Underground coal mining

In short

Underground coal mining wins coal from seams too deep or too thick-overburden to strip from surface, through shafts or inclines, using either bord and pillar (a grid of galleries leaving coal pillars, later extracted) or longwall (a long face retreated in one pass with powered supports while the roof caves behind). India works over 95% of its underground coal by bord and pillar, mostly semi-mechanised with SDL/LHD, with continuous miners and powered support longwalls at a growing number of faces. The subject is governed by the Coal Mines Regulations, 2017 under DGMS, and is examined heavily in GATE (Mining), university Mining Methods papers and in the Mining Sirdar/Overman/Manager certificate examinations. Employers: Coal India subsidiaries (ECL, BCCL, CCL, NCL, WCL, SECL, MCL), SCCL, NLC India, Tata Steel and JSW collieries, and contractors such as Thriveni, AMR and Sainik Mining.

Explained

Access, layout and terminology

• Access: vertical shaft (deep, gassy, high output), incline/drift (seam outcrops; conveyor or tub haulage), or adit in hilly terrain (NE India). Mine development and shaft sinking is a separate subject.
• Panel: a block of coal bounded by barriers/isolation stoppings, worked and then sealed as a unit.
• Gallery (bord): the roadway driven in coal; dip gallery / level / rise; split = gallery driven through a pillar; stook = the small remnant of a split pillar.
• Goaf (gob): the void left after extraction, either caved or stowed. Line of extraction, rib, barrier, snook are the standard terms.
• CMR 2017 limits a gallery in coal to not more than 4.8 m wide and 3.0 m high (greater dimensions need DGMS permission with a support plan).

Bord and pillar — development and depillaring

1. Development: drive galleries on two near-perpendicular sets to form square/rectangular pillars. Recovery at this stage is only 20–30%.
2. Cycle (conventional/semi-mechanised): drilling (jack hammer / UDM) → charging with permitted explosives (grades P1/P3/P5, the grade set by the degree of gassiness) and shotfiring → ventilating and dressing → loading by SDL (side discharge loader) or LHD into tubs/chain conveyor → supporting the roof per the Systematic Support Rules.
3. Continuous miner (CM): a drum-type cutter head wins and loads in one operation; coal goes by shuttle car or flexible conveyor train to a feeder breaker and belt. Bolter-miners install roof bolts on the face. India's first CM was at Anjan Hill (SECL, 2002); SCCL's VK-7 and GDK units followed; CM units routinely give 30,000–80,000 t/month against 3,000–6,000 t/month for an SDL section.
4. Depillaring (pillar extraction): pillars are split and then sliced from the boundary towards the shaft, keeping a straight or diagonal line of extraction, so the goaf is always behind the men.
   • With caving: roof is allowed to collapse; cheap, fast, causes subsidence; needs control of air blast at a main fall (break the roof in smaller, regular falls, keep goaf edge supported with chocks/props or mobile roof supports).
   • With sand stowing: hydraulically transported sand fills the goaf; recovery up to ~85–90%, subsidence reduced to a few per cent of thickness, fire risk cut — but costly and needs sand and water.
   • Partial extraction: pillars reduced to a safe stook size where surface features must be protected.
5. Advantages: flexible, low capital, suits disturbed/faulted seams, allows early production from development. Limits: low recovery, high manpower per tonne, coal locked in pillars, standing-support intensive, slow.

Longwall mining

• Layout: a panel 150–300 m wide and 1,000–3,000 m long, bounded by two gate roads (main gate for conveyor and intake, tail gate for return).
• Equipment: a double-ended ranging drum shearer (or plough) cutting a 0.6–1.0 m web, an armoured face conveyor (AFC), and powered roof supports (chock shields of 500–1,200 t capacity) that advance themselves and let the immediate roof cave behind.
• Retreating longwall is the norm — both gate roads are driven first, so geology is known and the goaf is behind the face; advancing longwall needs roadways maintained in the goaf and is rare now.
• Advantages: high concentrated output (one face can replace 10–20 bord and pillar sections), 85–95% recovery, no standing support, better strata and gas control, high productivity (OMS).
• Limits: very high capital, needs 2–4 km of undisturbed, gently dipping seam of fairly uniform thickness; inflexible when faults, washouts or rolls appear; subsidence is immediate and planned for. Indian experience has been mixed (Moonidih, Jhanjra, Churcha, GDK-10A, Adriyala) mainly because of geological disturbance and equipment availability.
• Shortwall: a 50–100 m face with continuous miner and powered supports — a compromise where panels are short.

Thick seam methods (an Indian specialty — many seams are 6–30 m)

• Multi-slice (horizon) working with stowing: extract 3 m slices in descending order, stowing each; safe but slow and costly.
• Blasting Gallery (BG) method: develop a gallery in the lower section, drill long ring holes upward and blast the seam down, loading by LHD — pioneered at GDK-11A (SCCL) with French technology.
• Multi-lift caving / integrated caving, Wongawilli and inclined-slicing variants; top-coal caving (LTCC) longwall where the top coal is drawn through the rear of the shields.
• Highwall mining (a remotely operated continuous miner pushed in from a surface highwall on added conveyor modules — distinct from auger mining) recovers coal locked under OC benches — used at several SCCL and MCL sites.

Strata control and support

• Rock Mass Rating (CMRI-ISM RMR, 0–100) from layer thickness, structural features, weatherability, rock strength and groundwater seepage, adjusted for depth and stress; RMR class gives the rock load and the support density from the CIMFR design tables.
• Supports: full-column resin/cement-grouted roof bolts (the standard, with W-straps and wire mesh), friction/hydraulic props, steel girders with chocks, cogs, and link-n-lock chocks at goaf edges. Pre-tensioned bolting gives beam-building; longer cable bolts at junctions.
• Systematic Support Rules (SSR) are statutory: a support plan for every working place, approved by DGMS, specifying support type, spacing, maximum unsupported width/span, and the testing of bolts (pull-out tests).
• Monitoring: telltale/roof extensometers, load cells, convergence stations, and micro-seismic monitoring on longwall faces.

Hazards specific to coal, and the law

• Firedamp (CH₄): seams are classed by specific emission — Degree I (<1 m³/t), Degree II (1–10 m³/t), Degree III (>10 m³/t of coal produced); higher degrees demand stronger ventilation, flameproof equipment, methane monitoring and methane drainage. Work stops and persons are withdrawn when methane in the general body of air reaches the limits set in CMR 2017 (danger level is reached well before the 5–15% explosive range).
• Other gases: blackdamp (CO₂+N₂, suffocating), whitedamp (CO, toxic), afterdamp (post-explosion mixture), stinkdamp (H₂S).
• Coal dust explosion: a methane ignition can raise and ignite coal dust. Controls — water infusion, wet drilling, dust suppression sprays, stone dusting to keep the incombustible content of roadway dust above the statutory minimum, and stone-dust/water barriers. (The exact percentages are set in CMR 2017 and DGMS circulars and have been revised — check the current circular.)
• Spontaneous heating / mine fire: Indian coals are mostly prone; each seam has an incubation period (commonly 6–10 weeks) within which a depillaring panel must be extracted and sealed. Indices: Crossing Point Temperature (CPT), Graham's ratio (CO/O₂ deficiency), Young's and Jones-Trickett ratios. Control — minimise leakage into goaf, pressure balancing, N₂/CO₂ inertisation, sand stowing, isolation stoppings, and sealing with dams.
• Inundation: advance boreholes ahead and flanking, statutory barriers against waterlogged workings and surface water bodies, dams and precautions as prescribed in CMR 2017; Chasnala (1975) remains the standard case study.
• Subsidence: surface lowering over caved panels; angle of draw, subsidence factor, strain and tilt damage. Controlled by panel size, stowing, leaving barriers under villages, railways and rivers.
• Legal frame: the Mines Act, 1952 was repealed by the Occupational Safety, Health and Working Conditions Code, 2020, in force from 21 November 2025, which carries mine-specific provisions; the Coal Mines Regulations, 2017 (which replaced CMR 1957) continue under the Code's savings clause until replaced, rescue and vocational training are prescribed by the OSH&WC (Central) Rules, 2026 (which superseded the Mines Rescue Rules, 1985 and Mines Vocational Training Rules, 1966), and DGMS circulars continue to apply. Regulator: DGMS, Dhanbad. Mineral rights flow from the MMDR Act, 1957 and the Coal Mines (Special Provisions) Act, 2015 / commercial auctions; environment clearance under EPA, 1986 and forest clearance under the Van (Sanrakshan Evam Samvardhan) Adhiniyam, 1980 (formerly the Forest (Conservation) Act, 1980).

Key points

• Bord and pillar - galleries form pillars; development recovers 20–30%, depillaring the rest.
• Longwall - one long face, powered supports, planned caving; 85–95% recovery, high capital.
• Retreating longwall - gate roads driven first; goaf always behind the face; preferred over advancing.
• Gallery limit (CMR 2017) - normally ≤ 4.8 m wide, ≤ 3.0 m high in coal.
• Pillar size - fixed by a statutory table of centre-to-centre distance against depth and gallery width; larger with depth.
• Tributary area theory - pillar stress rises as the square of the ratio of centre distance to pillar width.
• Safety factor - pillar strength ÷ pillar stress; about 1.5–2.0 or more for long-standing development pillars.
• Sheorey/CMRI formula - the Indian empirical pillar strength relation, using UCS, working height, depth and pillar width.
• Stowing vs caving - stowing gives higher recovery, little subsidence and lower fire risk, at much higher cost.
• Incubation period - the time within which a panel must be worked out and sealed to avoid spontaneous heating.
• Degree of gassiness - I, II, III by specific methane emission (<1, 1–10, >10 m³/t).
• SDL/LHD - semi-mechanised loading in bord and pillar; continuous miner cuts and loads in one machine.
• Blasting Gallery method - thick-seam extraction by long ring-hole blasting from a bottom gallery (SCCL).
• RMR (CMRI-ISM) - 0–100 rock mass rating that drives rock load and support density in the SSR.
• Air blast - sudden pressure surge at a main goaf fall; prevented by inducing regular, smaller falls.
• Barriers and advance boreholes - the statutory defence against inundation from old waterlogged workings.
• Underground share - under 5% of India's coal output today; policy aims to raise it, mainly through CM and highwall units.

Formulas and examples

Example 1 — pillar design and safety factor. Depth 300 m, gallery 4.2 m wide, working height 3.0 m, pillar centres 37.5 m, coal UCS (25 mm cube) 25 MPa.

• Pillar width w = 37.5 − 4.2 = 33.3 m.
• σv = 0.025 × 300 = 7.5 MPa.
• σp = 7.5 × (37.5/33.3)² = 7.5 × 1.268 = 9.51 MPa.
• S = 0.27 × 25 × 3^−0.36 + (300/250 + 1)(33.3/3 − 1) = 6.75 × 0.673 + 2.2 × 10.1 = 4.54 + 22.22 = 26.8 MPa.
• SF = 26.8/9.51 = 2.8 — comfortably safe for standing development pillars.

Example 2 — percentage extraction. Same layout: extraction = 1 − (33.3/37.5)² = 1 − 0.789 = 21.1% on development. If depillaring with caving later recovers 80% of the remaining 78.9%, overall recovery = 21.1 + 0.80 × 78.9 = 84.2%.

Example 3 — longwall face output. Face 150 m, web 0.8 m, height 3.0 m, ρ = 1.5 t/m³, recovery 95%, 10 web cuts per day.

• Per cut = 150 × 0.8 × 3.0 × 1.5 × 0.95 = 513 t; per day = 5,130 t; at 25 working days, about 1.28 lakh tonnes per month from one face.

Example 4 — ventilation for a gassy seam. A Degree III panel produces 3,000 t/day with specific emission 15 m³/t; permissible methane in the district return 1.25%.

• q = 15 × 3,000 = 45,000 m³/day = 45,000/1,440 = 31.25 m³/min.
• Q = 31.25/0.0125 = 2,500 m³/min of air through the district. With 40 persons in the district, the per-person minimum is 40 × 6 = 240 m³/min, so methane dilution governs.

Example 5 — subsidence. Panel 200 m wide, extracted 3.0 m thick at 200 m depth, caving, subsidence factor 0.6, angle of draw 35°.

• Smax = 0.6 × 3.0 = 1.8 m. Draw distance beyond the panel rib = 200 × tan 35° = 200 × 0.700 = 140 m, so surface protection must extend 140 m outside the panel outline. (Panel width 200 m against a critical width of about 1.4H = 280 m makes this subcritical, so actual subsidence would be less than 1.8 m.)

Where it is used

• Coal India subsidiaries: ECL and BCCL (deep Raniganj/Jharia seams, bord and pillar with stowing), SECL (CM units, Churcha longwall), longwall at Jhanjra in ECL and Moonidih in BCCL, MCL and NCL mainly opencast with UG at depth.
• SCCL, Telangana: the most mechanised UG operator — BG method at GDK, CM panels at VK-7, longwall at Adriyala; a standard recruiting ground for mining graduates.
• Captive and private collieries: Tata Steel (Jharia/West Bokaro), JSW, Hindalco and auctioned commercial blocks reopening underground capacity.
• Job roles: Mining Engineer trainee/Assistant Manager (needs the DGMS First/Second Class Manager's Certificate for statutory posts), Overman, Mining Sirdar, Surveyor, Ventilation Officer, Safety Officer, planning engineer doing panel and support design.
• Consultancy and R&D: CIMFR (support design, pillar strength, spontaneous heating tests), CMPDI (mine planning and feasibility reports), equipment vendors (Komatsu/Joy, Sandvik, Caterpillar, Eickhoff; Eimco Elecon for SDL/LHD).
• Related applications: coal bed methane drainage, mine-fire control projects in Jharia, highwall mining of OC locked-up coal, UCG pilots.

Interview questions

• Q: What decides whether a coal seam is mined underground or opencast? A: The stripping ratio, depth, seam thickness and gradient, surface features and land availability; when the overburden-to-coal ratio makes stripping uneconomic or the surface cannot be disturbed, underground is chosen.
• Q: Why is bord and pillar still dominant in India? A: Indian seams are often faulted, variable in thickness and already partly developed; bord and pillar is flexible, low in capital, gives production during development and tolerates disturbed geology, whereas longwall needs long undisturbed panels.
• Q: What is the difference between development and depillaring? A: Development drives galleries to form pillars and recovers only 20–30%; depillaring extracts those pillars by splitting and slicing, with caving or stowing, to raise overall recovery.
• Q: Why is retreating longwall preferred to advancing? A: Both gate roads are driven first, so the geology is proved, roadways are never maintained in the goaf, and the goaf stays behind the face, which simplifies ventilation and strata control.
• Q: How do you size a pillar? A: Start from the statutory centre-to-centre table in CMR 2017 for the depth and gallery width, then verify by computing pillar stress (tributary area) against strength from the Sheorey/CMRI formula for an adequate safety factor.
• Q: What is the incubation period and how does it govern panel design? A: It is the time a seam can be left exposed in a goaf before spontaneous heating begins (often 6–10 weeks for Indian coals); the panel size and extraction rate must allow it to be worked out and sealed within that period.
• Q: What is an air blast and how is it prevented? A: A violent pressure wave from a sudden large goaf fall; prevented by inducing regular small falls, keeping the goaf edge well supported and limiting the unsupported goaf area.
• Q: How are seams classified for gassiness and why does it matter? A: Degree I, II and III for specific emission below 1, 1–10 and above 10 m³ per tonne; the degree sets the ventilation quantity, methane monitoring, flameproof equipment and methane drainage requirements.
• Q: Advantages of sand stowing over caving? A: Higher recovery, almost no surface subsidence, far lower fire and spontaneous heating risk, and support to overlying seams — at a much higher cost and with the need for sand and water.
• Q: How is a continuous miner section different from an SDL section? A: The CM cuts and loads in one machine with belt/shuttle-car transport and bolter support, giving an order-of-magnitude higher output with far fewer people and no blasting.
• Q: What precautions are taken before approaching old waterlogged workings? A: Keep the statutory barrier, drive advance and flank boreholes from a safe distance, plan an escape route to a higher level, provide dams and pumping capacity, and work under a DGMS-approved scheme.
• Q: Which Indian method is used for very thick seams and why? A: The Blasting Gallery method or multi-slice stowing — a single 3 m lift would leave most of a 10–20 m seam unworked, so ring blasting from a bottom gallery or descending slices with stowing is used to recover the full thickness.

Common mistakes

• Confusing pillar width with centre-to-centre distance — pillar width = centre distance minus gallery width; using the wrong one wrecks the stress and extraction calculation.
• Forgetting that percentage extraction on development uses the square of the ratio for square pillars, not the ratio itself.
• Mixing up the statutory gallery limits (4.8 m wide, 3.0 m high) with the pillar table, or quoting old CMR 1957 regulation numbers instead of CMR 2017.
• Quoting air quantity from the per-person minimum alone when methane dilution demands much more.
• Assuming longwall is always better: it fails on faulted, variable Indian seams, and Indian longwall history shows the geological constraint is the binding one.
• Treating caving and stowing as interchangeable — they differ completely in recovery, subsidence, fire risk and cost.
• Ignoring the incubation period when proposing a large depillaring panel, which is the root cause of many Indian mine fires.
• Unit slips: γ = 0.025 MPa/m (not kPa/m), density in t/m³, and emission per tonne of coal produced, not per tonne in situ.

Related topics

Surface mining methods · Underground metal mining · Mine development and shaft sinking · Mine ventilation and air conditioning · Rock mechanics and ground control · Mine safety legislation, DGMS and rescue · Mine environment and subsidence engineering