Stope wall falloff and its subsequent influence on production efficiency is largely controlled by geology. Consequently, the stope design philosophy must consider the influence of any large geological feature intersecting a stoping area. The stopes must be designed to minimize falloff, rather than to maximize direct short-term cost savings by utilizing existing development that may force unfavorable positioning of blasthole rings, cutoff slots, retreat directions, and sequencing of the blast within the stope itself.

Priority must be given to the analysis of the geological structures and their influence on production blasting and stope wall stability. Short-term savings on development may subsequently lead to poor fragmentation, falloff, and production losses several orders of magnitude higher than the savings on development.

Production Rings

Ring blasting establishes the location of blastholes in relation to the drill drives, the orebody, and, most importantly, the planned stope outline. The designed volume and shape of the stope to be blasted as well as the positions and shapes of the drill drives and production mucking horizons are established for each ring section. Each individual ring design layout consists of a section through the orebody. The information presented in a ring design consists of the collar positions and the lengths and angles of the holes to be drilled and blasted. The hole size, the amount of explosives used on each
hole, and the tonnes fired in each ring can also be indicated. In addition, the lengths of any uncharged collars on the holes are also provided. A plan view of the drill drives is used to determine the position of the cutoff slot in relation to the rings as well as the burden on the rings.

A flexible blast design is one that allows the engineer a choice of single or multiple ring firings avoiding significant undercutting of stope areas. Blasting of the initial rings around the cutoff slot creates enough room for the remainder of the stope to be blasted. Considerations such as the level of the induced stresses and production and access constraint requirements are taken into account to determine the number of rings to be blasted together.

Important information such as the actual firing sequence, blasting results (fragmentation, freezing of holes, misfires, etc.), and any stope wall failures related to blasting must be recorded during ring blasting.

Diaphragm Rings

Diaphragm rings are used where there is a moderate to high probability of fill exposure failure. Diaphragm ring design is complicated by issues such as different drilling and blasting techniques, different exposure sequences, varying stress regimes, and containment of anything from cemented to uncemented fill. Diaphragms are potentially unstable where undercutting of the diaphragm by the main rings is experienced. This may occur due to poor drilling resulting in hole deviation. In addition, failure may occur when a weak geological structure intersects a diaphragm in an unfavorable orientation or when extraction from previous stoping has damaged the rock mass within the diaphragm sufficiently to reduce stability.

Another factor that assists diaphragm design is accurate knowledge of the backfill–rock interface. This knowledge would allow a proper determination to be made of the diaphragm thickness, in cases of uneven and sometimes overhanging fill masses. Stope surveys using the cavity monitoring system must be conducted following stope completion. However, stope wall falloff may still occur after the final stope survey, and probe drilling may be needed to accurately determine the actual rock–fill interface.

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