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In some cases, a major crown pillar is left in place to separate open pit and underground excavations within the same orebody. Consequently, crown pillar stability is then critical to ensure safe underground extraction. The crown pillar dimensions and stability are a function of a number of parameters. The most important are the width of the orebody, the stress regime, the blasting practices, the rock mass strength within the pillar, the overall extraction sequence (top-down or bottom-up), and whether backfill will be introduced into the system.

The actual crown pillar dimensions will depend upon the stress environment. Indications of high stress could include obvious signs of mininginduced stress fracturing or rock burst activity. High stresses may also be induced in otherwise low stress environments near the surface, due to the geometry of the orebody and the extraction ratio below and above the pil lar. In addition, if a crown pillar is situated within a stress shadow environment, consideration must also be given to potential unraveling due to loss of clamping across the pillar. As a general rule of thumb, for narrow orebodies
(<10 m), the crown pillar height or thickness is based on a width/height ratio of 1:1 plus 5–10 m. For orebodies wider than 10 m, the crown pillar heights are designed with a width/height ratio of 1:1 plus 20–25 m. However, numerical modeling is required to determine whether excessive stress concentra-
tions are likely to occur within a pillar.

A strategy to minimize the effect of stress and potential seismicity within crown pillars is to place cemented fill within the first stoping lift, thus allowing the recovery of all the ore and minimizing the buildup of stress. Alternatively, the crown pillar may be recovered early in the stoping life by incorporating the extraction of portions of the crown pillar above each individual stope extraction.

Sublevel Interval

The selection of a sublevel interval is controlled by a global economic deci sion that provides the lowest cost per tonne of ore for the mining method chosen for a particular stoping block. The selection of the sublevel interval is not always controlled by stope wall stability. In most cases, the sublevel interval is based on factors such as development cost, down dip orebody irregularity, the available drilling equipment, and considerations of rock mass damage from explosives (Figure 3.37).

The underlying criteria should be the control of dilution and the reduction of the ore loss, as increased sublevel intervals reduce the required sublevel development, but may increase dilution. Consequently, an assessment is required of the anticipated economic impact of ore loss and dilution for each particular sublevel interval. Although this is not an issue that is well understood, an attempt must be made during the economic evaluation to cost the additional development required to reduce the sublevel interval in order to minimize dilution and ore loss.

Access Crosscuts

Crosscuts are designed to provide access to the orebodies at the selected sublevel interval. In cases where the crosscuts are located within a regional pillar, they are designed to be directly above one another.

In single steeply dipping orebodies that are extracted using a single ramp access as shown in Figure 3.38, the access crosscuts are not fixed at a particular location along the strike of the orebody, but rather where the ramp intersects the sublevel elevation. In both cases, crosscut development must be maintained at minimum size and design shape in order to improve stability at the crosscut–orebody intersection. The best practice for construction is to anticipate the crosscut design position near the orebody hangingwall boundary, with the final mining cut taken under geological control. Probe drilling using a mobile drilling machine can be used for orebody delineation prior to the mining of the last development cut near the hangingwall of the orebody. Such a step may be required to avoid undercutting the hangingwall planes, and thus minimize any falloff during subsequent stoping operations.

Crosscuts also play a key role in orebody delineation and rock mass characterization as the orebody boundaries are delineated within the crosscut walls prior to the orebody drive breakoff. The geotechnical behavior of the stope boundaries can be predicted from the results of crosscut mapping.

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