The short answer
Soil and rock change volume as they are handled. One cubic metre in the ground becomes about 1.25 cubic metres in a tipper for ordinary earth and 1.5 to 1.6 for blasted rock. Compacted into an embankment, that same cubic metre of earth ends up as roughly 0.9. The bill of quantities pays in one state and the trucks carry another, so convert before counting loads.
Three people on one job can each be right about the quantity and each have a different number. The surveyor measured the cutting before it was dug. The transport clerk counted tipper loads. The engineer measured the finished embankment. They were measuring the same material in three states.
01The three states
- Bank. Undisturbed, as it lies in the ground. Cutting and rock excavation are measured here, from levels taken before and after.
- Loose. Dug or blasted and sitting in a bucket, a truck or a heap. Broken material has air between the pieces, so it takes up more room.
- Compacted. Placed in layers and rolled to the specified density. Embankment and subgrade are measured here.
02Typical factors
| Material | Loose, m3 | Compacted, m3 | Bank density, t/m3 |
|---|---|---|---|
| Sand and gravel | 1.10 to 1.15 | 0.95 to 1.00 | 1.7 to 1.9 |
| Ordinary earth, loam | 1.20 to 1.30 | 0.85 to 0.90 | 1.6 to 1.9 |
| Clay | 1.30 to 1.40 | 0.85 to 0.90 | 1.7 to 2.0 |
| Weathered rock, ripped | 1.30 to 1.45 | 1.05 to 1.20 | 2.0 to 2.4 |
| Hard rock, blasted | 1.50 to 1.65 | 1.25 to 1.40 | 2.6 to 2.9 |
Planning values. The compacted figure for earth depends on how dense the ground was to begin with: loose natural soil shrinks more, dense soil less. Test your own material before fixing a rate on it.
The pattern to remember is that earth ends up smaller than it started and rock ends up larger. A cubic metre of soil cut from a hillside fills about 0.9 of a cubic metre of embankment. A cubic metre of rock from the same cutting fills 1.3. A mass haul diagram that treats them alike will be out by a third.
03From BOQ to loads
- Compacted volume in the BOQ
- 10,000 m3
- Bank volume to dig, at 0.90
- 11,100 m3
- Loose volume to haul, at 1.25
- 13,900 m3
- Weight, at 1.8 t/m3 in the bank
- 20,000 t
- Loads by volume, 14 m3 body
- 993
- Loads by weight, 18 t payload
- 1,111
Both limits apply and the larger number of loads is the real one. Here the truck reaches its legal weight before its body is full, so weight governs. With a light, dry soil it can go the other way. Price the haulage on whichever is higher. Fleet sizing from that load count is in how many tippers a job needs.
04What the contract actually pays for
On highway work, embankment and subgrade are paid as compacted volume, measured from cross-sections of the finished layer. Borrow excavation is normally not paid separately. So the contractor is paid for 10,000 cubic metres and has dug, hauled, spread, watered and rolled 11,100. A rate built on the BOQ figure alone has left out a tenth of the work.
Cutting is the reverse. It is paid in the bank, and what leaves in trucks is a quarter to two-thirds more volume than the paid figure. Nobody is being cheated in either case. The rate simply has to be built in the unit the bill uses.
05Where the factors go wrong
- Assuming one factor for a whole cutting. Most cuttings pass from soil through weathered rock into hard rock, and each band behaves differently.
- Wet season moisture. Wet clay weighs more per load and may be unusable as fill until it dries.
- Rock in fill. Blasted rock placed as embankment leaves voids unless it is graded and the voids are filled, which is why specifications limit lump size and layer thickness.
- Topsoil. Stripped separately, stockpiled and re-spread. It is not embankment and should not be in the balance.