Underground Round Charge & Powder Factor
How much explosive a development round takes, and whether the powder factor
that falls out of it is sane for a heading that size.
Underground development headings only: a face with one free face, broken
to a cut. It is not surface bench blasting, where the burden breaks to an open face and the
arithmetic is different. It is not a blast design: no delay timing, no initiation sequence, no
vibration or airblast prediction, and no perimeter-control design. Blasting is licensed work.
Step 1: The heading and the round
The face area is what the powder factor is judged against.
Step 2: The explosive
The loading factor is an equation, not a lookup.
Step 3: The holes
Relief holes carry no explosive and are counted separately.
Step 4: Checks
Published limits. Outside them you are past what the sources cover.
| Quantity | Published | Yours |
|---|
The powder factor band and the V-cut width rule are both from SME Mining Engineering Handbook p. 452. The loading factor is Equation 11.2, SME Mining Reference Handbook 1st ed. p. 205, after IME 1997.
| Step | Value | Where it comes from |
|---|
Where the powder goes
Why small headings burn more powder
Why the powder factor is not a constant
SME Mining Engineering Handbook p. 452, in full:
"Because the same sized burn cut is used for a small-diameter tunnel or a large R&P
heading, the powder factor drops from around 6 kg/m³ for a very small heading to 1.2 kg/m³
for a large heading."
That sentence contains the whole idea. The cut is a fixed-size feature: it takes roughly the same
holes and the same powder to open a void whether the heading around it is 6 m² or 30 m².
In a small heading that fixed cost is spread over very little rock, so the powder factor is high.
In a large one the same cut is a rounding error against the face holes.
So a powder factor is only high or low relative to the size of the heading it came from,
and comparing two headings' powder factors without comparing their areas is close to meaningless.
This page always reports the two together.
The Reference Handbook plots the same relationship as Figure 11.4, "Relationship of powder factor
to heading area", with separate curves by rock type and hole diameter. That figure is a
picture, not a table. It is not in the text layer and cannot be read off as data, so this
page uses the two published endpoints and does not draw a curve between them. If you want the
curve, read it off p. 285 of the printed book.
The loading factor equation
SME Mining Reference Handbook 1st ed. p. 205, Equation 11.2, after IME 1997:
LF = 0.3405 d² ρ
where LF is the loading factor in lb/ft, d the explosive column diameter in inches, and ρ the
specific gravity of the explosive.
The book prints a 28-row table of this equation evaluated at 11 densities, and says in its own
words that "values not contained in Table 11.3 can be calculated" from the equation. So this page
computes it rather than carrying the table, which removes a whole class of transcription risk
from the page you are reading.
The constant is derivable and worth checking, because a constant you can derive is one you do not
have to trust: a cylinder of diameter d inches has area πd²/4 square inches, a foot of it
is 12πd²/4 cubic inches, and water weighs 0.0361273 lb/in³. Multiply those and you get
0.34049, which is the printed 0.3405. Nothing was taken on faith.
Spending more time on spreadsheets than on the job?
We build the reporting and admin automation for mining contractors, so the numbers keep themselves.
Book a 20 min callWorked example
Worked with the values this page loads with, so the method can be checked against the cited source without entering anything. Change any input above and the calculator recomputes; the numbers below stay as a fixed reference case.
- 234.5 kg
- 2.55 kg/m³
- 3.68 m
| Heading face | 5.00 m × 5.00 m | your input |
| = face area | 25.00 m² | width × height, arch treated as square |
| Drilled depth | 4.00 m | your input |
| × pull | × 0.92 | your input |
| = advance per round | 3.68 m | what the round actually wins |
| = volume broken | 92.0 m³ | face area × advance |
| = tons broken | 248 t | × 2.70 t/m³ |
| Explosive | ANFO poured | p. 279 |
| its specific gravity | 0.810 | midpoint of 0.77-0.85, p. 279 |
| Explosive column diameter | 45 mm | your input |
| = loading factor | 1.288 kg/m | 0.3405 × 1.772² × 0.810 lb/ft, Eq. 11.2, p. 205 |
| Charged length per hole | 3.50 m | depth − stemming |
| = charge per face hole | 4.5 kg | loading factor × charged length |
| × charged face holes | × 34 | your input |
| Perimeter share of a column | × 1.00 | your standard; not published |
| = charge per perimeter hole | 4.5 kg | face hole × perimeter share |
| × perimeter holes | × 18 | your input |
| = explosive per round | 234.5 kg | the sum of every charged hole |
| = powder factor | 2.55 kg/m³ | charge ÷ volume broken |
| = explosive per ton | 0.944 kg/t | charge ÷ tons broken |
| Drilling per round | 224.00 m | (34 + 18 + 4 relief) × 4.00 m |
Estimating aid only. This is not an engineering design and not a substitute for the judgment of a competent or qualified person. Verify every result against the cited source and your own site conditions. Full terms and disclaimer.