Question 4 of 6: Cut-and-Fill Stoping — Overhand/Underhand, Fill Products, Captive vs. Ramp Access, and Ramp-in-Vein Resuing

Question 4: Cut-and-Fill Stoping — Overhand/Underhand, Fill Products, Captive vs. Ramp Access, and Ramp-in-Vein Resuing (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.

4.1.1 — Overhand/underhand and open/tight cut-and-fill

The overhand versus underhand distinction describes which direction mining advances relative to the placed fill: in overhand cut-and-fill, each new slice is drilled and blasted from the top of the previously placed fill, working upward lift by lift, so the miner always stands and drills on the fill surface below the new cut — the far more common arrangement, because it lets the fill cure and provide a stable working floor before the next lift is taken. In underhand cut-and-fill, each new slice is instead taken downward from beneath an already-placed, cured fill "roof" (or beneath a previously mined and filled lift), so the fill above must have gained enough strength to safely self-span as an artificial back over the new working void — used where the immediate rock back is too weak or the ground too highly stressed to safely expose, so the miner works under an engineered fill roof instead. The open versus tight distinction describes the relationship between the stope walls and the fill: in an open stope configuration the fill is placed with a visible working gap left against one or both walls (or the fill surface is left somewhat below the back), giving working clearance and drainage but leaving that gap as a potential dilution and wall-instability source; in a tight configuration the fill is placed hard against the walls and up to the back with minimal gap, maximising wall support and minimising void-driven slough at the cost of more careful (often paste or hydraulic) fill placement to actually achieve full contact.

4.1.2 — Fill products, additives, and high early strength fill

The two major bulk fill products, together making up well over 80% of most cut-and-fill operations' fill volume, are: hydraulic (sand) fill — deslimed mill tailings or a dedicated sand product transported underground as a water slurry through a pipeline and dewatered in place through a barricade/decant system, valued for low cost and use of an otherwise-waste product; and rock fill (development waste or quarried aggregate, often combined with hydraulic or paste fill as a composite) — valued for high bulk strength and ready availability from the mine's own waste-development muck. Additives (typically Portland cement, and sometimes fly ash or other pozzolans), used at under 5% by weight, are blended into the fill primarily where the fill must develop enough standing or early strength to be safely exposed as a working floor or a self-supporting wall/roof, rather than simply occupying the void — plain uncemented hydraulic fill has essentially no cohesive strength and can only be left as a passive wall-support medium, never exposed on a free face.

4.1.3 — Cycle of operations in mechanised cut and fill

The mechanised cut-and-fill cycle repeats, lift by lift: (1) drill the current lift's blastholes from the cured fill floor of the previous lift, typically with a mechanised jumbo; (2) blast the drilled round, breaking a slice of ore typically 3–4 m high across the full stope width; (3) ventilate/clear the blast fumes before re-entry; (4) muck the broken ore with a scoop-tram/LHD, tramming it to an ore pass or direct to a haulage vehicle; (5) scale and support the freshly exposed back and walls (bolting/mesh/shotcrete as required) before personnel work beneath them for the next steps; (6) place fill — hydraulic, paste or rock fill (with cement additive as needed per 4.1.2) — back into the mined-out void up to the design fill level, typically leaving working clearance to the new back; and (7) allow the fill to cure/drain (dewater through a barricade for hydraulic fill, or cure structurally for cemented/paste fill) before the cycle repeats on top of it. Because step 7 imposes a real time delay before the next drill pass can safely proceed, the fill cure schedule is frequently the pacing item in the whole cycle, not the drill-blast-muck sequence itself — exactly the scheduling constraint flagged in Question 1.4.

4.1.4 — Captive cut-and-fill versus ramp access

Captive cut-and-fill confines a small, dedicated equipment set (drill, small LHD/slusher) permanently within one or a very few adjoining stopes, reached via a raise or small internal access rather than a full vehicle ramp — equipment is essentially "captive" to that stope, moved between stopes rarely if at all (sometimes lowered/raised by hoist or crane through a raise). Ramp access instead drives a continuous, gently graded ramp connecting the surface or main haulage level to every active lift of every stope, allowing standard rubber-tyred mobile equipment (LHDs, jumbos, trucks) to drive freely between stopes and lifts under their own power. Captive cut-and-fill has decreased substantially in favour of ramp access because: mechanised, rubber-tyred equipment is dramatically more productive per operator-hour than the small, often rail- or slusher-based captive equipment it replaces, but that equipment needs ramp access to be usable at all; ramp access lets a single mobile equipment fleet be shared and reallocated across many active stopes as production needs shift, instead of being stranded in one captive working area; and ramp access gives much better emergency egress and materials/personnel access than a raise-served captive working. The trade-off is that ramp development is a substantially larger up-front capital and time investment than a small captive raise — which is exactly why captive cut-and-fill persists in narrow, marginal, or short-life stopes where the extra ramp development cannot be justified.

4.2.1 — Resuing and its economics

Resuing is a selective mining technique for a narrow, high-grade vein thinner than the minimum practical mining width: rather than blasting the vein alone (which would leave an unworkably narrow, unsafe opening) or blasting the full minimum mining width in one pass (which dilutes the ore with a large proportion of barren wall rock), the miner blasts and mucks the barren hangingwall (or footwall) rock separately from the ore vein itself, in two distinct passes within the same slice — typically drilling and blasting the waste side first to open a full-width working space, then carefully drilling and blasting the narrow ore vein on its own round, keeping the two muck streams separate. For the 1 m rich vein here, resuing directly improves economics by keeping the diluting waste rock (blasted separately) out of the ore muck stream entirely, so the mill only ever sees near-vein-grade material rather than a diluted blend — for a "very rich" vein, avoiding dilution of high-value ore with low-value waste has a far larger economic impact than the modest extra drilling/mucking cost of the second, separate pass.

4.2.2 — Ramp-in-vein cut and fill mining sequence

The mining sequence develops a narrow ramp within or immediately beside the vein from the bottom of the panel upward, breaking out into a working slice at each lift; on each lift the resuing sequence of 4.2.1 is carried out (waste side blasted and mucked separately from the ore vein), the exposed back is supported, cemented fill is placed to re-establish a trafficable floor and the ramp is extended up through that new fill to reach the next lift — repeating lift by lift as the ramp switches back and forth up the narrow panel, always staying within the panel width the 1 m vein and its resuing allowance define, so no full-width level development is needed to serve each lift the way conventional cut-and-fill would require.

4.2.3 — Equipment and positioning for ramp-in-vein resuing

Because the vein is only 1 m wide (resued to perhaps 2–2.5 m of practical working width), all equipment must be narrow-profile: drilling uses small, narrow hydraulic single- or twin-boom jumbos (or, in the narrowest panels, a pneumatic stoper/jackleg drill) sized to the resued opening rather than a standard-width production jumbo; loading uses a compact, narrow-chassis LHD (commonly a 1–3 t bucket "mini-LHD" purpose-built for narrow-vein work) that can turn and muck within the resued width and travel the switchback ramp; and haulage from the ramp bottom to the shaft or main haulage level uses a correspondingly narrow-chassis underground haul truck, or in very confined panels the mini-LHD trams the ore out to a transfer point itself rather than loading a separate truck underground. All three machines are positioned and operated along the ramp axis running parallel to (and within) the vein strike, working the ore-vein face at the up-ramp end of each lift while the waste-side muck from resuing is kept and hauled separately, exactly as described in 4.2.1, so grade control at the loading step is preserved all the way to surface.

4.2.4 — Cemented fill placement and avoiding fill

Cemented fill placement. Cemented paste or hydraulic fill, batched at a surface or underground fill plant, is piped or poured down a fill hole/raise connected to the level above directly into the mined-out lift, filling to within working clearance of the back; a bulkhead/barricade at the lift's lower access retains the slurry while it drains and cures to the strength needed to carry the ramp and equipment loads of the next lift (as in 4.1.2/4.1.3). Avoiding fill is possible in a narrow, short-life panel by substituting dry waste-rock fill tipped directly from development muck (no fill plant, pipeline or cure-wait needed, though the resulting surface is less uniform and less immediately trafficable) or, where ground conditions and mine sequencing allow, by leaving the resued panel's small void entirely open and unfilled (relying on the narrow span and competent walls to stand unsupported) — a viable option only because the panel is narrow enough, and short-lived enough, that the stability and subsidence risk a wider or longer-life stope would carry from an open void does not apply.