Question 3 of 6: Vertical Crater Retreat (VCR) — Development Cycle, Crater Blasting, Ore Loading and Backfill

Question 3: Vertical Crater Retreat (VCR) — Development Cycle, Crater Blasting, Ore Loading and Backfill (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

3.1 — VCR development/production cycle

Development begins with a raise connecting the bottom-sublevel drawpoint level to the top-sublevel drill drift, followed by a slot cut at the base of the intended stope to create the first free face. Production then proceeds bottom-up: long vertical blastholes are drilled from the top sublevel down to just above the current free face, a horizontal crater slice is blasted against that free face, the broken ore is drawn from the bottom-sublevel drawpoints, and the newly exposed rock becomes the free face for the next slice above — the stope thus retreats progressively upward from the slot toward the top sublevel, one crater slice at a time, until the stope is mined out.

3.2 — Livingston crater blasting theory applied to VCR

Drilling. Large-diameter (typically 150–250 mm) vertical or near-vertical blastholes are drilled from the top-sublevel drill drift the full height of the intended slice, using down-the-hole (DTH) hammer drills or large-diameter rotary/percussive rigs capable of holding a straight, large hole over that length.

Explosive loading. Rather than filling the hole with a long, cylindrical column charge (which behaves as a series of small charges along its length), a concentrated charge is loaded only at the toe of the hole, with substantial air-gap or inert stemming decking left above it — this approximates Livingston's idealised spherical (point) charge, whose crater-breakage geometry is what the crater-blasting theory is built around, rather than the cylindrical-charge breakage behaviour of a fully-loaded blasthole.

Initiation. Each concentrated toe charge is fired from the existing free face below it (the previously broken and drawn slice), so the blast breaks toward that free face and forms the characteristic crater shape, rather than firing into unrelieved, fully confined rock.

Delay timing to avoid "frozen" rock. Adjacent holes/rings within a slice are fired on separate, sequenced delays (tens of milliseconds apart) rather than simultaneously, so that each successive charge detonates only after the rock broken by the previous charge has had time to displace and genuinely relieve toward the free face; firing too many holes together into rock that has not yet relieved over-confines the later charges (they have no free face to break toward) and "freezes" — fails to break or is poorly fragmented — instead of cratering cleanly.

3.3 — Ore loading equipment and ventilation in VCR

Remote/teleremote LHDs. Load-haul-dump loaders operated by remote control (or teleremote from a safe location) muck the drawpoints beneath an open, largely unsupported crater stope, keeping the operator out from under the unsupported back. Slusher/scraper systems use a cable-hauled scraper to drag broken ore from a constrained or awkwardly shaped drawpoint to the ore pass, useful where drawpoint geometry does not suit an LHD. Grizzly/finger-raise gravity draw lets broken ore gravity-flow to a grizzly screen, oversize is reduced (secondary breaking), and the undersize is drawn through a chute by an LHD or slusher below.

Ventilation to these machines. Because the stope above the drawpoints is an open, unsupported void and remote LHDs may be tramming without an operator physically present, auxiliary ventilation (flexible ducting and a booster fan) must be extended from the sublevel main airway to within a short distance of the active drawpoint to clear diesel exhaust and residual blast fumes at the working face, rather than relying on the natural through-flow of the main circuit alone.

Advantages/disadvantages. Remote/teleremote LHD operation removes personnel from beneath unsupported ground and sustains high mucking productivity, but line-of-sight/camera-based remote tramming is slower and more prone to downtime than direct operation; slusher systems are cheap and simple but slow and limited to a straight pull line; and auxiliary vent ducting run close to an active drawpoint is exposed to blast damage and must be regularly repaired or replaced, adding a recurring maintenance cost.

3.4 — Low-cement tailings fill for recovery and dilution control

A low binder ratio (typically 3–5% cement) keeps the tailings fill cheap enough to place routinely, immediately, in every mined-out primary stope rather than being reserved for a few critical panels. This delivers two distinct benefits. First, because it is placed promptly, the fill provides a confined wall against the adjacent (secondary/pillar) stope while that stope is later mined — limiting wall slough and dilution into the ore being extracted, in exactly the way an unfilled open stope would not. Second, because every primary stope is filled, the pillars originally left between primary stopes can subsequently be mined out as secondary stopes with fill already in place on one or more sides, recovering ore that a permanent, un-mined pillar in an open-stoping-only sequence would otherwise sterilise — raising overall extraction well above what VCR without backfill could achieve.