Excavation Classes: From Soft Soil to Hard Rock

The most disputed item in any earthworks quote is which class the material falls into. The difference is not academic: between soft soil and hard rock the unit rate changes several times over and output drops to a third.

Seven classes

Effect on output

With the same 20-tonne excavator, indicative figures:

The point: hard rock takes ten times as long as soft soil. Where the ground is uncertain, the quote is uncertain too.

Class changes mid-contract

Encountering different ground than expected is common. The contract should say how that is handled: who determines the change, how the new rate is set, and who signs the record. Without those clauses, work stops when rock appears and negotiation starts again.

Related: How to choose excavator size

How class affects price

The price difference arrives through two channels that compound each other:

1. Time. Hard rock takes roughly ten times as long as soft soil. The same machine and operator produce a tenth of the output in the same hour.
2. Equipment. The moment a breaker is needed, a separate attachment rental, a consumable line (breaker tools) and higher fuel consumption all begin.

This is why a single "earthworks unit rate" in a contract is not workable. A rate schedule by class protects both sides: when rock appears, work continues at the scheduled rate instead of stopping for renegotiation.

Batter slopes and real excavated volume

Class also determines whether the excavation can be cut vertically. Loose ground will not stand vertical; a batter must be cut, which increases the volume above what the drawing shows.

A useful consequence: rock costs more per cubic metre but you excavate fewer cubic metres, because no batter is needed. On deep excavations the two effects partly cancel.

How the class is determined on site

1. Geotechnical report. The most reliable route. If a soil investigation exists, share it at quotation stage.
2. Neighbouring works. Recent excavations in the same area are a good indicator.
3. Trial pit. On larger works, a pit dug before the main excavation removes most of the uncertainty for a few hours of plant time.
4. Local knowledge. Contractors working an area usually know what comes out of it.

Frequently asked

The contractor says rock has appeared — how do I verify it?

Ask for a record: date, level, photographs and, if possible, a sample. Where the contract sets out how a class change is determined and priced, this does not become a dispute. Where it does not, it becomes one.

Is blasting cheaper than breaking?

On large volumes, yes. But blasting requires permits, is often impossible near occupied buildings, and is a separate specialism. On small and medium works a hydraulic breaker is in practice the only option.

If the class changes, does the programme change too?

Yes, and usually more than the price does. A job that meets hard rock can run to three times its planned duration. The contract should set out how extensions of time are handled, not just rates.

Can rock be reused on site?

Often, yes — crushed and used as sub-base. That saves both haulage and imported material, and is worth planning for before excavation starts rather than after.