Ground in its natural state is packed tight. Dig it up and it loosens, taking up more space. This is swell, and it's why excavation jobs so often need more skip bins or truck loads than planned. The reverse also happens: loose fill compacts down when you roll it, so you need more of it than the hole's volume suggests.
Bank, loose and compacted volume
- Bank volume: the volume of the hole as measured, with the soil still in the ground.
- Loose volume: what the dug material occupies on a truck or in a pile: bank volume × swell factor.
- Compacted volume: what it occupies after rolling or tamping back into place, often less than the bank volume for granular material.
Typical swell factors
| Material | Swell |
|---|---|
| Sand and gravel | +10–15% |
| Common earth / loam | +20–30% |
| Clay | +30–40% |
| Rock, broken concrete | +50–80% |
These are typical figures. Wet clay and blasted rock can swell even more.
Working out loads
Loads = (bank volume × swell factor) ÷ vehicle capacity, rounded up. A 10 × 6 m dig 600 mm deep is 36 m³ in the bank. Common earth at +25% becomes 45 m³ loose: eight 6 m³ skip bins or tipper loads, weight permitting.
Pricing removal
Skip bins are hired by volume, commonly 2–10 m³, but bins for soil and rubble have weight limits and may need to be booked as heavy-waste bins, and mixed loads cost more. Tipper trucks, often about 6 m³ or 10 tonnes, can be cheaper for big digs. Check how each prices, and whether a bin on the street needs a council permit.
Clean, separated material is cheaper to get rid of than mixed waste. Topsoil is worth keeping separate: it can often be reused on site or sold.
Backfill and compaction
When filling, work the other way. To end up with a compacted volume, you need more loose material: typically 15–25% more for granular fill. Compact in layers. Most base materials should go down no more than 75–100 mm at a time under a plate compactor, or thicker under a roller.
Spoil you dug out isn't always good backfill. Clay and organic soils shrink, swell and settle, which is why specifications often call for imported granular fill under slabs and around pipes.
Trenches
Trenches for footings and services are the same calculation, but measure the real width. A 600 mm bucket digs wider than 600 mm, and trench sides often collapse in loose ground. Allow 10–15% extra on trench volumes.
Before you dig
Lodge a free Before You Dig Australia (BYDA) enquiry before digging, then locate services on site. Assume they're there until you've proved they aren't. Note any trees nearby, which affect foundation depths on clay, and plan where spoil will be stored before it's taken away.
The excavation calculator includes swell and loads by vehicle, and the material weight calculator checks loads against payload.
A worked example
A strip footing trench 18 m long, 600 mm wide and 1 m deep is 10.8 m³ in the bank. Add 10% for over-dig: 11.9 m³. Clay at +35% becomes about 16 m³ loose: three 6 m³ skip bins. Check the weight too: about 21 tonnes of clay, so a 10-tonne tipper needs three loads.
Planning the dig
Excavation is easy to underprice because the hole itself looks simple. Before you quote, think through the whole sequence:
- Machine size. A mini excavator that fits through a side gate digs far slower than a larger machine with open access. Ask how wide the gate is before you choose.
- Where the spoil goes. If a skip bin or truck can't be loaded straight from the machine, every bucket has to be moved twice: once to a pile and again into the vehicle.
- Ground conditions. Groundwater, buried rubble, old foundations and tree roots all slow digging and change what you're removing.
- Protection. Driveways and lawns crossed by machines may need boards or track mats.
- Disposal costs. Clean soil, mixed waste and contaminated material each cost differently to get rid of. Rubble is often cheaper than mixed waste, and clean topsoil can even be sold.
Price machine time, a spotter or labourer, spoil removal, and fuel as separate lines. If anything about the ground is uncertain, say so in the quote and price any extra work as a variation.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the NCC, Australian Standards and your building surveyor or certifier for your project.