Underground metal mining

In short

Underground metal mining is the extraction of metalliferous ore bodies (lead-zinc, copper, gold, uranium, manganese, chromite) by driving openings in rock and removing ore from excavations called stopes, with the overlying rock either supported, backfilled or deliberately caved. The method is chosen from ore geometry, dip, ore and wall rock strength, grade distribution, depth and subsidence tolerance - not by preference. It matters because India's base-metal and gold reserves are deep and steeply dipping, so surface mining is not an option: Hindustan Zinc's Rampura Agucha, Sindesar Khurd, Rajpura Dariba and Zawar, HCL's Malanjkhand and Khetri, UCIL's Jaduguda and Narwapahar, and Hutti Gold Mines all run underground. It is a high-weight area in GATE Mining (MN), in UPSC/state Mines Inspector and DGMS statutory competency examinations, and in campus interviews for Hindustan Zinc, Vedanta, Hindustan Copper, UCIL, NMDC, Tata Steel, SCCL and mining contractors such as Barminco, Byrnecut and Thriveni.

Explained

Terminology and the classification of methods

• Stope: the excavation from which ore is actually produced. Development: the permanent openings (shafts, declines, levels, crosscuts, raises) that give access, ventilation and haulage. Stoping ratio / development ratio: metres of development per tonne or per 1,000 t of ore - the key capital-efficiency number.
• Level: a horizontal network at a fixed elevation; sublevel, an intermediate one. Drive follows the ore; crosscut cuts across it. Raise is driven upward (or bored) between levels, winze downward. Ore pass moves broken ore by gravity; drawpoint is where it is loaded.
• Pillars: *rib* (vertical, between stopes along strike), *sill* (horizontal, below a stope), *crown* (horizontal, above a stope), *post/room* pillar (in flat deposits), *shaft pillar* (protects the shaft), *barrier pillar* (isolates workings).
• Classification by ground support:
  1. Unsupported (naturally supported): rock stands on its own or on pillars - room-and-pillar, sublevel stoping, VCR, shrinkage stoping.
  2. Supported (artificially supported): cut-and-fill, shrinkage with fill, square-set, stull stoping - the void is filled or timbered.
  3. Caving: the rock is induced to fail and flow - block caving, sublevel caving, longwall (rare in metal).
• Selective vs bulk (mass) mining: selective methods (cut-and-fill, shrinkage) follow narrow or irregular ore and keep dilution low at high cost per tonne; bulk methods (sublevel stoping, caving) give cheap, highly mechanised, high-tonnage output but accept more dilution and less selectivity.

Room-and-pillar (flat to gently dipping deposits)

• Used for ore bodies dipping up to about 20-30°, of moderate to good strength, with a competent roof - e.g. limestone/dolomite, some manganese and base-metal flat lenses.
• Layout: ore is removed in rooms (or panels) on a regular grid, leaving pillars of ore (usually square or rib pillars) to support the back. Openings 5-15 m wide; pillar size from tributary-area loading and pillar strength.
• Operations: jumbo drilling of horizontal face rounds, blasting, mucking by LHD (load-haul-dump) to a truck or ore pass; roof bolting and mesh as support. In thick deposits, benching (breasting the top, then benching the floor) or post-pillar mining with fill.
• Recovery: 50-80% initially; pillar robbing or backfilling (post-pillar cut-and-fill) recovers more. Advantages: simple, cheap, flexible, fully trackless. Limitations: ore locked in pillars, pillar failure risk at depth (progressive/domino collapse), roof falls.

Stoping methods for steeply dipping ore bodies

• Shrinkage stoping: mining proceeds upward in horizontal slices from a sill; the broken ore (which swells about 40-50%) is retained in the stope and used as a working platform, with only enough drawn off (≈30-35%) to keep head room. Final draw empties the stope. Suits narrow, strong, regular, steep ore with strong walls. Advantages: little development, no fill needed, low capital. Limitations: labour works under a blasted back, ore hangs up and oxidises/segregates, poor selectivity, low productivity - now largely obsolete, retained in some narrow-vein Indian gold and base-metal workings.
• Sublevel stoping (longhole open stoping, LHOS): sublevels are driven in the ore 15-40 m apart; longholes (fan or parallel, 64-115 mm) are drilled between them, a slot raise is opened, and the stope is blasted in rings into an open void. Broken ore is drawn from drawpoints on the undercut level by LHD. Needs strong ore and strong walls and a regular ore body with sharp boundaries. Advantages: large, cheap, safe (workers never in the stope), highly mechanised, high productivity. Limitations: inflexible, dilution from blast overbreak, large standing voids needing stability analysis, pillars or fill to recover adjacent blocks. Dominant method at Sindesar Khurd, Rampura Agucha underground and Rajpura Dariba.
• VCR (vertical crater retreat) / VRM: large-diameter (140-165 mm) vertical holes drilled from a top sill over the whole stope; spherical charges are fired in short benches from the bottom upwards, retreating vertically, with broken ore left in the stope (crater retreat) or drawn continuously (open VRM). Needs no sublevels - lower development than sublevel stoping - but demands accurate drilling and good ore strength.
• Cut-and-fill stoping: mined in horizontal (or slightly inclined) slices, each cut backfilled before the next is taken; the fill becomes the working floor and supports the walls. Overhand (bottom-up, usual) or underhand (top-down, mining under a cemented-fill back, used in weak or high-stress rock). Drift-and-fill for weak ore: mine a drift, fill it, mine the next alongside. Mechanised cut-and-fill (MCF) with jumbos and LHDs on ramps is the workhorse for irregular, high-grade, weak-walled ore - Zawar, Kayad, parts of Dariba and Hutti/UCIL. Advantages: near-total recovery, very low dilution, excellent selectivity and ground control, no surface subsidence, tailings disposed underground. Limitations: highest cost per tonne, fill plant and distribution system, cycle time dominated by filling.
• Backfill types: *hydraulic/sand fill* (deslimed classified tailings, placed as slurry, drained through barricades), *paste fill* (full tailings at 75-85% solids with 3-7% cement, pumped, no segregation, higher strength), *cemented rock fill / CRF* (waste rock + cement slurry, high strength, used for sills and under-cuts), *dry rock fill* (waste rock only, support but low strength). Fill reduces convergence, allows pillar recovery and disposes of tailings - a major reason Indian zinc mines favour it.
• Square-set and stull stoping: timber framework (square sets) or struts between walls; now historical, kept only for very weak, high-value, irregular ore.
• Sublevel caving (SLC): the ore is drilled in rings from closely spaced sublevels (20-30 m vertical, 10-15 m drift spacing) and blasted against caved waste; ore is drawn from the brow until waste dilution limits the draw. Top-down, continuous, highly mechanised, low development cost per tonne, but dilution 15-35% and recovery depends on draw control and gravity flow. Requires caving hanging wall and tolerance of subsidence.
• Block / panel caving: a large block is undercut so the ore caves continuously under its own weight into an engineered drawpoint grid (herringbone or El Teniente layout) and is loaded by LHD or grizzly/slusher. Lowest cost per tonne of any underground method and highest tonnage, but very high pre-production capital and lead time, 5-10 years to full production, needs a cavable, fragmenting ore body of large footprint and uniform low grade, and causes surface subsidence. Cavability is assessed with Laubscher's caving (MRMR) chart against the hydraulic radius of the undercut.

Method selection, design and rock mechanics

• Selection inputs: ore geometry (thickness, dip, plunge, continuity), grade and its variability, ore and wall rock strength (RMR, Q, RQD, UCS), depth and in-situ stress, water, subsidence permissibility, required production rate, capital available, labour and mechanisation.
• Nicholas method: numerical ranking - score each candidate method against geometry, grade distribution and the rock mass strength of ore/hanging wall/footwall, weight and sum; the highest total is the preferred method. Commonly asked as a descriptive answer.
• Rock mass classification: *RQD* (Deere), *RMR* (Bieniawski, 0-100, from UCS, RQD, spacing, joint condition, water, orientation), *Q* (Barton, from RQD/Jn × Jr/Ja × Jw/SRF), *MRMR* (Laubscher, RMR adjusted for weathering, joint orientation, blasting and stress) for caving and stope spans.
• Mathews-Potvin stability graph: plot stability number N' = Q′ × A × B × C against hydraulic radius HR = area/perimeter of the stope face to decide stable / transition / caving spans and the need for cable bolting. Q′ is Q with Jw = 1 and SRF = 1; A = stress factor, B = joint-orientation factor, C = gravity/failure-mode factor.
• Dilution is quantified by the equivalent linear overbreak/slough (ELOS) in metres of wall slough per stope wall.
• Support: friction/resin rock bolts, split sets, cable bolts (long tendons for large spans and stope backs), welded mesh, fibre-reinforced shotcrete, steel sets and shotcrete arches in bad ground, yielding support (cone bolts, dynamic bolts) where rockburst risk exists at depth. Pre-conditioning blasting and destressing are used in high-stress mines; India's deep gold mines (the former KGF) are the classic Indian rockburst case study.

Unit operations, services and productivity

• Development: drill-and-blast with electro-hydraulic jumbos, emulsion/ANFO charging, non-electric or electronic detonators, bolting rigs, LHDs and dump trucks; access by decline/ramp (trackless, up to about 1:7) for shallow-to-medium depth, by shaft for deeper or high-tonnage mines; raise boring for ore passes, ventilation and slot raises. Alimak climbers for conventional raises.
• Production drilling: top-hammer longhole rigs to about 89 mm, ITH/DTH rigs for 102-165 mm; drilling accuracy governs dilution and recovery.
• Loading and haulage: LHD (3-17 t bucket) to ore passes, 30-60 t underground dump trucks or skip hoisting via a shaft; conveyors and shaft crushing in high-tonnage mines; semi-autonomous and tele-remote LHDs are now operating in Indian zinc mines.
• Ventilation: mechanical, through-flow circuits with primary surface fans, raises as upcast/downcast, booster and auxiliary fans with ducting for development ends; dilution of diesel exhaust usually governs quantity (typically ~0.06-0.09 m³/s per kW of diesel, per the mine's approved plan). Secondary egress and refuge chambers are mandatory practice.
• Services: compressed air, service and process water, dewatering pumps in stages, paste/hydraulic fill lines, electrical reticulation, leaky-feeder/Wi-Fi/LTE communication, personnel and vehicle tracking.

Indian regulation, standards and practice

• The Occupational Safety, Health and Working Conditions Code, 2020 (which repealed the Mines Act, 1952 from 21 November 2025) with the Metalliferous Mines Regulations, 1961 (MMR 1961), continued under the Code, is the governing framework for underground metal mines, administered by DGMS (Directorate General of Mines Safety), Dhanbad, under the Ministry of Labour and Employment. MMR covers support, ventilation, blasting, winding, drawings, notices of accidents and statutory appointments (Manager, Assistant Manager, Mining Mate, Surveyor, Blaster) with first- and second-class competency certificates.
• Statutory essentials: an approved support/ground-control plan, systematic support rules, two means of egress from every working, standards for ventilation quantity and air quality, periodic survey plans and sections, examination of workings before shift, and permitted explosives/charging practice.
• OSH&WC (Central) Rules, 2026 (welfare, medical examination, occupational health, rescue rooms/stations; they superseded the Mines Rules, 1955 and Mines Rescue Rules, 1985), Explosives Act, 1884 and Explosives Rules, 2008 (PESO licensing for magazines and transport).
• Titles and tenure come under the MMDR Act, 1957 as amended (auction-based allocation of mineral concessions, National Mineral Exploration Trust, District Mineral Foundation); environment clearance under the EP Act, 1986 regime and forest clearance under the Van (Sanrakshan Evam Samvardhan) Adhiniyam, 1980 (formerly the Forest (Conservation) Act, 1980); IBM (Indian Bureau of Mines) approves mining plans and enforces conservation and systematic development.
• The four Labour Codes in force from 21 November 2025 (Code on Wages; Industrial Relations Code; Code on Social Security; Occupational Safety, Health and Working Conditions Code) replaced the Factories, Minimum Wages, Industrial Disputes and BOCW Acts; mines are covered by the OSH&WC Code, which repealed the Mines Act, 1952 (section 143); MMR 1961 continues under the Code's savings clause as the mining-specific technical law until its draft 2026 successor is finally notified.
• Indian Standards are quoted for rock testing and support (UCS and point-load test methods, rock bolts, wire ropes for winding, shotcrete) - several older IS numbers have been revised or withdrawn, so cite the current edition or name the subject rather than an uncertain number. DGMS circulars (technical, general, approval) carry practical force alongside the regulations.

Key points

• Stope - the excavation producing ore; development - permanent access/ventilation/haulage openings.
• Method selection - driven by dip, thickness, ore/wall rock strength, grade distribution, depth, subsidence tolerance and required tonnage, not by cost alone.
• Three families - unsupported (room-and-pillar, sublevel, shrinkage), supported (cut-and-fill, square-set), caving (block, sublevel).
• Room-and-pillar - flat deposits, dip under ~30°, recovery 50-80%, ore locked in pillars.
• Shrinkage stoping - broken ore held as a working floor; swell ~40-50%, only ~30-35% drawn during mining; obsolete for safety and productivity.
• Sublevel stoping - longholes between sublevels blasted into an open void; bulk, cheap, safe, needs competent ore and walls.
• VCR - large-diameter vertical holes, spherical charges fired in benches from the bottom up; saves sublevel development.
• Cut-and-fill - slice-by-slice with backfill; best recovery and lowest dilution, highest cost; overhand, underhand and drift-and-fill variants.
• Backfill grades - rock fill < hydraulic sand fill < paste fill < cemented rock fill in strength; paste fill avoids segregation and drainage barricades.
• Sublevel caving - top-down ring blasting against caved waste; dilution 15-35%, draw control is everything.
• Block caving - lowest operating cost, highest tonnage, longest lead time; cavability judged from MRMR vs hydraulic radius of the undercut.
• Hydraulic radius HR = area/perimeter - the size measure used in both caving and stope-stability charts.
• Stability number N' = Q′ A B C plotted against HR on the Mathews/Potvin graph sizes stope walls and backs.
• Dilution and recovery are the two numbers that decide a stope's economics; ELOS measures wall slough in metres.
• Pillars - rib, sill, crown, shaft, barrier; tributary-area theory gives average pillar stress.
• Support - rock bolts, split sets, cable bolts, mesh, fibre shotcrete; yielding/cone bolts where rockburst is a risk.
• Law - OSH Code, 2020 (replaced the Mines Act, 1952) + MMR 1961, enforced by DGMS; mining plans and conservation under IBM; concessions under the MMDR Act, 1957.
• Indian examples - Sindesar Khurd and Rampura Agucha (sublevel stoping with fill), Zawar and Kayad (cut-and-fill), Malanjkhand (underground block/sublevel development), Jaduguda (cut-and-fill), Hutti (narrow-vein gold).

Formulas and examples

• Example 1 - stope tonnage, dilution and diluted grade. A sublevel stope block is 30 m along strike, 12 m wide, 25 m high, in zinc ore of density 3.4 t/m³ at 10.5% Zn. Blast overbreak brings in 0.4 m of barren wall (zero grade, density 2.7 t/m³) on each of the two 30 m × 25 m walls.
  • In-situ ore: 30 × 12 × 25 = 9,000 m³ × 3.4 = 30,600 t.
  • Waste: 2 × 0.4 × 30 × 25 = 600 m³ × 2.7 = 1,620 t.
  • Dilution = 1,620/30,600 × 100 = 5.29%.
  • Diluted grade = (30,600 × 10.5 + 1,620 × 0)/(30,600 + 1,620) = 321,300/32,220 = 9.97% Zn.
  • With 94% ore recovery, metal recovered to ROM = 30,600 × 0.94 × 0.105 = 3,020 t Zn (contained).
• Example 2 - room-and-pillar pillar stress, extraction and safety factor. Square pillars 8 m × 8 m on 20 m centres (room width 12 m), depth 240 m, overburden density 2,700 kg/m³, mining height 5 m, cubical specimen strength S₁ = 40 MPa.
  • Vertical stress σ_v = 0.027 × 240 = 6.48 MPa.
  • σ_p = 6.48 × 20²/8² = 6.48 × 400/64 = 6.48 × 6.25 = 40.5 MPa.
  • Extraction ratio e = 1 − 64/400 = 0.84 (84%) - very high, hence the heavy pillar stress.
  • Pillar strength (Obert-Duvall, w/h = 8/5 = 1.6): S_p = 40 (0.778 + 0.222 × 1.6) = 40 (0.778 + 0.3552) = 40 × 1.1332 = 45.3 MPa.
  • FoS = 45.3/40.5 = 1.12 - unacceptable (design target typically 1.5-2.0). Widening pillars to 11 m on 20 m centres: σ_p = 6.48 × 400/121 = 21.4 MPa, w/h = 2.2, S_p = 40(0.778 + 0.4884) = 50.7 MPa, FoS = 2.37, with e = 1 − 121/400 = 0.70. Recovery falls from 84% to 70% to buy stability.
• Example 3 - shrinkage stoping draw during mining. A shrinkage stope holds 12,000 t of in-situ ore; the swell factor is 1.45.
  • Broken (loose) volume = 1.45 × in-situ volume, so the ore occupies 45% more space than the void created.
  • Fraction that must be drawn to keep head room = (1.45 − 1)/1.45 = 31.0% → 0.310 × 12,000 ≈ 3,720 t drawn during mining; the remaining ≈ 8,280 t comes out in the final draw.
• Example 4 - hydraulic radius and stope wall stability. A stope hanging wall is 35 m high and 20 m wide; the rock mass gives Q′ = 9, with factors A = 0.7 (stress), B = 0.6 (joint orientation), C = 5.5 (steep wall, sliding mode).
  • HR = (35 × 20)/[2(35 + 20)] = 700/110 = 6.36 m.
  • N' = 9 × 0.7 × 0.6 × 5.5 = 20.8.
  • On the Mathews-Potvin graph, N' ≈ 21 at HR ≈ 6.4 m plots in the stable zone (the stable/transition boundary at this HR sits roughly at N' ≈ 6-10), so the wall can stand unsupported; enlarging the stope face to raise HR, or a drop in N' from higher stress (lower A), would push it into the transition zone requiring cable bolting. Read the exact boundary off the chart supplied in the exam.
• Example 5 - powder factor, specific drilling and development ratio. A ring blast in a sublevel stope uses 14 holes of 89 mm diameter averaging 22 m, charged over 19 m each with emulsion of density 1.15 g/cm³, and breaks 2,600 t.
  • Charge per hole = (π/4)(0.089)² × 19 × 1,150 = 0.006221 × 19 × 1,150 = 135.9 kg; total = 14 × 135.9 = 1,903 kg.
  • Powder factor = 1,903/2,600 = 0.73 kg/t.
  • Drilled metres = 14 × 22 = 308 m → specific drilling = 308/2,600 = 0.118 m/t (8.4 t per drilled metre).
  • If the stope block of 2,600 t × 12 rings = 31,200 t needed 260 m of sublevel and access development, development ratio = 260/31.2 = 8.3 m per 1,000 t.

Where it is used

• Zinc-lead: Hindustan Zinc (Vedanta) - Rampura Agucha underground, Sindesar Khurd, Rajpura Dariba, Zawar group, Kayad: mechanised sublevel stoping and cut-and-fill with paste fill, declines and shafts, semi-autonomous LHDs.
• Copper: Hindustan Copper - Malanjkhand underground project, Khetri and Kolihan (Rajasthan), Surda (Jharkhand); block/sublevel methods and shaft hoisting.
• Gold and uranium: Hutti Gold Mines (Karnataka) narrow-vein stoping with backfill; UCIL - Jaduguda, Bhatin, Narwapahar, Turamdih, Tummalapalle - cut-and-fill and blasthole stoping with strict radiation/ventilation control.
• Job roles: graduate mining engineer / shift in-charge, mine planning engineer (Deswik, Datamine, Surpac, Vulcan), ventilation engineer, rock mechanics / geotechnical engineer, drill-and-blast engineer, backfill engineer, mine surveyor, DGMS-certificated Manager and Assistant Manager; contractors Barminco, Byrnecut, Thriveni, Shyama Power recruit for mechanised development and production.
• Consultancy and research: MECON, SRK, WSP/Golder, CSIR-CIMFR and NIRM (rock mechanics, support design, caving studies), IBM mining plan approvals.
• Equipment and services: Sandvik, Epiroc, Caterpillar, Normet, Komatsu/Joy, Orica and Solar Industries (explosives, charging systems) - roles in application engineering and underground service support.

Interview questions

• Q: How do you pick between sublevel stoping and cut-and-fill for a steep ore body? A: Sublevel stoping if ore and walls are competent, the ore body is thick and regular with sharp boundaries, and bulk tonnage is wanted; cut-and-fill if the ore is high-grade, narrow or irregular, the walls are weak, or near-total recovery with minimal dilution is required.
• Q: Why is broken ore left in a shrinkage stope? A: It serves as the working platform to reach the back and gives passive wall support; only the swell (about 30-35%) is drawn off during mining and the rest in the final draw.
• Q: What does the Mathews stability graph tell you? A: Plotting stability number N' = Q′ A B C against the hydraulic radius of a stope surface indicates whether that span will be stable, transitional or caving, and hence whether cable bolting or a smaller stope is needed.
• Q: Why does block caving have the lowest operating cost but is rarely the first choice? A: Gravity does the breaking and most of the handling, so cost per tonne is minimal, but it needs huge pre-production capital, 5-10 years lead time, a genuinely cavable large-footprint ore body, and it causes surface subsidence - all irreversible once committed.
• Q: Difference between paste fill and hydraulic fill? A: Hydraulic fill is deslimed tailings pumped as a dilute slurry that must drain through barricades; paste fill is full tailings at 75-85% solids with cement, placed without segregation or drainage, giving higher and more predictable strength.
• Q: What is dilution and how do you control it? A: Waste mined with the ore, which lowers mill feed grade; controlled by accurate drilling, correct blast design and charging, drill-hole surveying, sound stope sizing from rock mass data, good draw control and cable bolting of walls.
• Q: What is the mine call factor? A: The ratio of metal actually received at the mill to the metal predicted in situ from the block model - it exposes combined losses from dilution, ore loss, sampling and estimation error.
• Q: Which Indian regulation governs support in an underground metal mine? A: The Metalliferous Mines Regulations, 1961 (made under the Mines Act, 1952 and continued under the OSH Code, 2020), enforced by DGMS, requiring an approved support plan and systematic support rules; DGMS circulars give the practical standards.
• Q: Why are rib, sill and crown pillars left even in a backfilled mine? A: They carry load and separate working areas during the mining sequence; they are later recovered against cemented fill or sacrificed, depending on the extraction sequence and stress conditions.
• Q: A pillar's factor of safety comes out at 1.1 - what do you do? A: Reduce the extraction ratio (wider pillars or narrower rooms), reduce the mining height to improve w/h, add backfill confinement, or re-sequence the panel; 1.1 is too close to failure for a long-standing pillar.
• Q: Why is sublevel caving top-down while sublevel stoping is usually bottom-up? A: SLC blasts ore against already caved waste and relies on the cave propagating downward behind the retreat, so it must work from the top; open stoping needs a free face and void, which is created by starting at the bottom with an undercut and slot.
• Q: What usually sets ventilation quantity in a trackless metal mine? A: The installed diesel power underground - exhaust dilution normally governs, ahead of dust, heat and blasting fumes clearance, with the required quantity per kW set in the mine's approved ventilation plan.

Common mistakes

• Treating method choice as a cost ranking and naming block caving or sublevel stoping for an irregular, weak, narrow ore body it cannot work in.
• Confusing sublevel stoping with sublevel caving - one blasts into an open void bottom-up, the other against caved waste top-down.
• Forgetting to include the waste tonnage in the denominator when computing diluted grade, or quoting dilution and recovery interchangeably.
• Using the tributary-area formula with pillar width in place of pillar centre spacing (the numerator must be the full centre-to-centre dimension).
• Mixing in-situ and loose (swelled) tonnages/volumes - swell matters for draw, ore pass and LHD capacity, not for reserves.
• Ignoring the development needed to make a method work, then claiming a low cost per tonne for it.
• Quoting support and ventilation practice from coal regulations (CMR 2017) in a metal-mine answer, when MMR 1961 applies.

Related topics

• Surface mining methods · Underground coal mining · Mine development and shaft sinking · Rock mechanics and ground control · Mine ventilation and environment · Drilling, blasting and explosives.