Swell: Loose Volume from Bank Volume

Soil has three volumes and they are never equal: bank (in place, undisturbed), loose (after the bucket has broken it up) and compacted (after the roller). Excavating breaks the interlock between particles and introduces voids, so loose volume always exceeds bank volume. Typical swells are 12 % for damp sand, 25 % for common earth, 40–45 % for clay and 55–65 % for blasted rock. The Panama Canal's Culebra Cut is the monument to this arithmetic: the engineers moved something like 76 million cubic metres of bank material, and every haul, spoil bank and rail schedule had to be sized in loose measure, not bank.

The trap is mixing measures inside one estimate. Excavation is paid by bank cubic yard, trucking is priced by loose cubic yard, and the two differ by a quarter or more. A worked example: a basement of 1,000 BCY in common earth with 25 % swell yields 1,000 × 1.25 = 1,250 LCY to haul; at 12 LCY per truck that is 105 loads, not 84. Running the formula backwards turns a measured truck count back into the bank quantity the owner actually pays for.