Question 5 of 6: Innovative Shaft Sinking — Deepening while Hoisting

Question 5: Innovative Shaft Sinking — Deepening while Hoisting (20 marks, optional — Section C)

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.

5.1 — Equipment for internal ore-pass hoisting and cross-cut mucking

5.1.1 Hoisting broken waste up the internal ore pass. Rock broken beneath the present shaft is drawn from the ore pass through a chute/gate onto a small AUXILIARY hoisting system dedicated to the sinking work — typically a single-drum sinking hoist and Kibble/skip (or, on a raise-type ore pass, a slusher/scraper drawing to a loading pocket feeding a small skip) running in the 1,000 m internal ore pass, entirely independent of the production Koepe winder in the main shaft. Because this equipment only needs to move the (much smaller) volume of development waste, not full production tonnage, it can be a modest-capacity single-rope hoist, sized and scheduled without any interaction with the production hoisting cycle above it.

5.1.2 Moving rock from the cross cuts under the present shaft. Within the cross cuts (which are limited-headroom, confined workings directly beneath active production infrastructure), rock is moved with low-profile LHDs (load-haul-dump loaders) or, where headroom is even more restricted, scraper/slusher mucking to a transfer point, then trammed to the internal ore pass loading point described in 5.1.1. Standard production-scale haulage equipment is deliberately avoided here both because of the confined geometry and because introducing large mobile equipment traffic directly under the live, hoisting shaft pillar would itself be a hazard the whole scheme is designed to avoid.

5.2 — Drilling and blasting the shaft extension in short segments

5.2.1 Opening ground in short vertical segments. Because the extension is being sunk from BELOW an already-standing, still-in-service shaft pillar rather than conventionally from the top down, the ground is taken in short (a few metres) vertical lifts, each one drilled, blasted and mucked as a self-contained segment before the next is started — effectively treating the shaft-bottom advance as a sequence of small benches rather than one continuous sinking round, which limits the unsupported span/height exposed beneath the live shaft at any one time and lets each segment's stability be confirmed before proceeding to the next.

5.2.2 Drilling, loading and blasting method. Because the geometry (breaking rock upward or in confined vertical lifts toward a fixed opening above, working from cross cuts) closely resembles stope mining rather than conventional top-down shaft sinking, the same techniques are used: ring or fan drilling from the cross cuts/access drives into each short vertical segment (rather than a full-face shaft-sinking jumbo drilling straight down), loaded with packaged or bulk explosive via the same techniques used in longhole stope blasting, and fired on a delayed sequence so each segment breaks toward the opening created by the segment below it. This stope-mining approach is chosen specifically because it lets the segment be opened from the SIDE (via the cross cuts), rather than requiring the sinking crew to work directly beneath the unsupported ground under the live shaft the way top-down sinking would.

5.2.3 Avoiding the ground "freezing" between segments. If each short blasted segment is not fully broken through to the segment below/beside it, the remaining solid rock "freezes" (locks) the broken muck in place and prevents it from drawing freely, jamming the sequence. This is avoided by ensuring each blast round's toe holes are drilled and charged to break FULLY through to the previously opened void (confirmed by survey/probe-hole before loading), by sequencing the blast timing so each new segment's break connects cleanly into the existing opening rather than leaving an unbroken pillar between lifts, and by immediately mucking each segment before drilling the next so broken rock never has the chance to re-compact or bridge across the opening.

5.3 — Removing the top section last without disrupting production

The top section of the shaft extension — directly beneath the STILL-OPERATING original shaft bottom and its loading pocket — cannot be removed until every other part of the new extension AND the new deeper loading pocket/hoisting arrangement are complete and commissioned, because that top plug is the only remaining rock structurally separating (and load-bearing for) the live shaft above from the new void being created below. Once the new shaft steel, timber, rope and deeper loading pocket are fully installed and the Koepe system is ready to hoist from the new pocket, hoisting is switched over to the new arrangement (a short, planned production changeover) and only then is the final top plug removed — by the same short-segment drill-blast-muck sequence as the rest of the extension — now that it no longer needs to support anything and production is already running through the new, deeper loading point.

5.4 — Replacing the rope and shaft infrastructure while still hoisting

Because the winder is a Koepe hoist, replacing the head rope(s) and installing new shaft steel/timber for the extension is scheduled as a SEPARATE, short-duration hoisting outage taken only once the new shaft steel, guides and loading pocket infrastructure in the extension are already fully installed and inspected — not attempted incrementally while hoisting continues. The new (longer) rope is pre-cut, pre-socketed and staged at surface so the changeover itself is a single, minimised-duration operation: production hoisting is stopped, the existing rope is disconnected at the capel, the new longer rope is run out and capped to the conveyance at its new (deeper) position, sheave alignment and rope tension/equalization are checked, and hoisting resumes — all shaft steel and timber installation for the extension itself is completed BEFOREHAND, during normal production hoisting, using the internal ore-pass/cross-cut access developed in Questions 5.1–5.2, so that only the rope-and-final-tie-in work needs an actual hoisting stoppage.

5.5 — Why hoist from the new pocket 200 m deeper

The mine has decided to hoist from the new loading pocket 200 m deeper (rather than continue using the old, shallower pocket once the extension below it is complete) because the entire purpose of the deepening project is to access ore and waste BELOW the original 600 m bottom — ore hoisted from the extension level must either be hoisted directly from its own (new, deeper) pocket, or first tramme/hoisted up an intermediate system to the old pocket, adding a redundant transfer step, extra handling cost and an additional point of mechanical failure/bottleneck for every tonne mined below 600 m. Hoisting directly from the new, deeper pocket also means the single Koepe system serves the FULL depth of the extended shaft going forward (not just the original 600 m), which is the entire economic point of extending the shaft rather than sinking and equipping an entirely separate new shaft.

5.6 — Plans and sections

A complete plan/section set for this work shows, at minimum: a LONG SECTION through the shaft from surface to 1,000 m depth showing the original 600 m shaft bottom, the internal ore pass down to 1,000 m, the cross-cut connections to the future shaft-extension location, and the sequence of short vertical segments comprising the extension itself; PLAN views at the original shaft-bottom level, at the cross-cut/ore-pass level, and at the new (200 m deeper) loading-pocket level, showing the ore pass, cross cuts and their relationship to the shaft centreline; and a section through the final loading-pocket/skip-dumping arrangement at the new depth, matching (or, where design has improved, updated from) the original pocket's geometry. The governing design principle shown on all of these is that the internal ore pass and cross cuts are offset laterally from the shaft's own load-bearing rock so that broken-ground development at depth never undermines the shaft pillar still carrying live production loads above it.

5.7 — Schedule and cost (critical path)

5.7.1 Flow sheet / critical path. The controlling (critical) path runs: sink the internal ore pass to 1,000 m → drive cross cuts to the future shaft location → drill/blast/muck the shaft extension in short segments (5.2) → complete the new loading pocket → the short rope-changeover hoisting stoppage (5.4) → finally remove the top plug (5.3), the only activity that MUST wait for everything else to finish. Shaft-steel and timber installation (5.4) is scheduled in PARALLEL with the segment blasting, using the same cross-cut access, so it does not extend the critical path provided its own duration stays shorter than the drill-blast-muck sequence it runs alongside — the classic critical-path technique of identifying which activities can be de-coupled from the sequential (rope, top-plug) work that genuinely cannot start early, as summarised in Fig. 5.7.

5.7.2 Time and cost estimates. Representative order-of-magnitude durations and costs for a project of this scale (600 m extension, competent ground, no lining): ore-pass sinking to 1,000 m raise-equivalent advance, several months at typical raise/shaft-sinking advance rates of 30–60 m/month; cross-cut development, weeks per cross cut at typical development advance rates (60–100 m/month per heading); the drill-blast-muck segment sequence for the ~400 m of new shaft length (1,000 m less the original 600 m), the longest single item, on the order of 8–14 months at a conservative sinking-from-below advance rate (slower than conventional top-down sinking because of the short-segment method in 5.2); steel/timber installation running concurrently with that item at no separate schedule cost; the rope-changeover stoppage itself measured in days, not weeks, precisely because all steel/timber/pocket work is finished beforehand; and the final top-plug removal, a few weeks. Costs scale with the same items — development/sinking advance rate cost per metre (typically the largest single cost category, driven by drilling, explosives, ground support and mucking labour/equipment), the auxiliary hoisting/mucking equipment described in 5.1, and the new rope/loading-pocket capital cost — and the whole schedule/cost estimate should be presented against the flow sheet of 5.7.1 so the client can see which activities genuinely drive the completion date versus which have schedule float.