Repeated 3D terrestrial digital photogrammetric surveys were conducted on hard rock mine pillars to characterize damage during secondary mining. Damage was quantified by measuring Depth of Failure, DOF, from spalling material and crack intensity, D21. These damage values were then correlated to visually assessed Pillar Damage Ratings and numerically estimated pillar loads. Measured damage, particularly Depth of Failure, correlated well with Pillar Damage Ratings. Numerical predicted pillar stress vs. damage exhibited the expected trend with shorter pillars being stronger. High resolution photographs and 3D models were also used to characterize damage mechanisms, which were found to be structurally constrained.
The Doe Run companies Southeast Missouri Operations consists of six room and pillar lead mines (Brushy Creek, No. 29, Casteel, Buick, Fletcher and, Sweetwater) (The Doe Run Company, 2014). The mines are hosted in the Carbonates and Shales of the Viburnum trend with mining depths varying from 180 to 300 m. Pillars are commonly cut square with widths varying from 7 to 9 m and heights generally between 5 and 18 m, resulting in extraction ratios between 80 and 85% (Roberts et al., 1999). Figure 1 presents an idealized stratigraphic section through the Viburnum trend and Figure 2 shows a plan view of the five areas spanning the three mines included in this study.
Secondary mining of pillars contributes significantly to Doe Run's total tonnage and lead output (28% and 37% in 2013, respectively). During secondary mining, load from extracted pillars is shifted onto adjacent pillars which can manifest as damage in the form of material spalling and cracking. To reduce the risk of secondary mining causing excavation instability, pillar conditions are assessed using the Pillar Damage Rating system by Roberts et al., (2007) (Figure 3). Mining engineers assess Pillar Damage Ratings prior to commencing extraction in an area and at key points during extraction sequences. These damage ratings are then used as input to calibrate Displacement Discontinuity numerical models using the NFOLD code (Golder Associates, 2007). The Pillar Damage Rating system takes into account the magnitude of spalling and the presence of large cracks in the pillar. Current understanding of pillar stress vs. damage response suggests that pillars which exhibit a Pillar Damage Rating of 4 or higher have begun to shed load to adjacent pillars.
