NFOLD modeling were initially based loosely on Hedley's formula, in which
Strength= Kx UCS x (Wi/2/H 4),
where Strength =ultimate pillar strength
K= constant
UCS= uniaxial compressive strength of rock
W= pillar width.
H= pillar height
This strength equation was found to represent satisfactorily the observed response of Missouri lead belt pillars between 10 and 15 m high. However, when the equation was used for pillars outside this range, it was thought to be conservative, particularly for pillars greater than 15 m high. This was resolved by further calibration