The short answer
Soft and weathered rock is ripped with a dozer or dug with a heavy excavator. Hard rock in open country is drilled and blasted, which is the cheapest method per cubic metre by a wide margin. A hydraulic breaker is for small volumes, trimming and places where blasting is not permitted. Controlled blasting, with small charges fired on delays, covers most of the ground in between.
The method is chosen by two things: how hard the rock is, and what is standing next to it. The first decides what will physically work. The second decides what will be allowed.
01How the specification classifies it
MoRTH Clause 301 sorts excavation into classes, and the class sets the rate. The engineer decides the class on site, and that decision is worth more money than almost any other on the earthwork bill.
| Class | What it covers | Usual method |
|---|---|---|
| Soil | Earth, sand, gravel, murrum and the like | Excavator, dozer |
| Ordinary rock, not requiring blasting | Weathered rock, soft laterite, boulders that can be dug or split | Ripper, heavy excavator, breaker |
| Hard rock, requiring blasting | Sound rock that cannot be removed economically without explosives | Drilling and blasting |
| Hard rock, blasting prohibited | The same rock where explosives may not be used | Breaker, splitter, chiselling |
| Marshy soil | Soft, wet ground | Excavator with special measures |
The fourth class is paid at a much higher rate than the third, with good reason. If a stretch near a village is likely to fall into it, establish that before the bid and not at the first complaint.
02Ripping
A heavy dozer with a single-shank ripper tears weathered and well-jointed rock out of the ground without explosives. It is quick and cheap where it works. Whether it works depends on how fractured the rock is more than on how strong it is. A seismic refraction survey along the cutting gives a wave velocity, and rock under roughly 2,000 metres a second usually rips with a large machine. Above that the dozer polishes the surface and wears out tips.
03Drilling and blasting
For sound rock in volume, nothing else comes close on cost. A single shot loosens hundreds or thousands of cubic metres in an instant, and the excavators then load broken rock all week. The pattern is designed to give the fragment size the next user wants: small for embankment fill, coarser for crusher feed. The ratios are in reading a drilling pattern.
Blasting needs a licensed crew, a licensed magazine or delivery arrangement, and a cleared danger zone for each shot. Those are fixed costs, so the method favours volume. For a few hundred cubic metres in a culvert foundation it is rarely worth mobilising.
04Controlled blasting
Close to houses, roads, pipelines or the final face of a cutting, the same explosives are used with tighter limits. Holes are smaller and closer, the charge fired at any one instant is capped by a vibration limit, and the shot may be covered. Along the final slope, a line of closely spaced, lightly charged holes is fired first to crack a clean plane, which is pre-splitting. It leaves a smooth face that stands, and it stops the main blast shattering rock that has to remain.
This is most highway rock cutting in practice. The limits and the design response are in controlled blasting near structures.
05Breaking and splitting
A hydraulic breaker on an excavator needs no licence, no danger zone and no notice. Its drawback is output. In hard rock a 20 tonne machine may remove only a few cubic metres an hour, and it wears chisels and its carrier doing so. The cost per cubic metre is many times that of blasting.
Where even a breaker is unsuitable, holes are drilled and filled with an expanding grout that cracks the rock overnight, or a hydraulic splitter is used. Both are slow and both are for small, awkward quantities: a foundation beside an old structure, a trench under a live line.
06Choosing
| Method | Best for | Relative cost per m3 | Main limit |
|---|---|---|---|
| Ripping | Weathered, jointed rock in open cut | Lowest | Fails in sound rock |
| Production blasting | Sound rock, large volume, open country | Low | Needs clearance and licences |
| Controlled blasting | Sound rock near structures or final faces | Moderate | Slower, more drilling per m3 |
| Hydraulic breaker | Small volumes, trimming, oversize | High | Low output |
| Expanding grout, splitter | No-vibration zones | Highest | Very slow |
07Overbreak is not paid
Excavation beyond the lines on the drawing is not measured, and where it weakens a slope or a foundation it has to be made good at the contractor's cost. A heavy blast that saves a day and takes an extra half metre off the design slope has cost money twice. Careful drilling along the final line is cheaper than backfilling in concrete.