Short answer
The commonest mistake when comparing blasting with mechanical excavation is costing the blast on explosives and drilling alone. The cost of a blast is a chain: drilling, explosives, permits and notification, production stopped for the exclusion zone, scaling afterwards, and secondary breaking of the blocks that will not enter the crusher. Mechanical excavation has fewer items but a higher hourly rate: machine, attachment, wear parts and time. The comparison only means something when both chains are laid out in full; a comparison made on one item almost always makes blasting look cheap.
Items in the blasting chain
- Drilling: number of holes, depth and drilling rate. In hard, abrasive rock drill bit consumption is an item of its own.
- Explosives and the initiation pattern.
- Permits, notification and records: near settlements in particular, administrative time is a cost.
- Exclusion zone time: production stopped is not just the blast itself but the time to set up and clear the zone.
- Scaling after the blast: bringing down loose blocks left on the face, which has to be done before loading starts.
- Secondary breaking: reducing the blocks that will not enter the crusher. Where the size distribution is poor, this item grows quickly.
- Vibration and airblast management: measurement, complaint handling and, where needed, a change of design.
Items in the mechanical excavation chain
That last point is the most overlooked side of the comparison: in mechanical excavation the size distribution of the material is controlled, so crusher feed is regular and the need for secondary breaking is small.
- Hourly cost of machine and attachment: depreciation, maintenance, fuel and operator.
- Wear parts: in abrasive rock this item grows markedly and is the most variable part of the comparison.
- Time: moving the same volume takes longer than blasting, and that time feeds into the other items of the project.
- Downtime: an attachment failure stops production directly; spare parts availability sets this item.
- On the credit side: the scaling and secondary breaking items largely disappear, because the material comes out already sized.
When does mechanical excavation win?
- Rock of medium hardness and fractured: mechanical excavation speeds up and blasting’s advantage narrows.
- A quarry close to settlement: permit time, vibration management and complaint risk all lengthen the blasting chain.
- Work needing profile accuracy: final slopes, profiles or a limited excavation volume; blasting cannot deliver that accuracy.
- Small and scattered excavation volumes: when blasting’s fixed costs (permits, zone, preparation) are divided over a small volume, the cost per cubic metre rises.
- Operations that cannot stop production: where exclusion zone time creates continuous lost output, mechanical excavation offers predictability.
We print no figure here, because the comparison is yours
Cost per cubic metre varies with the hardness and abrasivity of the rock, your drilling rate, explosive prices, how close your quarry is to settlement and your machine fleet. A number offered as an industry average does not improve your decision. What to do is fill in both chains with your own data; this guide gives the method, your own records give the numbers.
What does the rock itself change?
What drives the cost of mechanical excavation most is not the hardness of the rock but its abrasivity. Hard but not very abrasive rock gives a predictable cost in mechanical excavation; medium hard but highly abrasive rock inflates the wear parts item.
The second driver is the fracture pattern. A fractured mass offers mechanical excavation a free face and progress speeds up; in a massive, uncracked mass mechanical excavation slows markedly.
How those two properties change the attachment decision is covered in the rock hardness and abrasivity guide; the same two properties are also inputs to the blasting versus mechanical comparison.
Do and don’t
Do
- Cost blasting over the whole chain; drilling and explosives alone mislead.
- Put secondary breaking and scaling on the blasting side; in mechanical excavation they are largely absent.
- Calculate the wear parts item with your own rock data; abrasivity sets that item.
- Make the comparison per cubic metre, not per hour.
- Consider a hybrid: mass excavation by blasting, final slopes and sensitive areas mechanically.
Don't
- Do not call blasting cheap on the explosive cost alone.
- Do not call mechanical excavation expensive on the hourly rate alone.
- Do not leave permit and exclusion zone time out of the cost.
- Do not ignore the effect of size distribution on crusher feed.
- Do not take the decision once and for all; the rock changes within a quarry and the decision may change by area.