Dig volume, muck-away and skips
In-situ volume = length × width × depth. Dug soil bulks up, so the spoil to remove = in-situ volume × bulking factor.
Estimates only. Bulking and weight depend on the actual ground and moisture; skip and grab hire firms set their own weight limits — check before you book.
Bulking factors, skips and grab lorries
Typical published earthmoving ranges. Use site investigation data where you have it.
| Soil | Bulking used | Typical range | Loose t/m³ |
|---|---|---|---|
| Clay | 1.30 | 1.20–1.30 | 1.54 |
| Sand | 1.15 | 1.05–1.15 | 1.54 |
| Gravel | 1.20 | 1.10–1.20 | 1.6 |
| Topsoil / loam | 1.25 | 1.20–1.25 | 1.31 |
| Mixed subsoil / made ground | 1.30 | 1.20–1.30 | 1.48 |
| Skip | Volume | Clay dug (in situ) to fill it |
|---|---|---|
| 4-yard | 3.06 m³ | 2.35 m³ |
| 6-yard | 4.59 m³ | 3.53 m³ |
| 8-yard | 6.12 m³ | 4.71 m³ |
Skip sizes are nominal cubic yards (1 yd³ = 0.765 m³). Soil and rubble are heavy: many hire firms will only let a 6-yard skip or smaller be filled completely with soil, and larger skips have weight limits. An 8-wheel grab lorry typically carries about 16 tonnes, so grab loads are worked out by weight.
Excavation safety — CDM 2015
Regulation 22 of the Construction (Design and Management) Regulations 2015 requires all practicable steps to stop an excavation collapsing, material falling on anyone, and people, plant or materials falling in — with support or battering where necessary.
- Inspections: where support or battering is provided, a competent person must inspect the excavation at the start of every shift, after any event likely to affect its stability, and after any material falls or is dislodged — before anyone works in it. Reports are needed under Regulation 24.
- No safe depth: the old "1.2 m rule" is not in the current regulations. Any trench can collapse; one cubic metre of soil weighs over a tonne. Decide on support before you dig and have trench sheets, props or boxes on site.
- Edges and spoil: keep spoil heaps, plant and vehicles back from the edge, and use barriers where people could fall in.
- Services: locate buried cables and pipes before digging, with drawings and a cable locator, and dig carefully by hand near them.
Bulking and muck-away
Example: a 10 m trench, 600 mm wide and 1 m deep is 6 m³ in the ground. In clay with a bulking factor of 1.3 that becomes 7.8 m³ of loose spoil, about 12 t — two 6-yard skips by volume, or one 16 t grab lorry.
Questions groundworkers ask
How do I calculate excavation volume?
Multiply length × width × depth in metres. A 10 m trench 0.6 m wide and 1 m deep is 6 m³ in the ground. Multiply by a bulking factor (about 1.2–1.3 for clay) for the volume of loose spoil to remove.
What is a bulking factor?
Dug soil takes up more room than it did in the ground. Typical bulking factors are about 1.20–1.30 for clay, 1.05–1.15 for sand, 1.10–1.20 for gravel and 1.20–1.25 for topsoil.
How many cubic metres in an 8-yard skip?
About 6.1 m³ (8 × 0.765 m³). A 6-yard skip is about 4.6 m³. For soil and rubble, check the hire firm's weight limit — many only allow a skip up to 6 yards to be filled completely with soil.
When does an excavation need to be inspected?
Under CDM 2015 Regulation 22, an excavation with support or battering must be inspected by a competent person at the start of each shift, after any event likely to affect its stability, and after any accidental fall of material, before work is carried out in it.
More free tools for groundworkers
Concrete calculator — slabs, footings, post holes — m³, bags, mix› Aggregate calculator — Type 1, sand, gravel, topsoil — tonnes and bags› All free calculators for the trades›Muck, plant and money — all accounted for
Excavation inspections, plant and materials logged per job, signed dayworks, delay records and live costing.
See Graftum for groundworksEstimates only; follow CDM 2015 and HSE guidance for excavation support and inspections.