Excavation and soil bulking
Volume of soil in the bank and once dug out (bulked), and transport trips based on the vehicle's capacity.
How it is calculated
The "bank" volume is what the soil occupies undisturbed, as it sits in the ground: length × width × depth. Once dug out, the soil loosens up and takes more volume than in the bank, because air gaps remain between the clods: this is bulking (also called the swell factor).
Loose volume = bank volume × (1 + bulking). Typical bulking coefficients used here: sand 12% (10-15%); topsoil 25% (20-30%); clay 35% (30-40%); blasted or very compact rock 55% (50-60%). This is what actually has to be transported, not the bank volume.
Transport trips = loose volume (in litres) ÷ vehicle capacity, rounded up.
Example: a 4 × 2 × 1 m trench in topsoil, with a 100 L wheelbarrow. Bank volume = 4×2×1 = 8 m³. Loose volume = 8 × 1.25 = 10 m³ = 10,000 L. Trips = 10,000/100 = 100 trips.
Keep in mind
- If you are later going to backfill the same hole with part of that soil (for example, around a footing or a pipe), keep in mind that once recompacted it does not go back to exactly the original bank volume: there is leftover soil even after backfilling and compacting, in a proportion that depends on the ground and how it is compacted.
- Bulking varies a lot with moisture and the exact ground type; the percentages in this calculator are reference figures for an estimate, not for an earthworks contract.
- Before digging, locate and mark any buried services (water, gas, electricity, drainage); digging blind can cut a supply line or be dangerous.