Bank volume and bulking
The hole you dig has one volume. The pile of dirt that comes out of it has a bigger one, because digging breaks the ground up and fills it with air. Engineers call the in-ground figure the bank volume and the dug figure the loose volume, and the ratio between them is the bulking factor. It matters because trucks and tip sites deal in loose volume and tonnes, while the drawing gives you the bank volume.
bank (m³) = length × width × depth loose = bank × bulking factor tonnes = bank × in-ground density
| Ground | Bulking | In-ground density | 10 m³ cut becomes |
|---|---|---|---|
| Sand | ×1.12 | 1.6 t/m³ | 11.2 m³, 16 t |
| Gravel or compacted fill | ×1.15 | 1.9 t/m³ | 11.5 m³, 19 t |
| Topsoil | ×1.25 | 1.4 t/m³ | 12.5 m³, 14 t |
| Clay | ×1.30 | 1.8 t/m³ | 13.0 m³, 18 t |
| Ripped rock | ×1.50 | 2.3 t/m³ | 15.0 m³, 23 t |
Trucks
A small tipper takes about 3 m³, a tandem tipper about 10 m³ and a truck and dog about 20 m³. Those are volumes; the trucks also have a weight limit, roughly 12 to 14 tonnes for a tandem and 25 for a truck and dog, and wet clay reaches the weight limit with room to spare in the tray. Tip sites charge by the tonne and by what the material is: clean fill is cheap or free, mixed soil costs more, and anything with concrete, roots or rubbish in it costs more again. If the ground has been built on before, budget for the dearer rate.
Machine time
Digging rates depend on the machine, the ground and whether the trucks keep up. A 1.7 tonne mini excavator moves 5 to 8 m³ an hour in soft ground and far less in clay with roots. A 5 tonne machine does 15 to 20, an 8 tonne 25 to 30, and a 13 tonne machine 40 or more. The figure in the calculator is the digging rate only; add time for setup, for the machine waiting on trucks, for trimming to level, and for the things you find, which on an old block means everything from a septic tank to a car body.
Safety
Any excavation deeper than 1.5 m is high-risk construction work under Australian WHS and OHS regulations and needs a safe work method statement, and either battered sides or shoring before anyone gets in it. Trenches collapse without warning and the soil is heavy enough to kill. Keep spoil and machines back from the edge, and treat every excavation as if there is a service in it until proven otherwise.
Worked example
A 10 × 8 m house site cut 300 mm deep in clay. Bank volume 24 m³, which becomes 31.2 m³ loose and about 43 tonnes. Four tandem tipper loads by volume, but at 43 tonnes it is closer to four loads by weight with the last one light, so plan for four. A 5 tonne excavator digs it in about 1.6 hours plus setup, so half a day with the truck cycle. At a tip fee of $60 a tonne the tipping alone is about $2,600, which is why keeping fill on site is worth thinking about.
Common questions
How much does dug soil expand?
Sand 10 to 15 percent, topsoil 25, clay 30, ripped rock up to 50.
How many tonnes per cubic metre?
Sand 1.6, topsoil 1.4, clay 1.8, rock over 2, measured in the ground.
How much can a tipper carry?
Small tipper 3 m³, tandem 10 m³ or 12 to 14 t, truck and dog 20 m³ or 25 t.
How long does a dig take?
Mini 5 to 8 m³/h, 5 t 15 to 20, 13 t 40 or more in easy ground with trucks available.
Planning figures only. Ground conditions, access, services and tip classifications change everything; get a site-specific quote from the excavation contractor.