Explain the Swell Factor of Soil

A soil's swell factor represents the fact that the volume of soil placed by nature in the ground is not the same as the volume of the same mass of dirt excavated by the contractor and placed in the dump truck. The same mass of soil occupies more volume in the truck (loose cubic yards) than it does in the ground (bank cubic yards). The swell factor is an adjustment representing this increase in volume. However, the swell factor plays no part in the calculation of an earthwork's balance. The swell factor is used to determine the subsequent hauling and stockpiling requirements.

Swell is the percentage increase in volume caused by the excavation of soil. Physically, the act of excavation breaks up the soil into particles and clods (lump of earth) of various sizes. This creates more air pockets and results in an effective increase in the soil's void volume. An increase in volume also results in a decrease in density. This decrease in density and increase in volume varies between soil types and is not proportional due to the initial, natural void volume of the bank soil. The swell factor equations are found in the Table below:

Swell: A soil increases in volume when it is excavated.

Applying the equation, soil with a swell of 25% would have a load factor of 80% (the inverse of 1.25). The load factor can be used to show the relationship between Loose and Bank density by dividing the loose density by the load factor (i.e., 2100 / .79 = 2650).