Load, haul & blast Guide
Powder factor and fragmentation: the basics for contractors
Powder factor is explosive mass per bank cubic metre or tonne blasted. The formula, a worked blast pattern, and what fragmentation does to load and haul.
Contents 7 sections
Powder factor is the mass of explosive used in a blast divided by the rock it breaks, written in kilograms per bank cubic metre or kilograms per tonne. For one hole it is the charge in the hole divided by burden × spacing × bench height, and it has a strong influence on how fine the rock breaks.
That matters to a load and haul contractor. Fragmentation sets how well the bucket fills, how many passes a truck needs, how much the crusher takes in an hour and how much rock has to be broken a second time. A study of the Aitik open pit, published by the SAIMM, found that blast fragmentation “has a significant influence on the operational efficiency of open pit mines, especially on loading and crushing, the two immediate tasks after blasting” (Beyglou, Johansson and Schunnesson 2017, read 25 Sep 2026).
Powder factor definition and formula
A blasting paper from the University of Mines and Technology in Tarkwa gives the standard definition: “the ratio between the total weights of the explosives detonated in a blast divided by the amount of rock that is broken”, and adds that it “has a vital influence on the resultant fragmentation” (Agyei and Owusu-Tweneboah 2019, read 25 Sep 2026). The US Office of Surface Mining sets out the same calculation hole by hole: “powder per hole divided by rock volume per hole” (OSMRE, Blast Design Rules of Thumb, read 25 Sep 2026).
Charge length (m) = hole depth (m) - stemming (m)
Linear charge (kg per m) = π ÷ 4 × hole diameter² (m²) × explosive density (kg per m³)
Charge per hole (kg) = linear charge × charge length
Rock per hole (bank m³) = burden (m) × spacing (m) × bench height (m)
Powder factor (kg per bank m³) = charge per hole ÷ rock per hole
Powder factor (kg per tonne) = powder factor (kg per bank m³) ÷ in situ rock density (tonnes per m³)
A few points on using it:
- Use bench height for the rock volume. Subdrill below the floor is charged but breaks rock that belongs to the next bench. OSMRE’s coal sheet uses hole depth in its formula. Say which one your figure uses, so blasts are compared like for like.
- Per bank cubic metre or per tonne. Blast design works in bank cubic metres, the rock in place before it swells. Contracts are often paid per tonne. Converting needs the in situ density, the same number used to turn bank cubic metres into tonnes.
- Use the explosive actually loaded. OSMRE’s advice on flyrock includes: “always measure explosive quantities or tape the holes while loading” (OSMRE Module 8, read 25 Sep 2026).
- Geology changes the target. “Competent rock or formation with few or no pronounced planes of weakness require higher powder factor than a formation with numerous closely spaced structural weakness” (Agyei and Owusu-Tweneboah 2019). A figure that works in one bench may be wrong in the next.
Worked example: powder factor for a blast pattern
The right figure for a given bench comes from test blasts, measured fragmentation and the explosives supplier’s advice. Under regulation 4.15 (GN R. 953 of 2018), the employer must take reasonably practicable measures to ensure “the maximum amount or mass of explosives used per shot hole is according to the manufacturer’s or supplier’s recommendations”.
What blast fragmentation does downstream
Loading
Caterpillar’s Performance Handbook gives wheel loader bucket fill factors for blasted rock of 80% to 95% when well blasted, 75% to 90% for average, and 60% to 75% when poorly blasted. For hydraulic excavators it gives 60% to 75% for well-blasted rock and 40% to 50% for poorly blasted rock (Caterpillar Performance Handbook 50, sections 5 and 20, read 25 Sep 2026). For hydraulic mining shovels it notes that “material fragmentation and penetration resistance make a big difference” to cycle times.
Each extra pass adds to the load time in every haul truck cycle. Each light truck raises the cost of every tonne it carries, which is worked through in payload management. At Aitik, the researchers found the shovel’s fill factor was highest in medium-fine fragmentation and fell steadily towards the coarsest class. An earlier Aitik study by the same authors concluded that “the influence of muckpile diggability on loading is greater than that of operator skills” (cited in Beyglou et al. 2017).
Crushing
At Aitik, “the crushing energy consumption gradually increases as fragmentation shifts from fine to coarse. Meanwhile, throughput shows a consistent decrease with increasing fragment size.” The drop was not gradual all the way: the study found “a sudden reduction in efficiency for P80 values coarser than 800 mm” and suggested a target P80 of 600 to 800 mm for that mine (Beyglou et al. 2017). P80 is the size that 80% of the rock passes. Every mine’s threshold differs; the pattern is that throughput holds up until the feed passes it, then drops.
Secondary breaking and oversize
Boulders cost twice. The Tarkwa study notes that “the presence of boulders requires secondary fragmentation to further reduce the broken materials to the acceptable sizes, thereby increasing the cost of production” (Agyei and Owusu-Tweneboah 2019). Caterpillar’s loading advice for its large trucks says large rocks should be broken up to limit loading damage and spread the load evenly, and warns that extreme rock drops “will cause frame and body damage” (Caterpillar, 6 loading tips, 2020, read 25 Sep 2026).
Too much of a good thing
A higher powder factor is not free. OSMRE lists “burdens and spacings too close together (resulting in high powder factors)” among the causes of flyrock (OSMRE Module 8). Fine fragmentation also has limits for loading: at Aitik, the finest class of material filled the dipper less well than medium-fine. Caterpillar’s truck body guide notes that fragmentation size “varies by material type (such as overburden vs. coal) and the intended downstream processing” (Cat Truck Bodies, 2023, read 25 Sep 2026).
Predicting fragmentation: Kuz-Ram in brief
The best-known prediction tool is the Kuz-Ram model (Cunningham, 1983), which builds on an equation by Kuznetsov (1973) and uses the Rosin-Rammler function for the spread of sizes (Beyglou et al. 2017; Agyei and Owusu-Tweneboah 2019). Its inputs include a rock factor, the powder factor in kg per m³, the charge per hole and the explosive’s strength relative to ANFO. Cunningham adjusted the model in 2005.
The Aitik authors point to its limit: it treats fragmentation as a function of burden and spacing, while in practice “variations in lithology, structural geology, explosive performance and precision of drilling and blasting” make run-of-mine fragmentation vary from blast to blast (Beyglou et al. 2017). Use it to compare patterns, then check the result with photographs or scans of the muckpile and with the loader’s own payload and pass counts.
Who may blast in South Africa
Blasting on South African mines is governed by Chapter 4 of the regulations under the Mine Health and Safety Act, 1996 (Act 29 of 1996). Chapter 4 was substituted in full by Government Notice R. 953 in Government Gazette 41904 of 14 September 2018, to come into operation three months after publication (GN R. 953, read 25 Sep 2026). The Mine Health and Safety Council’s amendment note records that the new definition of competent person took effect on 14 December 2018 (MHSC, Explosives Regulations Amendment Note 2019, read 25 Sep 2026).
The points a contractor working next to a blasting team should know:
| Regulation | What it requires (summary, with the regulation’s words in quotes) |
|---|---|
| 4.4(1) | Where “primary or secondary blasting takes place”, the employer must appoint “a competent person … in writing” to control the explosives for the working places in their charge, prepare primers, deal with misfires and sockets, “connect blasting rounds or circuits” and “charge shot holes with explosives or place explosive charges” |
| 4.4(3) | The employer may appoint competent persons in writing to assist with preparing primers, charging holes, connecting rounds and handling and transporting explosives |
| 22.4.1 | For 4.4(1), the competent person must hold a certificate “recognised by the Department for this purpose, valid for the class of mine”: a Department blasting certificate issued until 30 June 2009, a rock breaker or equivalent certificate from the Mining Qualifications Authority issued from 1 July 2009 to 31 May 2017, or a “Blasting certificate issued by the Department with effect from 01 June 2017” |
| 4.15 | The employer must take reasonably practicable measures so that the maximum mass of explosives per shot hole follows “the manufacturer’s or supplier’s recommendations” |
| 4.14(3) | Stemming material must comply with SANS 120:2009, “Stemming for use in blasting” |
| 4.16(2) | No blasting within 500 metres of “any public building, public thoroughfare, railway line, power line, any place where people congregate” or other structure that needs protection, unless a risk assessment identifies a lesser safe distance and the Principal Inspector of Mines gives written approval |
The duty sits with the employer. On a contract mining site, that means the contract and the mine’s appointments must be clear about whose competent person charges and fires each blast. The regulations define blasting as “the initiation of explosives for the purposes of fragmenting of rock or ore body”. Check the current consolidated text of Chapter 4 before relying on this summary, in case of later amendments. The wider duties under the Act are covered in the Mine Health and Safety Act for mining contractors.
Common questions
Is powder factor measured per cubic metre or per tonne?
Either. Per bank cubic metre is the usual blast design figure. Per tonne is useful where the contract pays per tonne. Divide the kg per bank cubic metre by the in situ rock density in tonnes per cubic metre to convert.
Does a higher powder factor always give better fragmentation?
No. Powder factor has a strong influence on fragmentation, but geology, stemming, timing and drilling accuracy change the result too. Burdens and spacings set too tight are a known cause of flyrock, and at Aitik the finest class of rock filled the shovel dipper less well than medium-fine rock. Aim for the fragmentation the loader and crusher need.
Who may charge and fire a blast on a South African mine?
The employer must appoint in writing a competent person to control the explosives, prepare primers, charge holes, connect the round and deal with misfires. The competent person must hold a certificate recognised by the Department for the class of mine: a blasting certificate issued by the Department, or a rock breaker certificate issued by the Mining Qualifications Authority between 1 July 2009 and 31 May 2017 (MHSA regulations 4.4(1) and 22.4.1).
Sources
- Department of Mineral Resources, Mine Health and Safety Act, 1996: Regulations relating to explosives, GN R. 953, Government Gazette 41904, 14 Sep 2018. Read 25 Sep 2026.
- Mine Health and Safety Council, Explosives Regulations: Amendment Note - 2019. Read 25 Sep 2026.
- US Office of Surface Mining Reclamation and Enforcement, Blast Design Rules of Thumb - Coal. Read 25 Sep 2026.
- US Office of Surface Mining Reclamation and Enforcement, Blaster training Module 8: Controlling the Adverse Effects of Blasting. Read 25 Sep 2026.
- Agyei, G. and Owusu-Tweneboah, M., A Comparative Analysis of Rock Fragmentation using Blast Prediction Results, Ghana Mining Journal 19(1), 2019, pp. 49-58. Read 25 Sep 2026.
- Beyglou, A., Johansson, D. and Schunnesson, H., Target fragmentation for efficient loading and crushing: the Aitik case, Journal of the Southern African Institute of Mining and Metallurgy 117(11), 2017. Read 25 Sep 2026.
- Caterpillar, Caterpillar Performance Handbook, Edition 50 (SEBD0351-50), June 2022, sections 5, 13 and 20. Read 25 Sep 2026.
- Caterpillar, Cat Large Mining Trucks: 6 loading tips to help improve truck reliability, 2020. Read 25 Sep 2026.
- Caterpillar, Cat Truck Bodies, PEDJ0489-01, 2023. Read 25 Sep 2026.
