Bank, loose and compacted — three different cubic metres
The same soil occupies three different volumes. Bank volume is the ground in place, and it is what you measure from the drawing. Loose volume is that soil after it has been dug, and it is bigger — the swell factor is typically 20–25% for common earth, 10–15% for sand and 40–60% for rock. Compacted volume is what you get after it is placed and rolled back down, and it is smaller than bank. The consequence is unavoidable: you excavate 10 m³, you haul away roughly 12.5 m³, and if you then need 10 m³ of fill you must import more than 10 m³ of loose material to compact down to it. Quote the wrong volume and the haulage line is wrong by a quarter.
Trenches, pads and pier holes
A trench is length × width × depth, but the width is rarely the drawing width: a shored or battered trench opens out, and the bucket width sets a practical minimum — a 600 mm bucket digs a 600 mm trench even if the pipe only needs 300 mm. Pier and post holes are cylinders (π r² × depth) and a bell-out at the base adds more than people expect. Pads and slabs are simple rectangles until you add the over-dig, and there is always over-dig. When the excavation is stepped or the ground slopes, split it into prisms and sum them rather than averaging a depth across the whole footprint — averaging hides exactly the error you are trying to catch.
Getting the spoil off site
Spoil is priced by the load, so the question is how many loads. A standard tandem dump truck carries 10–14 yd³ loose but is usually weight-limited before it is volume-limited: wet clay at about 1.7 tonnes per cubic metre will hit the axle limit long before the body is full. A 6-yard builder's skip takes about 4.5 m³ of loose soil, and a 1-tonne bulk bag holds around 0.6 m³. Always calculate loads from loose volume, never from bank volume, and always confirm the weight limit as well as the volume — hauliers charge for the one you exceed first.
Cut and fill, and the balance you are aiming for
On a sloping site the goal is to balance cut against fill so that no soil has to be bought or removed — every cubic metre you can move across the site rather than off it is a cubic metre you do not pay twice for. Set the finished level, measure cut and fill volumes separately, and remember the swell and compaction factors work against each other: 100 m³ of cut becomes about 125 m³ loose, but compacts back to only about 90 m³ of engineered fill. Topsoil is stripped and stockpiled separately before any of this, because it is worth money and it is worthless as structural fill. And before anything moves: a service check. The cheapest cubic metre of excavation is the one you did not put a bucket through a gas main to reach.
