8.5.2 Calibration
Numerous NFOLD models, including models of previous pillar failures, have provided the opportunity to calibrate estimated strength with actual underground observations. This calibration involves two independent steps.
The first step is to rate the condition of the pillars through underground observations. For this purpose, a pillar rating system has been developed whereby each pillar (or in some cases portions of pillars) is assigned a number between 1 and 6. A of represents a pillar for which no signs of stress are evident while a 6 is given to pillars that have completely failed
The second step is to model the area in question using the NFOLD program. This allows a pillar load to be estimated for the pillars, or portions of pillars, that have been assigned a pillar condition rating.
Once pillar condition ratings and pillar loads have been established, pillar load as a ratio of the UCS value of the rock is plotted against pillar height for standard pillar widths. Figure 8.7 shows the curves that have been developed by applying this method to hundreds of pillars in various areas at four of Doe Run's mines. This calibration is used to predict the progress of pillar dete. rioration and ultimately complete failure as adjacent pillars are mined uat. Mood