Question 7 of 7: Cut-and-Fill Stoping Variants and Backfill Practice

Question 7: Cut-and-Fill Stoping Variants and Backfill Practice (15 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.

7.1 Overhand/underhand and open/tight. Overhand cut-and-fill mines successive horizontal slices upward from a level, placing fill beneath each completed slice to serve as the working floor for the slice above — the common, default variant. Underhand cut-and-fill mines slices downward instead, so the (usually cemented) fill placed above the current slice must itself serve as a self-supporting artificial roof over the working face — used where the immediate back is too weak to stand unsupported, i.e. mining "under" a competent, artificially created crown. Open cut-and-fill leaves the current slice's span unfilled/exposed for an extended working period before filling, tolerable only in more competent ground; tight cut-and-fill fills each slice promptly and snugly against the back before the next lift starts, giving more continuous ground support at the cost of cycle time, and is used in weaker ground.

7.2 Major mine products used for fill. (1) Development waste rock generated by lateral development, shaft sinking and other unavoidable excavation, placed as rockfill (sometimes cemented) — a byproduct with essentially no incremental mining cost. (2) Mill tailings from the processing plant, deslimed/thickened and placed hydraulically or as (cemented) paste fill — using the mine's own tailings as underground fill both disposes of a portion of the tailings underground, reducing the surface tailings-storage footprint and associated long-term liability, and returns the material as engineered backfill.

7.3 Cement/binder use and high-early-strength fill. Portland cement (sometimes blended with fly ash or slag) is added to hydraulic or paste tailings fill to develop the compressive and shear strength needed for the fill to (a) stand as a temporary exposed vertical face once an adjacent stope is mined next, and (b) in underhand mining, act as a self-supporting artificial roof over the slice mined beneath it. High-early-strength (accelerated) cement mixes are specifically needed where the schedule cannot tolerate the weeks a standard mix takes to cure: Example 1 — underhand cut-and-fill under a weak back, where mining cannot proceed beneath the fill roof until it has gained adequate strength, so an accelerated mix minimises the schedule delay. Example 2 — a filled stope with an adjacent (not overlying) primary stope scheduled to be mined next, exposing the fill as a vertical face; without high early strength the mine must wait an uneconomically long cure period before the adjacent stope can be blasted.

7.4 Cycle of operations, mechanised overhand cut-and-fill. Drill the uppers (mechanised long-hole or jumbo drilling from the current fill/muck floor) → blast → ventilate to clear blast fumes → scale and install ground support (bolts/mesh) on the newly exposed back → muck the broken ore by LHD to the ore-pass → extend access ways (man-ways, ore/waste passes, ventilation raises) and construct fill bulkheads for the completed lift → place fill (hydraulic, rock or paste) to the design elevation → allow cure time if cemented → repeat the cycle for the next lift.

7.5 Captive versus ramp-access cut-and-fill. "Captive" cut-and-fill shares mobile drilling/mucking equipment among a small cluster of two to four adjacent stopes served by a local ore-pass/man-way system, without a continuous vehicle ramp reaching every lift — equipment is often smaller or specialised (raise-serviced, crane-assisted) and stays dedicated to that cluster. Ramp-access cut-and-fill instead develops a continuous ramp connecting every active stope and lift across an entire mining sector, allowing standard trackless LHDs and trucks to be driven directly to any working face in the sector (not just a captive cluster) and redeployed elsewhere in the mine as production needs shift.

7.6 Decline of captive cut-and-fill. Ramp access lets a mine run large, standard, fully mobile trackless equipment across an entire sector (or the whole mine) instead of committing smaller, specialised equipment to one captive cluster of stopes, giving far higher equipment utilisation, productivity and redundancy at lower unit cost; ramp access also improves egress and simplifies services distribution. As mechanisation with diesel, rubber-tired equipment became the industry standard, that equipment is poorly suited to a confined, non-ramped captive layout, so ramp-access cut-and-fill has displaced captive methods almost everywhere except very narrow-vein, small or short-life orebodies where a full ramp cannot be justified.

7.7.1 Rock strength and choosing cut-and-fill. Cut-and-fill is selected over cheaper bulk methods (open/longhole stoping, caving) specifically when the wall rock and/or ore is too weak to stand unsupported over the larger spans a bulk method requires; the fill provides continuous ground support that makes otherwise unmineable weak ground extractable, at the cost of the lower productivity and higher unit cost cut-and-fill carries relative to bulk methods.

7.7.2 Rock strength and choosing a cut-and-fill variant. The specific variant follows the location and degree of weakness: a weak back specifically (rather than generally weak walls) drives underhand mining under a cemented-fill roof; generally weak walls needing support with minimal delay drive tight (rather than open) cut-and-fill; and very weak or highly stressed ground may additionally require a higher cement content or high-early-strength fill (7.3) and smaller stope panels, independent of which overhand/underhand or open/tight variant is otherwise chosen.