Sublevel open stopes are created by the sequential blasting of production rings into an initial expansion slot, called the cutoff slot. This initial opening is used to create sufficient void for the remaining portion of the stope to break into (Figure 2.2). The cutoff slot is usually located on a side or in the center of a stope either transversally (across) or longitudinally with respect to the strike of the orebody. An important point relates to whether the cutoff blasting will expose a critical stope wall, such as a hangingwall or a fill mass, very early in a stope blasting sequence.

The cutoff slot raises are blasted upward from sublevel to sublevel in order to expose the full stope height. At each level, the expansion slots are formed by sequentially blasting parallel holes into an LHW or a raise-bored hole. The slot must be expanded to the full width of the plane defined by the production holes that will be subsequently blasted into this initial opening.

High powder factors are normally used during slot blasting in order to ensure breakage and thus have a free face and a void available into which the remainder of the stope is to be blasted. The choice of slot location depends upon rock mass conditions, stope access, and the extraction sequence chosen. In a steeply dipping orebody, where the critical stope boundary is usually an inclined hangingwall, transversally oriented slots are used to ensure sequential hangingwall exposure by the production rings. In large, massive orebodies, the choice of slot orientation is also controlled by factors such as fill exposures, stress regime, and preestablished access.

In general, a slot must be designed so that failure within the main or production rings is minimized. In highly stressed pillars, a slot can be oriented normally to the major principal stress to shadow the main production holes. This is likely to minimize hole squeezing or dislocation due to stress-related damage. In cases where a stope access can be redesigned, the slot should be placed normal to any large-scale geological features likely to fail and damage the main ring geometries.

Damage to fill masses from cutoff slot blasting can be minimized by placing a cleaner ring between a cutoff and a fill boundary (Figure 2.4). The rock mass adjacent to a fill mass is usually preconditioned by stress redistribu tions and is likely to fail following a cleaner ring blasting.

In order to minimize hangingwall failures, cutoff slots are oriented transversally to the orebody strike. This allows the hangingwall plane to be sequentially exposed within a predetermined stable range. In secondary stope extractions, where longitudinal cutoff slots may be located parallel (and adjacent) to a stope hangingwall, the expansion slot exposes a full hang ingwall plane early in a blasting sequence. This usually limits the size of exposures that can be safely excavated, as this critical wall of the stope may fail when subjected to repetitive dynamic loading by the rest of the stope firings as shown conceptually in Figure 2.5. In addition, when the stopes are accessed centrally, drill design requires that the holes toe into any adjacent fill masses, thereby increasing the likelihood of fill dilution.

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