Excavation and muck-away
Bulked spoil volume and skip loads
- where
- bulked (loose) volume (m³)
- volume in the ground (m³)
- bulking or swell (%)
- number of skips
- skip capacity (m³)
Explain this equation
This turns the volume in the ground into the loose volume you actually load and cart away, and how many skips it fills.
V is the in-ground (bank) volume. s is the swell: how much bigger the soil gets once broken up, as a percentage. Vb is the loose volume, Cs the skip capacity and Ns the number of skips.
Digging breaks the soil into lumps with air gaps between them, so the same material takes more room. Multiplying by (1 + s/100) adds that extra. The ⌈ ⌉ brackets round up, because a part-full skip is still a skip.
Sense check: clay swells about 20–25%, so 5 m³ dug fills about 6 m³ of skip. Topsoil swells more, sand and gravel less.
Watch out: skip firms limit weight as well as volume; heavy clay can hit the weight limit first.
Turns the in-ground volume into the loose volume you actually have to take away, and the number of skips that fills.
Enter your values to see the result.
Good to know
Bulking from Caterpillar’s bank and loose densities: clay in natural bed 2020/1660 kg/m³ ≈ 22%; dry clay ≈ 24%; wet excavated earth ≈ 26%; dry sand ≈ 13%; dry gravel ≈ 12%; topsoil ≈ 44%. Broken rock and concrete bulk much more. 1 cubic yard = 0.7646 m³, so a 6 yd³ skip is 4.59 m³ and an 8 yd³ skip 6.12 m³. Skip firms limit the weight of soil and hardcore, often only allowing it in smaller skips and never above the rim: check weight limits.
Common mistake
Filling a large skip with wet clay: it can exceed the legal and lifting weight long before it is full.
Limits
Bulking varies with soil, moisture and how it is dug; the figures are averages. Volume only; check weight with the muck-away tonnes calculator.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS 8500, BS EN 206, BS EN 752, Building Regulations Part H, manufacturers’ data and your engineer’s design. Structural concrete and foundations need a competent designer.