In an opencast mine, a good blast is not luck. It starts with where you drill the holes and the order in which you fire them. Get the pattern right and you get well-fragmented rock, less secondary breaking and a safer bench. Get it wrong and you end up with boulders, fly rock and wasted explosives.
This post walks through the basic drill hole patterns used in opencast mines, the common blasting patterns, the charge calculation formulas and the deck charging techniques. It is useful revision for GATE Mining, DGMS and Mining Sirdar aspirants as well as working supervisors.
Basic Drill Hole Patterns at a Glance
The patterns used in opencast mines are:
- Square pattern
- Staggered (triangular) pattern
- Single row blasting pattern
- Multi row blasting pattern
- Wedge cut pattern
- Diagonal blasting pattern
The first two describe how the holes are laid out on the bench. The remaining four describe how the blast is organised and fired. Keeping that difference in mind makes the whole topic much easier.
1. Square Pattern
The square pattern is also called a multi row pattern. Holes are drilled in rows, one directly behind the other, so the holes line up in both directions.
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| Square pattern |
2. Staggered (Triangular) Pattern
Here too the holes are drilled in rows, but each hole in the second row sits midway between two holes of the first row. This offset spreads the explosive energy more evenly across the rock, which is why many mines prefer it.
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| Staggered or triangular pattern |
Blasting Patterns
Once the holes are drilled, the blast itself can be classified by the number of rows and the firing sequence. There are two main types by rows: single row and multi row. Multi row blasts are then fired using either a wedge cut or a diagonal sequence.
3. Single Row Blasting Pattern
- All holes are drilled in a single row.
- Specific explosive consumption is low.
- The danger from fly rock is higher, so extra care is needed.
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| Single row blasting pattern |
4. Multi Row Blasting Pattern
- Holes are drilled in several rows.
- The holes may be laid out in a staggered or a square pattern.
- After drilling, the blast is fired in either a wedge cut or a diagonal sequence.
5. Wedge Cut Pattern
- It is used in hard and medium hard rocks.
- Delays are fixed from the middle towards the ends, as shown in the figure.
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Wedge cut pattern
The sequence goes like this: the centre hole of the first row fires first. Then the holes connected to the neighbouring holes on either side fire with delay No. 1, and the rest follow in the sequential order of the delays.
6. Diagonal Blasting Pattern
- It is used in benches where the cleavage plane of the rock runs parallel to the length of the face.
- It helps reduce secondary blasting.
- Holes can be drilled in a square or a staggered pattern.
- Delays must be planned so that blasting proceeds from one side to the other.
- Delays must also be in the proper sequence to avoid any cut-off.
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| Diagonal blasting pattern |
Quick Comparison
| Pattern | Key Point |
|---|---|
| Square | Holes in rows, directly one behind the other |
| Staggered | Second row sits midway between first-row holes |
| Single row | Low explosive consumption, more fly rock risk |
| Multi row | Several rows, square or staggered holes |
| Wedge cut | For hard and medium hard rock, firing from the middle outwards |
| Diagonal | Cleavage parallel to face, fired from one side to the other |
How Much Explosive Do You Need?
The explosive required for a round of holes is only an estimate. In practice, mines usually arrive at the right quantity by trial and error, because the properties of the rock mass vary from place to place. Still, the charge per hole can be worked out with a formula:
Q = E × (H − St) × πD² / 4
- Q = quantity of charge per hole
- H = depth of the hole
- St = height of the stemming column
- D = diameter of the hole
- E = specific weight of the explosive
Two more terms are used to judge how efficient a blast is:
- Specific consumption (C) = Q / (B × S × H), in kg/m³
- Powder factor (P) = (B × S × H) / Q, in m³/kg
Here B is the burden and S is the spacing. In simple words, B × S × H gives the volume of rock each hole is expected to break. The usual order of work is to calculate the charge per hole first, based on the blast geometry and the powder factor, and then multiply to get the total charge for the blast.
Deck Charging
Deck charging is a controlled technique that spreads the blasting energy along the whole length of the drill hole, instead of packing it all at the bottom. It is followed in benches where the strata and grade vary.
The primer is placed at the bottom of the hole, followed by the base charge. A part of the hole is then filled with stemming material, and the same procedure is repeated for the next deck.
Air Deck Method
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Air deck method
Multi Deck In-Hole Delay Method
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| Multi deck in-hole delay method |
Why Use Deck Charging?
- It produces fewer oversize boulders and a smaller zone of uncontrolled rock breaking.
- About two-thirds of the charge generally goes in the lower part of the hole, and the remaining one-third is spread over the upper two decks.
- The explosive in the upper decks is usually of lower strength than the bottom charge.
- Each deck is primed separately.
Deck charging with air gaps is a modified version of the same idea. Air gaps are created wherever stemming would normally go, using expanding-type air bags. The number of decks and the length of the stemming column or air gap depend on the nature of the rock strata. The air gap usually runs from 20 to 35 percent of the hole length.
Quick Revision Points
- Square pattern: holes directly behind each other. Staggered pattern: second row midway between the first-row holes.
- Single row blasting uses less explosive but carries a bigger fly rock risk.
- Wedge cut is for hard and medium hard rock, with delays running from the middle to the ends.
- Diagonal pattern suits benches where the cleavage runs parallel to the face, and it reduces secondary blasting.
- Powder factor = (B × S × H) / Q, in m³/kg.
- In deck charging, roughly 2/3 of the charge goes in the lower deck and 1/3 in the upper decks. Air gaps are typically 20 to 35% of the hole length.
Blasting calculations and patterns change from mine to mine, so always follow the approved blasting plan and DGMS rules at your site. If this post helped, share it with your study group and drop your questions in the comments.
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