What is swell factor, and why it changes your truck count
A super wants to know how many truckloads it'll take to clear a cut. The excavator operator says "about forty loads." The pay app says the cut volume is 1,200 cubic yards. Somebody does the division, gets a number that doesn't match the ticket count at the end of the week, and now there's an argument about who's wrong. Usually nobody is. The math is just missing swell.
Bank yards vs loose yards
Soil in the ground sits packed. Particles are interlocked, moisture's settled in, there's no air between the clumps that isn't supposed to be there. That's a bank yard: a cubic yard measured as it exists undisturbed, which is what a cut volume on a grading plan or a pay app usually refers to.
The second you rip it out with a bucket, it loosens up. Air gets trapped between clods, the particle structure breaks apart, and the same mass of dirt now takes up more space. That looser, dug-up state is a loose yard, the measure that fills the bed of the truck.
Swell factor is the ratio between the two. If one bank yard expands to 1.25 loose yards once it's excavated, that's a 25% swell. It's reported as a percentage increase in volume, bank to loose, and it tells you how much truck bed a given cut needs. A cross-section number on the plan won't tell you that by itself.
There's a third state too: compacted yards, where the material gets placed and rolled back down, often to a density tighter than it started at. That's shrinkage, the mirror image of swell, and it matters on the fill side. For hauling material offsite, swell is the one that costs you truck count.
Swell percentage by soil type
Swell isn't a fixed constant. It moves with moisture content, how the material was excavated, and how long it's been sitting. But the commonly cited ranges by soil type give you a starting point:
- Sand and gravel: roughly 10-15% swell
- Common loam or topsoil: roughly 15-25%
- Clay: roughly 20-35%, sometimes higher if it's wet
- Dense or well-graded soil, hardpan: roughly 25-35%
- Blasted or ripped rock: 40-70%, varies a lot by fracture and fragment size
Those ranges are wide enough that picking a single number off a chart and trusting it for a whole site is how the truck count argument starts in the first place. A cut through mixed clay and hardpan on the same job can swell closer to 20% in one area and closer to 35% in another, and the chart won't tell you which is which.
Why it matters for truck counts and pay apps
Run the numbers on a 1,200 bank-yard cut. At 25% swell, that's 1,500 loose yards leaving the site. If your trucks haul 12 loose yards a load, that's 125 loads, where dividing bank yards straight into truck capacity would only give you 100. Quote the wrong number to a trucking sub and you're either short on trucks mid-shift or paying for idle ones that showed up for a count that was never going to happen.
It cuts the other way on a pay app too. If the earthmoving sub reports truckloads and backs into a bank-yard figure using their own assumed swell factor, that number is only as good as their assumption. A surveyed cut volume from before-and-after terrain data gives you the bank-yard figure independent of anybody's load count or swell estimate, and that's the figure that belongs on the application for payment. That's the kind of surface-derived cut volume a tool built for exactly this can hand you straight off weekly drone capture, no cross-section takeoff involved.
Get the bank volume right from survey, then apply the swell percentage you've actually observed on that specific material. Chart values are a starting point, and the truck count and the pay app figure should land within a load or two of each other. If they don't, the gap is worth checking before it turns into a dispute with the hauling sub or a short pay on the draw.
If you're tired of reconciling cut volumes against someone else's load count, see what Earthwork Volumes delivers off your next flight.