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

Question 6: Vertical Crater Retreat (VCR) — Development, Blasting Cycle, Tailings Fill and Ore Loading (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.

6.1.1–6.1.4 — Development sequence: cross-cuts, stope access, starter stope, peak production

6.1.1 — Initial cross-cuts and drifts from the shaft. Before any stope development, horizontal cross-cuts are driven off the shaft (or shaft station) at each level the VCR stope requires — typically a top-sublevel drill/access cross-cut and a bottom undercut/extraction-level cross-cut — establishing the horizontal access from the shaft to the future stope location at both elevations.

6.1.2 — Initial stope access: the "raise" and "slot." A vertical or steeply inclined raise is developed between the top and bottom cross-cuts, giving the first physical connection through the full height of the future stope and, critically, a vertical opening the initial blastholes can break toward. From that raise, a full-height slot is slashed (blasted outward) to the full width (or a substantial fraction of the width) of the planned stope, creating a genuine open vertical slot face across the stope footprint — this slot face is what every subsequent crater blast in the stope breaks toward, since a crater charge needs a free face reasonably close by to break effectively (Part 6.2.2 develops why).

6.1.3 — Starter stope. The first horizontal crater blasts are fired against the open slot face created in 6.1.2, breaking the first one or two horizontal slices across a limited portion of the stope footprint immediately adjacent to the slot — this small initial blasted volume is the starter stope: it is deliberately kept small (a fraction of the eventual full stope footprint) because the first slices have only the narrow slot as a free face to break toward, and it establishes the working void the stope then retreats and widens from as blasting proceeds outward across the rest of the footprint.

6.1.4 — Stope at the peak of production. Once the starter stope has opened a full-width working void, successive horizontal crater slices are blasted retreating upward across the entire planned stope footprint (not just the area adjacent to the original slot), each new slice breaking freely down into the growing open void beneath it exactly as a crater charge is designed to do; production is at its peak once the stope has reached this full-footprint, full free-face condition and multiple drill/blast/draw cycles can proceed across the width of the stope essentially without the initial slot-face constraint of 6.1.3.

6.1.5 — Stability support and ore losses

Because VCR leaves an open void as blasting retreats upward, stability is managed primarily by orebody/wall-rock selection and pillar design rather than installed ground support within the stope itself: rib and/or crown pillars are left, sized from a stability-graph span analysis (e.g. Mathews/Potvin) against the stope's actual dimensions and rock mass rating, to keep the unsupported open span within what the host and ore rock can stand for the stope's exposure life; installed support (cable bolting, shotcrete) is concentrated in the permanent top and bottom sublevel access and drawpoint drifts, which must remain serviceable for the stope's full life, not in the open stope walls themselves. Temporary ore losses are broken ore left inside the stope void that is not immediately drawn — caught in irregular muck-pile geometry or behind a temporarily inaccessible drawpoint — and can often still be recovered later as draw continues or during final cleanup. Permanent ore losses are ore left in rib/crown pillars that are never recovered (or only partly recovered, per 6.1.7) because full pillar removal would compromise stability of the mined-out area, plus any ore buried under wall slough/dilution that cannot economically be separated back out once mixed with waste.

6.1.6 — Sequence of mining a set of stopes

Across a set of VCR stopes, primary stopes are mined first, generally sequenced to advance away from existing infrastructure toward the orebody extremity (or top-down/bottom-up per the specific mine's stress-management plan), leaving rib pillars standing between each primary stope and its neighbours; each primary stope is fully drilled, blasted in retreating crater slices and drawn out (6.1.1–6.1.4) before its neighbour on the far side of a pillar is started, so that at any time an active stope has an already-mined-and-backfilled (or still-standing-pillar) boundary on one side, limiting how many simultaneously open, unsupported voids exist adjacent to one another. Only once enough primary stopes (and, where used, their backfill) are in place to provide adequate ground support does secondary pillar recovery (6.1.7) begin on the pillars left between them.

6.1.7 — Pillar reclamation and mining

Rib and crown pillars left between primary stopes are recovered, where the mine plan allows, once the primary stopes on both sides are mined out and (commonly) backfilled — the backfill in the adjacent stopes gives the pillar lateral confinement it did not have during primary mining and allows a higher-risk secondary extraction to proceed. Pillars are typically drilled and blasted with the same crater or longhole technique used on the primary stopes, often in a single retreating pass working away from the mine's main infrastructure toward the edge of the reclaimed area, so that ground relaxation/subsidence from the pillar's removal propagates away from, rather than toward, the areas still needed for access. Not every pillar is fully recoverable: a pillar needed as permanent crown support beneath surface infrastructure, or one whose recovery risk (given adjacent backfill quality or overall ground conditions) is judged too high, is left as a permanent loss (feeding back into the "permanent ore losses" of 6.1.5).

6.1.8 — 'Permanent' stabilization of the mined-out area

Once a VCR panel (and any recoverable pillars within it) is fully mined out, permanent stabilization is achieved by one or a combination of: backfilling the void with cemented rock or hydraulic/paste fill to re-establish long-term regional support and limit subsidence propagation toward overlying or adjacent workings; controlled caving/subsidence, where the void is deliberately left open (or partially filled) and allowed to relax and eventually cave to surface within a pre-planned, monitored subsidence zone, used where surface impact is acceptable and full backfill is not economically justified; and ongoing geotechnical monitoring (extensometers, seismic microphones, periodic survey) of the mined-out area and any remaining crown/rib pillars for as long as the area could still influence active workings or surface infrastructure, since a void left open (even a filled one, to a lesser extent) can continue to redistribute stress into adjacent ground for years after mining stops.

6.2.1 — Low cement ratio tailings fill to improve recovery and reduce dilution

Low cement ratio (just enough cement to give the tailings fill a stable, load-bearing but still low-cost surface, not full structural strength) tailings fill is placed to keep pace with the draw as the stope is progressively mucked out, so each successive crater slice is blasted onto a firm, reasonably level fill floor instead of onto an irregular pile of loose, previously-broken muck. This directly improves ore recovery because broken ore sitting on a firm fill surface stays recoverable at the drawpoints rather than working its way down into voids within a loose muck pile (a known VCR ore-loss mechanism), and it reduces dilution because a confined blast against a solid floor breaks more predictably to its intended geometry, rather than venting unevenly into the gaps of an irregular muck surface and pulling extra wall rock with it.

6.2.2 — Crater blasting cycle: drilling, loading, initiation and delay timing

C.W. Livingston's crater-blasting theory describes the ideal case of a single spherical (point) charge buried at the theoretically optimum depth below a free surface, which breaks a symmetric, cone-shaped crater outward to that surface with maximum efficiency; VCR approximates this ideal within the practical constraint of a large, flat horizontal slice rather than one buried sphere, and every element of the field cycle below exists specifically to get as close to that spherical-charge behaviour as a production blast pattern allows.

Drilling. Long, large-diameter (typically 150–250 mm) vertical blastholes are drilled downward from the top sublevel on a rotary-percussive or down-the-hole (DTH) hammer longhole drill rig, spaced on a square or staggered pattern across the slice being blasted; the large diameter is chosen specifically so that a single, compact, roughly cubical/spherical charge placed in each hole can approximate Livingston's ideal point charge, rather than the long, distributed column charge a smaller-diameter hole would force.

Explosive loading. Rather than filling the entire hole (a column charge), only a short, concentrated charge — a compact "powder factor"-calculated slug of bulk explosive (ANFO or a pumped emulsion) — is loaded at the calculated optimum burden depth below the current free surface (the top of the previously blasted slice or, for the very first slices, the slot face of 6.1.2/6.1.3), with the remainder of the hole above the charge left as inert stemming; this concentrated, compact charge geometry, positioned at the crater-theory optimum depth rather than distributed the full hole length, is the practical approximation of Livingston's spherical charge.

Initiation. Each hole's charge is fired using a non-electric (shock-tube) or electronic detonator with an in-hole primer/booster, tied into a surface trunk-line or electronic blast-control system that fires the pattern across the slice on a designed sequence rather than all holes simultaneously.

Delay timing to avoid "frozen" rock. Holes are fired on a staged delay sequence — commonly working outward from a free-face edge or in a diagonal/V pattern across the slice — so that each hole's charge always has a genuinely free, already-relieved surface to break toward from an adjacent hole fired a few milliseconds earlier, rather than firing into rock still fully confined by its neighbours. "Frozen" rock is the failure mode where a charge detonates against rock that has NOT yet been relieved by an adjacent blast (insufficient delay, a misfire in an adjacent hole, or too short/wrong-direction a delay pattern), so the charge's energy is absorbed in crushing and cracking locally rather than displacing a clean crater slice — giving poor fragmentation, a rough, uneven new free surface for the next slice, and sometimes leaving unbroken "frozen" remnants embedded in the muck pile that must be re-drilled and re-blasted at extra cost. Correct delay sequencing, calibrated to the pattern's actual burden and spacing, is what keeps every hole's charge breaking against genuinely relieved rock and is the single most important practical control over how closely a real VCR blast approximates Livingston's ideal spherical-charge crater.

6.2.3 — Ore loading methods, ventilation, advantages and disadvantages

Load-haul-dump (LHD) loaders are the dominant VCR ore-loading machine: a diesel or battery-electric LHD works the bottom-sublevel drawpoints, mucking broken ore directly into an ore pass, transfer point or waiting haul truck. Advantages: highly mobile and flexible, able to service many drawpoints across a panel from a single machine, and productive at the tonnage rates a VCR stope's large blasted slices deliver. Disadvantages: diesel units are a significant heat and diesel-particulate/exhaust-gas source right at the draw level, driving a large fraction of the panel's total ventilation air demand, and repeated LHD passes across an unstable or wet drawpoint brow carry real entrapment/collapse risk.

Slusher (scraper) systems — a scraper bucket hauled back and forth across the drawpoint floor by a double-drum winch and cable — are used in narrower or older VCR operations, or as a secondary system feeding a central ore pass. Advantages: the winch and operator can sit remotely from the draw point itself, reducing personnel exposure to an active, potentially unstable drawpoint brow, and slusher systems are simple, low-maintenance and effective at consolidating ore from several closely spaced drawpoints to one loading point. Disadvantages: much lower productivity (tonnes/hour) than an LHD, cable wear and snagging on broken rock are a recurring maintenance burden, and the fixed winch/cable layout is far less flexible to relocate as the draw pattern shifts than a mobile LHD.

Gravity draw through remote-controlled drawpoint gates/chutes feeding directly into an ore pass (no loading machine physically enters the draw area at all) is used where drawpoint ground conditions are too poor or the geometry too constrained for safe LHD or slusher access. Advantages: removes personnel and equipment entirely from the highest-risk zone (directly beneath an active drawpoint brow), and needs minimal mobile equipment investment at that specific point. Disadvantages: gives essentially no control over draw sequencing or blending between drawpoints beyond simple gate open/close timing, and hang-ups in the chute above a remote gate are far harder to clear safely than a hang-up an LHD operator can see and work around directly.

Ventilation for these methods is provided primarily by auxiliary ducting (rigid or flexible fan-forced ducting) extended from the main sublevel airflow directly to each active drawpoint/loading area, sized to clear diesel exhaust and blast fumes from the specific equipment in use — a much larger auxiliary air allocation is needed where diesel LHDs are the loading method (to dilute engine exhaust and particulate at the draw point itself) than for a slusher or a remote gravity gate, where the main sublevel ventilation current alone, supplemented by lighter local auxiliary ducting, is normally sufficient since no diesel engine or continuously present operator is at the draw point.