Question 4 of 6: Room-and-Pillar and Vertical Crater Retreat (VCR) Mining

Question 4: Room-and-Pillar and Vertical Crater Retreat (VCR) Mining (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 — Geology, geometry and rock-strength comparison

Room-and-pillar is suited to flat-to-gently-dipping, tabular, laterally extensive orebodies (coal seams, flat-bedded evaporite or industrial-mineral deposits, some flat vein systems) of moderate thickness, where the roof and the ore itself are competent enough to stand unsupported over a room span while a regular grid of pillars is left to carry the overburden load; pillar size and room span are set directly by tributary-area pillar-strength design against the overburden depth. VCR, by contrast, is suited to steep-to-vertical, thick, massive and reasonably competent orebodies (the walls must stand open, unsupported, over the full height of a lift between sill drifts) where a near-vertical large-diameter blasthole can be drilled the full lift height; VCR needs no permanent pillar grid because the ore column itself is retreated upward in full, so its rock-strength requirement is different in kind — competent WALL rock to stand an open stope, rather than a competent ORE pillar left standing after mining, is what the method depends on.

4.2 — Development and production cycle: room-and-pillar vs. VCR

4.2.1 Initial cross-cuts and drifts from the shaft. Both methods start identically: a main haulage drift is driven from the shaft (or decline) toward the orebody, with cross-cuts branching off at intervals to intersect it. In room-and-pillar this access drift is driven directly into the flat orebody at the mining horizon; in VCR the same access drift instead reaches an undercut/haulage level below the orebody, since VCR stopes are mined from the bottom up.

4.2.2 Initial stope access. Room-and-pillar: development headings simply fan out into the seam itself, each heading becoming the first "room." VCR: a vertical or near-vertical slot raise is driven the full height of the planned stope, plus a top sill drift above the stope and an undercut/drawpoint level below it — this slot is the essential free face every subsequent VCR ring needs to break into.

4.2.3 Starter stope. Room-and-pillar: the first room is simply the first heading driven wide enough to constitute a production opening, with an adjacent pillar left immediately. VCR: the slot raise is widened into a full-width slot by blasting a first set of vertical blastholes into it, creating the open void the first production ring can then retreat against.

4.2.4 Stope(s) at the peak of production. Room-and-pillar: many rooms are mined simultaneously across the panel on a regular grid, each a self-contained working face, so peak production comes from a large NUMBER of small, parallel openings. VCR: peak production comes from a small number of LARGE stopes, each retreating upward one full ring at a time through blasthole rings drilled the entire lift height from the top sill — a single stope can sustain very high tonnage per blast because each ring breaks the full stope cross-section at once.

4.2.5 Support for stability; ore losses. Room-and-pillar: stability is provided entirely by the LEFT PILLARS (a permanent, planned ore loss — often 25–50% of the in-situ tonnage, depending on depth and rock strength) plus roof bolting of the room backs; temporary ore loss also occurs in rib/barrier pillars left between panels. VCR: stability during mining relies on the open stope walls' own competence (no permanent pillars are planned within a stope), with cable bolting of the walls/back where spans are marginal; ore loss is mainly TEMPORARY, in sill/crown pillars left above and below an individual stope until backfill allows their eventual recovery (Part 4.2.7), plus unavoidable stope-wall dilution.

4.2.6 Sequence of mining a set of stopes. Room-and-pillar: rooms are typically advanced on a regular retreat or advance grid across the panel, often leaving every second or third room initially unmined to preserve panel-scale stability until pillar recovery begins. VCR: stopes are sequenced with UNMINED "buffer" stopes left between actively mining stopes (a primary/secondary sequence, akin to a checkerboard), so that an actively retreating stope is never immediately adjacent to another open void — each primary stope is backfilled before its neighbouring secondary stope is mined.

4.2.7 Pillar reclamation and mining. Room-and-pillar: once a panel's rooms are exhausted, pillars are recovered on a final retreat pass (pillar robbing/second mining), progressively removing pillars from the panel boundary inward while allowing the roof to controllably cave behind the retreat line — the highest-risk phase of the method. VCR: the temporary sill and crown pillars left in Part 4.2.5 are recovered as a final ring or set of rings once the stope has been backfilled and the fill has gained sufficient strength to support the adjacent, now-open pillar recovery blast.

4.2.8 "Permanent" stabilization of the mined-out area. Room-and-pillar: the mined-out panel is either left with a permanent barrier-pillar grid never recovered (true "permanent" stabilization, common where surface subsidence must be prevented) or is allowed to cave/subside in a controlled manner once pillar recovery is complete. VCR: the fully mined and pillar-recovered stope void is backfilled (commonly with cemented rock or paste fill) to provide permanent regional support and, where the stope adjoins a future stope, a stable, minable wall for that neighbour.

4.3 — Low-cement tailings fill in VCR

4.3.1 Backfilling a mined-out VCR stope with a LOW-cement-content tailings (or paste) fill, cured to only modest strength, gives the adjacent (secondary) stope's wall the support it needs to be mined with a WIDER open span and a steeper, tighter blast against the fill face than an unsupported rock wall would allow — because the fill actively confines and supports that wall during the neighbouring blast, less of the neighbouring stope's ore needs to be left unmined purely for wall stability, directly improving recovery.

4.3.2 The same confined fill face also lets the blast pattern be designed tighter to the fill/ore contact with less standoff, because the fill (rather than unsupported rock) absorbs blast-induced wall damage on that side — reducing the amount of WASTE rock pulled into the muck pile from an over-broken contact, which directly reduces dilution. Together, a modest, low-cost cement addition (just enough for handling/curing strength, not full structural strength) buys most of the recovery/dilution benefit of a fully engineered fill at a fraction of the cement cost, which is why low-cement tailings fill is the standard economic choice for this role rather than a high-strength paste.

4.2.1 (thick-seam) — Stope-and-pillar mining of thick seams

In seams thicker than about 6 m, "stope-and-pillar" mining differs from conventional (thin-seam) room-and-pillar chiefly in taking the FULL seam height in one pass with large, high-clearance production equipment rather than mining a thin bench and leaving the rest as roof/floor coal or ore — a practice that became viable in the 1960s once large-bucket LHDs and high-clearance drill jumbos capable of working a full 6+ m face height became available, in place of the low-profile continuous miners and shuttle cars suited to a thin seam. This produces much larger, higher rooms and correspondingly larger pillars (since pillar strength must now support a much taller overburden load-transfer column relative to pillar width), and it typically needs a benched or multi-pass extraction sequence within each room to blast and muck the full height safely, rather than the single-pass cut of thin-seam room-and-pillar. The trade-off is a much higher production rate and lower cost per tonne (fewer, larger openings extract the same reserve) against a harder roof-control problem (a taller unsupported span is inherently less stable) and coarser selectivity if the seam's grade varies with height.

4.2.2 (VCR crater blasting) — The Livingston crater and practical VCR blast design

C.W. Livingston's crater-blasting theory shows that a single spherical (point) charge, detonated at the correct "critical depth" of burial below a free face, breaks rock in a roughly conical crater whose volume is maximised at that specific burial depth — deeper and the charge's energy is wasted confining the rock; shallower and gas simply vents without breaking a full crater. Because a real production blasthole cannot carry a true spherical (point) charge, VCR practice APPROXIMATES it: a large-diameter (typically 150–250 mm) blasthole is loaded with a short, dense, roughly cubical/near-spherical charge (rather than a long, continuous column) positioned at the calculated critical depth below the current stope back (the free face), with substantial STEMMING (inert stemming material) above the charge confining it and separating it from the next charge up the same hole.

Drilling: large-diameter, vertical, full-lift-height production drills (down-the-hole or in-hole percussive rigs) drill the ring pattern from the top sill; ring spacing and burden are set from the crater-theory critical-depth/critical-radius relationship for the rock type and charge diameter. Explosive loading: each hole receives a discrete, short powder-factor-controlled charge (bulk emulsion or ANFO, pumped or poured to the calculated length) at the critical depth, not a continuous column, with the stemming placed immediately above it. Blast-pattern initiation: each ring is fired as a full ring in one blast (all holes in that ring simultaneous or closely sequenced), retreating the crater one ring's thickness upward with each blast, so the free face for the NEXT ring is always the freshly broken face left by the previous one. Delay timing: successive rings (and, on a wide stope, successive holes within a ring) are fired on carefully sequenced delays so each charge always sees a genuine free face to break toward; firing rings too close together in time (or with too little delay between them) leaves later charges detonating against rock that has not yet had time to relieve and move, producing "frozen" (unbroken or poorly fragmented) rock exactly as an over-confined charge in Livingston's own theory would predict.

4.3 (ore loading) — Modern loading equipment, services, and trade-offs

4.3.1 VCR ore loading. Broken ore reports by gravity to drawpoints at the base of the stope, where a load-haul-dump (LHD) loader, often diesel or increasingly battery-electric for a deep/hot mine, mucks it from the drawpoint to an ore pass or truck transfer point; remote/tele-operated LHDs are now standard for mucking directly under an open stope back, keeping the operator out from under unsupported ground. Services: ventilation must supply enough air to clear diesel emissions or, for a battery LHD, still enough for heat and dust; compressed air and water support drilling and dust suppression at the drawpoints; power is delivered by trailing cable to a fixed loading area or is self-contained on a battery machine.

4.3.2 Room-and-pillar ore loading. A shuttle car or, in a trackless operation, a rubber-tyred LHD/haul truck combination loads directly from the working room face (fed by a continuous miner or, in hard-rock room-and-pillar, from a drill-and-blast muck pile) and trams it to a central belt or rail loading point; because room-and-pillar openings are numerous, low and widely distributed across a panel, services (ventilation, power, compressed air, water) must be distributed to MANY simultaneously active low-clearance headings rather than concentrated at a small number of drawpoints, favouring a permanent, grid-like services layout advanced with the panel rather than the concentrated, stope-specific services layout VCR uses.

Advantages/disadvantages. VCR's drawpoint-concentrated loading needs far less loading equipment and far less services distribution per tonne mined (a major cost and productivity advantage at scale) but concentrates all of a stope's tonnage through a handful of drawpoints, so any drawpoint hang-up or equipment failure can stop an entire stope's production. Room-and-pillar's distributed loading across many simultaneous rooms is inherently more resilient to a single equipment or heading failure (production simply continues from other rooms) but needs proportionately far more loading units and a much larger services footprint to keep every active room supplied.

4.4 — Productivity, capital, rate and support-cost comparison