Blasting & explosives

SpecificationTwo consistent published rules. By hole diameter in inches: B (ft) = 2 to 3 times d (in), typically 2.5 x d. By charge diameter as a multiple: for ANFO at 0.85 g/cm3 in rock of about 2.7 g/cm3 density, start at 25 times the charge diameter; for denser explosives at about 1.2 g/cm3, 30 to 35 times the charge diameter. USBR gives it by rock density with ANFO at 53.1 lb/ft3: 28 x charge diameter in light rock, 25 in average rock, 23 in dense rock; for slurry and dynamite products, 33, 30 and 27 respectively. The burden-to-charge-diameter ratio is seldom less than 20 or seldom more than 40. Worked examples from OSMRE's Surface Blast Design module (Module 3, https://www.osmre.gov/sites/default/files/inline-files/Module3_0.pdf), not from the rules-of-thumb sheet cited below: 9-inch holes give a good starting point of a 19-foot burden, and 12.25-inch holes a burden of 25 feet. Both sit near 2.1 x d, at the low end of the 2-to-3 range rather than at the 2.5 x d typical value.

Where it falls downInsufficient burden on a highwall face is identified by NIOSH as a primary cause of flyrock. Burden is only nominal once drilled: hole deviation, a face that is not surveyed, and an abrupt decrease in rock resistance at a joint, bedding plane, fault, mud seam or void all change the real burden on a hole, and those are the conditions under which flyrock has travelled 600 to 800 feet in documented incidents.

SpecificationS (ft) = 1 to 2 times B, typically 1.5 x B (OSMRE rules of thumb). USBR: where holes in a row are initiated simultaneously, a spacing equal to twice the burden usually will pull the round satisfactorily. Where holes in a row are separated by millisecond delays the ratio is reduced, and USBR keys the reduction to hole diameter rather than to the timing alone - large diameter blast holes require lower spacing-to-burden ratios, usually 1.2 to 1.5 with ms delays, than small diameter blast holes, usually 1.5 to 1.8 - with 1.5 a good first approximation. For sequential firing down the row in a box cut or V pattern, OSMRE's Surface Blast Design module gives 1 to 1.2 times the burden, or close to a square pattern (https://www.osmre.gov/sites/default/files/inline-files/Module3_0.pdf).

Where it falls downWhere airblast is a problem the delay between holes in a row should be at least 2 ms per foot of spacing, which couples spacing to the timing design and caps how tightly holes can be spaced without producing an audible front. Spacing too wide leaves toe and oversize between holes; too tight wastes explosive and raises the powder factor without improving fragmentation.

SpecificationT (ft) = 0.5 to 1.0 times B, typically 0.7 x B. OSMRE's surface blast design module gives a range of 0.5 to 1.3 times the burden with 0.7 x B as the recommended starting point. USBR: field experience has shown that a collar distance equal to 70 percent of the burden is a good first approximation, and collar distances greater than the burden are seldom necessary. NIOSH's Flyrock Issues in Blasting reports the same figure but attributes it rather than making the recommendation itself: Konya and Walter (1990) recommend a stemming length of about 0.7 times the burden. Under 30 CFR 56.6304, tamping shall not be done directly on a primer, and rigid cartridges 4 inches or larger shall not be dropped on primers except in water-protected blastholes.

Where it falls downImproper or inadequate stemming causes collar zone fragmentation problems and is a documented cause of flyrock and airblast. Stemming that is too long leaves an unbroken collar and oversize. Typical practice, not a published standard: angular crushed rock stems better than drill cuttings because it locks in the hole rather than fluidising.

SpecificationIn most construction blasting, subdrilling is generally limited to 10 percent or less of the bench height. For structural excavations the final lift is usually limited to 5 or 10 ft (1.5 to 3 m); subdrilling is not allowed in a 5-foot lift and is limited to 2 feet (0.6 m) for the 10-foot lift. In OSMRE's powder column rule of thumb, PC = H - T - F - Td, the four variables are hole depth, stemming, backfill and decking; subdrill is not a term in that formula and Td does not mean subdrill. The separate instruction not to load explosives into the subdrill, on the grounds that excessive confinement drives high peak particle velocity ground vibration, is from OSMRE's Surface Blast Design module (Module 3), https://www.osmre.gov/sites/default/files/inline-files/Module3_0.pdf.

Where it falls downSubdrill in a coal operation risks damaging the seam below; in a final wall or a foundation it damages the rock that is supposed to remain, which is why it is reduced to zero on the last lift and replaced by controlled blasting. Too little subdrill gives toe, a humped floor, poor loading conditions and higher drilling costs on the next bench.

SpecificationHole diameter from bench height (OSMRE): d (in) = H (ft) / 5 to H (ft) / 10, typically H/7 - so a 35 ft bench points to a 5 inch hole. The stiffness ratio proper is bench height divided by burden. It is a different quantity from USBR's hole depth-to-burden ratio, because hole depth includes the subdrill, and USBR never calls its ratio the stiffness ratio. USBR's rule of thumb for bench blasting is that the hole depth-to-burden ratio should be between 1.5 and 4.0. Construction blasting practice (USBR): hole diameters usually 3.5 to 4.5 inches (90 to 114 mm), normal drilling depth less than 40 ft (12 m), and patterns from 6 by 8 ft (1.8 by 2.4 m) to 8 by 15 ft (2.4 by 4.6 m).

Where it falls downA bench that is short relative to its burden is over-confined and breaks badly, and the fix is to change the design rather than the explosive loading. A bench that is slender relative to its burden breaks freely, so on the stiffness ratio scale a high value is the favourable case, not a ceiling - the earlier statement here that a ratio above about 4 is a limit inverted the scale. USBR's separate hole depth-to-burden guidance of 1.5 to 4.0 is a different quantity and should not be read as an upper bound on stiffness. Large diameter holes concentrate charge weight per delay, which is what drives ground vibration, so the vibration limit on a site can cap hole diameter independently of fragmentation needs.

SpecificationPF (lb/yd3) = CW (lb) / (B x S x H / 27), where CW is charge weight per hole. Expanded from OSMRE's loading density LD (lb/ft) = 0.3405 x density (g/cm3) x D2 (in): PF (lb/yd3) = 9.19 x L x d x D2 / (B x S x H), with L the charge length in ft, d the explosive density in g/cm3 and D the charge diameter in inches. The coefficient is 27 x 0.3405 = 9.19 because the volume term converts from cubic feet to cubic yards; using 0.3405 without that conversion returns lb/ft3, not lb/yd3, and is wrong by a factor of 27 against the ranges below. USBR surface bench ranges by blasting difficulty: low 0.25-0.40 lb/yd3 (0.1-0.18 kg/m3), medium 0.40-0.75 lb/yd3 (0.18-0.34 kg/m3), high 0.75-1.25 lb/yd3 (0.34-0.57 kg/m3), very high 1.25-2.50 lb/yd3 (0.57-1.14 kg/m3). Underground powder factors are far higher because there is much less free face: 1.5 to 12 lb/yd3 (0.7 to 5.5 kg/m3). A worked OSMRE example produces 0.75 lb/yd3. Presplit lines are loaded to a powder factor of about 0.1 to 0.3 lb/ft3 of face area.

Where it falls downA powder factor alone says nothing about distribution: the same figure achieved with a few heavily loaded holes gives worse fragmentation and more flyrock than the same figure spread across a tighter pattern. Overloading due to powder loss into voids or cracks is a documented flyrock cause, so a high measured powder factor on a hole is itself a warning sign rather than a design choice.

SpecificationDelay time between holes in a row should be between 1 and 5 ms per foot (0.3 m) of burden, with 3 ms per foot of burden giving good results in many types of rock. Delay between rows should be two to three times the delay between holes in a row. Where airblast is problematic, the delay between holes in a row should be at least 2 ms per foot of spacing. For ground vibration control, most regulatory authorities consider two charges to be separate events if they are separated by a delay of 9 ms or more, which is the basis for computing charge weight per delay in a scaled-distance calculation.

Where it falls downInadequate delays between holes in the same row or between rows is a documented cause of flyrock. The 9 ms separation rule is only as good as the detonator's actual firing time, so a timing design built on nominal pyrotechnic delay numbers can collapse two intended events into one if scatter is large. Under 30 CFR 56.6501, detonating cord multiple-row blasts must be laid so detonation can reach each blasthole from at least two directions, with connections tight and at right angles to the trunkline.

Specification94 percent ammonium nitrate, 6 percent fuel oil. Bulk density 0.82 to 0.95 g/cm3 (53.1 lb/ft3, 0.85 g/cm3 as a design value). Detonation velocity 10,000 to 14,500 ft/sec. Detonation pressure 20 to 25 kbar. Water resistance poor; not water-resistant. Non-cap-sensitive, classified as a blasting agent rather than a high explosive. Burden for ANFO is taken at about 25 times the charge diameter in average rock.

Where it falls downLoses sensitivity and can fail to detonate once it takes up water, which is why it is confined to dry holes or must be sleeved. Low detonation pressure makes it poorly suited to hard, dense rock where a higher-pressure product is needed at the toe. A blasting agent by definition cannot be detonated by a No. 8 blasting cap when unconfined, so a booster or cast primer is mandatory.

SpecificationBlend range 20-80 percent emulsion to 80-20 percent ANFO. Bulk density 1.15 to 1.30 g/cm3. Detonation velocity 16,700 to 17,500 ft/sec. Detonation pressure 40 to 55 kbar. Water resistance improved over straight ANFO. At the denser end (about 1.2 g/cm3) the design burden rises to 30 to 35 times the charge diameter, versus 25 for straight ANFO.

Where it falls downWater resistance is improved but not absolute — at lower emulsion percentages the product still behaves substantially like ANFO in standing water. Higher density raises charge weight per delay for the same hole, which tightens the vibration constraint at a given scaled distance.

SpecificationBulk density 1.15 to 1.45 g/cm3. Detonation velocity 14,500 to 18,500 ft/sec. Detonation pressure 100 to 120 kbar — four to six times ANFO. The oxidiser is dispersed as micron-sized droplets within a continuous oil phase, which is what gives the high reaction rate. Extremely water-resistant.

Where it falls downCosts substantially more per pound than ANFO, so it is normally used selectively — in the wet portion of the column or on the toe — rather than as the whole charge. Sensitisation depends on the microsphere or gas-bubble population, which can be degraded by pressure from an adjacent detonating hole (dead pressing) or by long storage in the hole.

SpecificationBulk density 1.0 to 1.35 g/cm3. Detonation velocity 13,000 to 19,000 ft/sec. Detonation pressure 50 to 100 kbar. Water gels resist water by chemical cross-linking; slurries are miscible in water. Less effective at low ambient temperatures. USBR's burden table treats slurry and dynamite-class products at 33, 30 and 27 times charge diameter for light, average and dense rock.

Where it falls downTemperature-sensitive: reduced performance at low ambient temperature limits cold-weather and high-altitude use. Slurry products that are miscible in water lose the water resistance that justified them if the hole stands long enough.

SpecificationDynamite family densities 0.8 to 1.7 g/cm3, detonation velocities 6,500 to 25,000 ft/sec and detonation pressures 9.7 to 190 kbar. Straight dynamite is rated at 50 percent NG weight strength; ammonia dynamite 40 to 60 percent; straight gelatin 20 to 90 percent; ammonia gelatin 40 to 60 percent; semi-gelatin variable. A high explosive is defined as an explosive material that can be caused to detonate with a No. 8 blasting cap when unconfined.

Where it falls downHandled as high explosives throughout, with the transport, storage and magazine consequences that follow. Under 30 CFR 56.6304, rigid cartridges 4 inches or larger shall not be dropped on primers except in water-protected blastholes. Cost per pound and the manual loading rate rule them out for bulk bench work.

SpecificationCast boosters are typically pentolite at 50 percent PETN and 50 percent TNT, with some formulations as high as 60 percent PETN. USBR quotes a cast primer (Composition B) with a density of 1.65 g/cm3 and a velocity of 25,000 feet per second (7,622 m/sec). Definitional distinction: a primer is a booster (cast or packaged high explosive) into which a detonator has been inserted; a booster does not contain a detonator. Under 30 CFR 56.6303, primers shall be made up only at the time of use and as close to the blast site as conditions allow, and the detonator must be securely and completely within the explosive.

Where it falls downCap-sensitive high explosives requiring careful handling, so they are made up at the point of use rather than in advance. An undersized or badly seated primer can leave the column detonating below its rated velocity or failing entirely, which is one of the routes to a misfire in an otherwise correctly loaded hole.

SpecificationUnder 30 CFR 56.6501, in multiple-row blasts the detonating cord trunkline layout shall be designed so that detonation can reach each blasthole from at least two directions, connections shall be tight and shall be kept at right angles to the trunkline.

Where it falls downA detonating cord downline detonates the full length of the column on the way down, so it desensitises some products and contributes directly to airblast from the surface trunkline. Heavier core loads increase that airblast contribution. Because the cord fires the instant the trunkline reaches it, all timing has to be built from surface connectors rather than in-hole delay.

SpecificationRegulatory numbers from 30 CFR Part 56 Subpart E, which differentiate the types directly. Misfire waiting periods under 56.6310: 30 minutes if safety fuse and blasting caps are used; 15 minutes if any other type of detonator is used; 30 minutes if electronic detonators are used, or the manufacturer-recommended time, whichever is longer. Electric blasting circuits shall not be grounded (56.6601). Electrical distribution circuits within 50 feet of electric detonators at the blast site shall be deenergized (56.6402); such circuits need not be deenergized only between 25 and 50 feet of the detonators, and only where stray current tests show a maximum stray current of less than 0.05 amperes through a 1-ohm resistor measured at the blast site. Within 25 feet there is no test exception and deenergizing is mandatory; where extraneous electricity is suspected, loading is suspended until tests confirm stray current does not exceed 0.05 amperes through a 1-ohm resistor (56.6402, 56.6600). At least a 15-foot air gap shall be provided between the blasting circuit and the electric power source (56.6603). Minimum safety fuse burning times by hole count under 56.6502: 1 hole 2 minutes 1 second, 2-5 holes 2 min 40 sec, 6-10 holes 3 min 20 sec, 11-15 holes 5 minutes.

Where it falls downElectric systems are vulnerable to stray current, static and radio frequency energy, which is why the 0.05 ampere thresholds and the 15-foot air gap exist. Pyrotechnic delay elements — in both electric and shock tube detonators — have inherent firing-time scatter, so nominal delay numbers cannot be relied on to keep two charges 9 ms apart at the long end of a delay series. 30 CFR 56.6310(c) sets the standing time for electronic detonators at 30 minutes, or the manufacturer-recommended time, whichever is longer.

SpecificationPresplit spacing and charge by hole diameter (USBR): 1.50-1.75 in holes at 1.00-1.50 ft spacing and 0.08-0.25 lb/ft; 2.00-2.50 in at 1.50-2.00 ft and 0.08-0.25 lb/ft; 3.00-3.50 in at 1.50-3.00 ft and 0.13-0.50 lb/ft; 4.00 in at 2.00-4.00 ft and 0.25-0.75 lb/ft. Smooth blasting: 1.50-1.75 in holes at 2.00 ft spacing on a 3.00 ft burden, 0.12-0.25 lb/ft; 2.0 in at 2.50 ft spacing on 3.50 ft burden. Cushion blasting: 2.00-2.50 in at 3.00 ft spacing on 4.00 ft burden, 0.08-0.25 lb/ft, rising to 6.00-6.50 in at 7.00 ft spacing on 9.00 ft burden and 1.00-1.50 lb/ft.

Where it falls downPresplitting works only where the rock is competent and reasonably free of open structure: in heavily jointed or soft ground the fracture plane follows the structure rather than the drilled line. Subdrilling is not permitted in the final lift of a structural excavation, so the last bench must be taken with controlled blasting rather than with a conventional pattern. Hole deviation over depth destroys the line, so presplit depth is limited by drilling accuracy.