Drilling and blasting

In short

Drilling and blasting is the controlled use of a drilled hole as a container for chemical energy, so that rock is broken to a size and shape a mine's loading and crushing plant can handle. It governs the cost and productivity of almost every Indian mine - roughly 60-70% of mined rock in India is still loosened by explosives - and errors show up immediately as oversize, high-cost secondary breaking, damaged highwalls, flyrock or villagers' complaints of vibration. In exams it is a scoring area: blast design computations, cut patterns for underground drivage, explosive properties and statutory provisions are standard GATE (Mining) and university questions. In industry it is examined in interviews for Coal India subsidiaries, SCCL, NMDC, Hindustan Zinc, Tata Steel, JSW, UltraTech, NLC and explosive suppliers such as Solar Industries, IDL and Orica, and in DGMS competency examinations for Shot-firer, Blaster, Mining Mate and Manager.

Explained

Rock drilling - methods and machines

• Percussive drilling: a piston hammers the bit; the bit indexes between blows and chippings are flushed out. Energy transfer is by stress wave - best in hard, brittle rock. Sub-types: hand-held jackhammer (32-40 mm holes), drifter/airleg for underground, wagon drill, and crawler-mounted top-hammer surface drills (89-127 mm).
• Down-the-hole (DTH) hammer: the hammer travels with the bit inside the hole, so energy is not lost in rod joints - straighter and faster holes beyond about 20 m; typical 100-200 mm.
• Rotary drilling: high pull-down thrust plus rotation with tricone roller bits (hard rock) or drag/PDC bits (soft rock, coal, shale). Used for large surface blastholes 150-311 mm in opencast coal and iron ore.
• Rotary-percussive combines both; diamond core drilling is for exploration, not blasting.
• Bits: tungsten-carbide button (insert) bits for percussion, cross/chisel bits for soft rock, tricone and PDC for rotary. Bit wear rises steeply with silica content and abrasiveness.
• Flushing: compressed air (surface, with dust suppression as mandated), water (underground, to control respirable dust and silicosis risk), or air-water mist/foam.
• Drillability depends on compressive strength, abrasiveness, texture and fracture; measured by the Drilling Rate Index (DRI) and correlated with Protodyakonov's coefficient.
• Mechanisation trend: hydraulic jumbos (two- or three-boom Boomers) in underground metal mines and tunnels; surface miners (e.g. in SCCL and CIL blocks near habitations) remove blasting altogether in soft coal.

Explosives and accessories

• Definition: a chemical mixture that, on initiation, decomposes at supersonic speed (detonation) producing a shock wave plus high-pressure gas. A deflagrating explosive (black powder) burns subsonically - used only for special work.
• Classification by base: (a) nitroglycerine (NG) based - gelatines, semi-gelatines; high density, excellent water resistance, costly, being phased out; (b) slurry/watergel - aqueous ammonium nitrate thickened and cross-linked with gums, sensitised by aluminium, TNT or gas bubbles; (c) emulsion - water-in-oil emulsion sensitised by gas bubbles; now the Indian workhorse, pumpable as bulk or SMS/SMEX (site-mixed slurry/emulsion) from trucks; (d) ANFO - 94% porous prilled ammonium nitrate + 6% fuel oil, cheapest, no water resistance, for dry holes only; heavy ANFO blends emulsion with ANFO for water tolerance and higher energy density.
• Key properties: density (0.8-1.3 g/cc), velocity of detonation (VOD) - ANFO 3,000-3,500 m/s, emulsion 4,500-5,500 m/s, NG gelatines 5,500-6,500 m/s; detonation pressure; relative weight strength (RWS, ANFO = 100); water resistance; fume class; sensitivity; shelf life.
• Permitted explosives for gassy coal mines (classes P1, P3, P5) have additives that cool the flame; they are tested and certified by CIMFR, Dhanbad, and mines are classified by degree of gassiness I, II and III under the coal mines regulations. Solid blasting of coal with P5 explosives is allowed only under specific DGMS permission.
• Initiation accessories: plain detonator with safety fuse (burning ~0.5 m per 60 s, i.e. about 110-130 s per metre, now largely obsolete); detonating cord (PETN core, ~6,500-7,000 m/s); electric delay detonators; shock-tube/Nonel non-electric detonators, immune to stray currents, static and radio frequency (blasting must still be stopped during lightning) - the Indian standard for opencast; electronic detonators with 1 ms programmable delay for precise vibration control; cast boosters to initiate bulk emulsion; relays/down-the-hole + surface connectors for sequencing.
• Charging terms: coupled (explosive fills the hole) vs decoupled; deck charging with an inert deck to avoid pulverising coal; air decking to improve energy distribution; stemming with angular drill cuttings or sand to confine gases.

Surface (bench) blast design

• Geometry: hole diameter *d*, bench height H, burden B (distance to free face, normal to row), spacing S (between holes in a row), subgrade drilling J (below floor level, to avoid toe), stemming T, charge length, number of rows.
• Thumb rules: B = 25-40 d (lower for strong rock); S/B = 1.0-1.4 (staggered patterns up to 1.4); H/B ≥ 2 for good breakage (stiffness ratio); T = 0.7-1.0 B; J = 0.3 B; hole inclination 10-20° improves fragmentation and reduces backbreak.
• Patterns: square, rectangular, and staggered (equilateral) - the staggered pattern distributes energy best. Firing patterns: row-by-row (line), V or echelon (rotates effective burden and spacing, improves fragmentation and throws muck inward).
• Delay timing: inter-hole about 3-7 ms per metre of spacing, inter-row about 10-30 ms per metre of burden. Delays reduce the charge weight per delay (hence vibration), provide relief for each hole, and improve fragmentation; too short a delay causes cut-offs and freezing, too long causes flyrock and poor interaction.
• Powder factor (specific charge) typical: coal overburden 0.25-0.40 kg/m³, hard-rock quarry benches 0.40-0.80 kg/m³, iron ore 0.30-0.60 kg/m³.
• Controlled blasting at final walls: presplitting (closely spaced, lightly loaded holes fired first), smooth/cushion blasting (fired last), line drilling, decoupled charges, reduced sub-drilling.

Underground drilling and blasting

• A development face has no free face except the face itself, so an artificial void - the cut - must be created first.
• Cut types: wedge (V) cut, pyramid (cone) cut, drag/fan cut (angled holes - need wide faces), and parallel-hole cuts - burn cut, Coromant cut, large-hole (Fagersta/spiral) cut - which use one or more uncharged relief holes and suit jumbo drilling and long pulls.
• Hole groups and firing order: cut → cut spreaders → easers/relievers → side (rib) holes → roof (back) trimmers → lifters (fired last, to throw muck forward on the floor).
• Pull is the advance per round; 85-95% of hole depth is good practice. Specific charge in drivage is high (1.2-2.5 kg/m³) and specific drilling 2-5 m/m³, because confinement is severe.
• Stoping: long-hole open stoping, sublevel stoping with rings drilled by ITH/top-hammer longhole rigs, VCR, and blasthole/ring blasting in Hindustan Zinc's Rampura Agucha and Rajpura Dariba, Hutti Gold and Malanjkhand.
• Coal: conventional drill-and-fire in board-and-pillar with permitted explosives and a shot-firing cycle after the ventilation/gas check; mechanised faces (continuous miners, longwall shearers) avoid explosives.

Blast outcomes and environmental control

• Energy split: only a fraction of explosive energy fragments rock; the rest appears as ground vibration, air overpressure (air blast), noise, flyrock, back-break, dust and fumes.
• Ground vibration is measured as peak particle velocity (PPV, mm/s) and predicted from scaled distance. The dominant design variable is the maximum charge per delay, not the total charge in the blast.
• DGMS Technical Circular No. 7 of 1997 prescribes permissible PPV at structures - broadly 5/10/15 mm/s for domestic houses of kutcha/brick/cement construction at dominant frequencies below 8 Hz, 8-25 Hz and above 25 Hz, with higher limits for industrial RCC structures. Treat the exact table as the circular's; verify the current DGMS text before use.
• Air overpressure is commonly limited around 134 dB(L); it is controlled by adequate stemming, no exposed detonating cord, and avoiding blasting in adverse atmospheric inversion.
• Flyrock causes: insufficient burden, weak zones/cavities, poor stemming, overcharging, incorrect delay. Controls: proper burden survey (including hole-by-hole depth and face profile by laser/boretrack), blast mats, danger zone of at least 300-500 m with sirens and sentries per the statute.
• Fragmentation assessment: digital image analysis (WipFrag/Split), boulder count, shovel/crusher throughput, excavator dig rate.

Statutory and safety framework (India)

• The Occupational Safety, Health and Working Conditions Code, 2020 (in force 21 November 2025) is the parent mine-safety statute; it repealed the Mines Act, 1952 (section 143) and the OSH&WC (Central) Rules, 2026 superseded the Mines Rules, 1955. The Metalliferous Mines Regulations, 1961 and Coal Mines Regulations, 2017 (which replaced CMR 1957) continue under section 143(3) of the Code until their successors are notified, and govern drilling, explosive handling, shot-firing, misfires and danger zones. DGMS remains the enforcing authority. The same four Labour Codes replaced the Factories, Minimum Wages, Industrial Disputes and BOCW Acts.
• Explosives Act, 1884 and Explosives Rules, 2008, administered by PESO (Chief Controller of Explosives), Nagpur, govern manufacture, licensed magazines, transport (licensed vans, Form licences) and possession. Explosives and detonators are stored and carried separately.
• Competency: only a holder of a Shot-firer's/Blaster's certificate (or a Mining Mate/Overman/Manager authorised in writing) may charge and fire shots; vocational training is now prescribed under section 72 of the Code by the OSH&WC (Central) Rules, 2026, which superseded the Mines Vocational Training Rules, 1966.
• Misfire practice (regulatory): after firing, no one approaches the face for the stipulated interval - commonly 5 minutes for electric/electronic initiation and about 30 minutes where safety fuse is used. A misfired hole is never re-drilled; a fresh hole is drilled parallel to it, not closer than about 300 mm, and fired. Misfires must be recorded and reported to the next shift and to management. Confirm the exact intervals and clearances from the applicable regulation.
• Records and permissions: explosive consumption register, shot-firing report, DGMS permissions for SMS vans, solid blasting, blasting near structures/railways, and vibration study reports (CIMFR/NIRM are the usual agencies).
• Relevant Indian Standards include the safety code for blasting and related drilling operations (IS 4081) and the criteria for safety against blast-induced vibration in structures; since several older IS numbers have been revised or withdrawn, quote only the current edition after checking.

Key points

• Blasting - the use of confined chemical energy; the shock wave cracks the rock and the gas pressure heaves and displaces it.
• Burden - the single most critical dimension; too large gives oversize and toe, too small gives flyrock and air blast.
• Spacing ratio S/B - 1.0-1.4; staggered patterns distribute energy more evenly than square ones.
• Stiffness ratio H/B - should be 2 or more; low benches with large burden blast badly.
• Powder factor - kg of explosive per m³ or per tonne of rock; the yardstick of blast efficiency and cost.
• Subgrade drilling - 0.3 B below floor to prevent toe and hard digging.
• Stemming - 0.7-1.0 B of angular cuttings; poor stemming causes stemming ejection, air blast and flyrock.
• VOD - detonation velocity; higher VOD means higher shock energy, better for hard rock.
• ANFO - cheapest explosive, zero water resistance, needs a booster; emulsion is water-resistant and pumpable.
• Permitted explosives (P1/P3/P5) - mandatory in gassy coal mines; tested at CIMFR, Dhanbad.
• Delay detonators - cut vibration by reducing charge per delay and improve fragmentation by giving each hole relief.
• Scaled distance - distance divided by the square root of charge per delay; the basis of all vibration control.
• DGMS Circular 7 of 1997 - prescribes permissible PPV by structure type and dominant frequency.
• Cut - the artificial free face in underground drivage; burn cut and other parallel-hole cuts suit jumbo drilling and long pulls.
• Firing sequence underground - cut, easers, ribs, roof holes, lifters last.
• Misfire - never re-drill the hole; drill a parallel hole at least about 300 mm away, fire it, and record the misfire.
• Presplitting and smooth blasting - controlled techniques that preserve final wall stability.
• Kuz-Ram model - the standard examinable fragmentation prediction, giving mean size and a Rosin-Rammler distribution.

Formulas and examples

Example 1 - Bench blast design (limestone quarry, 115 mm DTH holes, bulk ANFO 0.85 g/cc, rock density 2.6 t/m³, bench 10 m).

• Burden B = 30 d = 30 × 0.115 = 3.45 m → take 3.5 m; spacing S = 1.15 B ≈ 4.0 m.
• Subdrilling J = 0.3 × 3.5 ≈ 1.0 m → vertical hole length L_h = 10 + 1 = 11 m; stemming T = 0.7 × 3.5 ≈ 2.5 m → charge length = 8.5 m.
• l = 7.854 × 10⁻⁴ × 115² × 0.85 = 7.854 × 10⁻⁴ × 13,225 × 0.85 = 8.83 kg/m.
• Charge per hole Q = 8.83 × 8.5 = 75.0 kg.
• Rock per hole V₀ = 3.5 × 4.0 × 10 = 140 m³ = 140 × 2.6 = 364 t.
• PF = 75/140 = 0.54 kg/m³ = 75/364 = 0.206 kg/t (≈ 4.85 t of rock per kg of explosive) - normal for medium-strong limestone.
• Specific drilling = 11/140 = 0.079 m/m³ (12.7 m³ per metre drilled).

Example 2 - Fragmentation by Kuz-Ram (same blast, A = 7 for medium jointed rock, E = 100 for ANFO).

• (V₀/Q)^0.8 = (140/75)^0.8 = (1.867)^0.8 = 1.648; Q^1/6 = 75^0.1667 = 2.054; (115/100)^0.95 = 1.142.
• x₅₀ = 7 × 1.648 × 2.054 × 1.142 = 27 cm.
• n = (2.2 − 14 × 3.5/115) × √[(1 + 4.0/3.5)/2] × (8.5/10) = (2.2 − 0.426) × 1.035 × 0.85 = 1.56 (drilling-accuracy term taken as 1).
• Oversize above 80 cm: R = exp[−0.693 × (80/27)^1.56] = exp[−0.693 × 2.96^1.56] = exp[−0.693 × 5.45] = exp(−3.78) = 2.3% - acceptable for a 900 mm primary crusher feed.

Example 3 - Vibration check and blast size. Nearest village house is 300 m away; site constants K = 1,140, B = 1.6; permissible PPV at the structure 5 mm/s (low-frequency case).

• With single-hole delays, Q_max = 75 kg → SD = 300/√75 = 300/8.66 = 34.6 m/kg^0.5.
• PPV = 1,140 × 34.6^−1.6 = 1,140/290.7 = 3.9 mm/s → within 5 mm/s, blast is permissible.
• Allowable Q_max for exactly 5 mm/s: (K/PPV)^1/B = (228)^0.625 = 29.8 → √Q = 300/29.8 = 10.1 → Q_max = ≈ 100 kg per delay. So two holes could even share a delay, but single-hole delays are kept for fragmentation.
• Blast for a 20,000 t production round: 20,000/364 = 55 holes (5 rows × 11), total explosive 55 × 75 = 4,125 kg, with 55 separate delays.

Example 4 - Underground drivage round. Face 4.0 m × 3.0 m, holes 3.0 m deep, 45 charged holes plus 1 relief hole, pull 2.7 m, 1.1 kg emulsion cartridges per metre charged over 2.4 m per hole.

• Solid rock blasted = 4.0 × 3.0 × 2.7 = 32.4 m³ (≈ 87 t at 2.7 t/m³).
• Explosive = 45 × 2.4 × 1.1 = 118.8 kg → PF = 118.8/32.4 = 3.67 kg/m³. This is high; reducing cartridge loading to 0.6 kg/m of hole gives 64.8 kg and 2.0 kg/m³, the normal drivage range - showing how easily drivage rounds are overcharged.
• Specific drilling = 46 × 3.0/32.4 = 4.26 m/m³; pull efficiency = 2.7/3.0 = 90%.

Example 5 - Drill-and-blast cost per tonne (indicative rates; actual rates vary by contract, state and year).

• A 10% increase in powder factor adds ₹0.93/t; if it removes ₹2/t of secondary breaking and crusher power, it pays - the usual drill-and-blast optimisation argument.

Where it is used

• Opencast coal and lignite: overburden and coal benches in CIL subsidiaries (NCL, MCL, SECL), SCCL and NLC, with 150-311 mm rotary drills, bulk SMS emulsion and Nonel/electronic initiation; blast-free surface miners where villages are close.
• Iron ore, limestone and aggregate: NMDC Bailadila and Donimalai, Tata Steel Noamundi, JSW and UltraTech/ACC/Ambuja limestone quarries - fragmentation control to suit crusher and kiln feed.
• Underground metal mines: Hindustan Zinc (Rampura Agucha, Sindesar Khurd), Hutti Gold, Malanjkhand - longhole ring blasting, paste-fill cycles, drill-rig operation and ring design.
• Civil tunnelling and hydropower: NHPC, THDC, NHAI and metro projects - controlled drill-and-blast with jumbos, smooth blasting of the periphery, vibration limits in urban reaches.
• Job roles: Graduate Engineer Trainee/Mining Engineer, Blasting Engineer, drill-and-blast engineer with explosive suppliers (Solar Industries, IDL, Orica, Keltech), vibration consultants (CIMFR, NIRM), and DGMS-certified Shot-firer/Blaster/Overman/Manager positions.
• Allied: demolition and controlled implosion of structures, explosive magazine and PESO licensing compliance, and blast monitoring and litigation support for mines near habitation.

Interview questions

• Q: Why is burden the most critical blast dimension? A: It fixes the confinement a charge works against - too large and the gas cannot break the rock, giving oversize, toe and high vibration; too small and the gas vents early, giving flyrock and air blast.
• Q: What is powder factor and what does a high value tell you? A: Explosive consumed per unit volume or tonne of rock; a high value may mean good fragmentation deliberately bought, or an overcharged, badly designed blast - it is meaningful only alongside fragmentation, vibration and digging rate.
• Q: Why does a delayed blast cause less vibration than an instantaneous one of the same total charge? A: Vibration depends on the maximum charge detonating per delay interval, not on total charge; delays split the blast into small independent events whose wave trains do not add in phase.
• Q: Define scaled distance and state how you use it on site. A: SD = distance divided by the square root of the charge per delay; with site constants K and B from monitored blasts, PPV = K·SD^−B lets you back-calculate the maximum charge per delay allowed for a given structure and distance.
• Q: Why is ANFO unsuitable for wet holes, and what is used instead? A: Ammonium nitrate is highly soluble, so water desensitises ANFO and causes misfires or partial detonation; bulk emulsion or heavy ANFO with high emulsion content is used, or the hole is dewatered and lined.
• Q: What is a permitted explosive and where is it mandatory? A: A cooler-flame explosive (classes P1, P3, P5) certified for use in gassy coal mines, tested at CIMFR Dhanbad; it is mandatory in coal mines classified as gassy, with P5 needed for solid blasting under DGMS permission.
• Q: Why is a cut needed in an underground face and which cut would you choose for a jumbo? A: The face has only one free surface, so a void must be created for the rest of the holes to break towards; a parallel-hole burn or large-hole cut, since jumbos drill parallel holes accurately and allow longer pulls.
• Q: What is the correct procedure for a misfire? A: Wait the statutory interval, keep the danger zone enforced, do not withdraw the charge or re-drill the hole; drill a fresh parallel hole not closer than about 300 mm, charge and fire it, then search the muck for undetonated explosive, and record and report the misfire.
• Q: How do you protect a final highwall during blasting? A: Presplit or cushion-blast the last row with closely spaced, lightly loaded, decoupled charges, reduce subdrilling and charge per delay near the wall, and fire the perimeter holes in the correct sequence.
• Q: Stemming is only drill cuttings - why fuss over it? A: It confines the gas long enough to do work on the rock; short or fine stemming lets gas vent up the hole, cutting fragmentation energy and producing air blast, noise and stemming ejection flyrock.
• Q: Which single change would you make first if a blast gives excessive oversize in the top of the bench? A: Reduce stemming length or add a small satellite/top charge and check the collar burden from the surveyed face profile - oversize at the top almost always means the collar zone is under-energised.
• Q: Electronic versus Nonel detonators - when is the extra cost justified? A: When vibration limits are tight near structures, or when fragmentation must be tuned precisely, since millisecond-accurate programmable delays allow single-hole firing and inter-hole timing that pyrotechnic scatter cannot deliver.

Common mistakes

• Mixing units in the charge-concentration formula - putting hole diameter in mm into (π/4)d²ρ with density in kg/m³, which inflates the charge per metre by a factor of a million.
• Forgetting to subtract stemming (and add subdrilling) when converting hole length into charge length and charge per hole.
• Using total blast charge instead of maximum charge per delay in the scaled-distance equation - this makes a safe blast look unsafe and vice versa.
• Quoting K and B as universal constants; they are site-specific regression values and must come from monitored blasts at that mine.
• Treating powder factor as a quality target on its own, without checking fragmentation, vibration, backbreak and digging rate.
• Writing that burden and spacing are interchangeable, or that a square pattern is as good as a staggered one for energy distribution.
• Saying a misfired hole should be re-drilled or the charge withdrawn - both are prohibited and a known cause of fatal accidents.

Related topics

Rock mechanics and ground control · Mine ventilation · Mine machinery and haulage · Mine planning and design software · Mine surveying · Mineral processing: crushing and comminution