Shrinkage and Swellage
Let us say that you are a gardener. You want to plant some colorful flowers that you purchased at the local nursery. So, what do you do? Of course, you find a nice spot in your yard, dig some holes and set your flowers in them. Then you cover the roots with the dirt you dug out of the hole. Repeat until all the flowers are planted. (And don’t forget to water them). Easy right?
Now let’s make it a little more complicated. You are a contractor hired to build a foundation for a client. You have to calculate how much dirt will be required to fill the area of the foundation. Before you can do that though you need to become familiar with two terms, Shrinkage and Swellage. Let’s break it down.
Let’s start with an example. The plans call for a foundation that measures 30 X 50 x 2 feet. It will be necessary to calculate how much dirt will be required to fill that area if it is already level. The area to be filled in is measured in cubic yards so if you figure the area (in this example 3000 c.y.) then you presumably would know how much material will be required to fill the hole. However, you also have to figure the shrinkage factor. Why? Because without figuring shrinkage there will not be enough material to fill the hole. When you bring in dirt from a site and compact to 95 percent you will need an additional 35 percent of fill material (35 percent is an approximation, depends on fill material density). If only digging dirt out and refilling because planting a tree that would be sufficient. However, for construction jobs such as a foundation, the dirt will have to be compacted.
If you had the same area that needed digging out (excavating) you will have to figure swellage as you truck in the material from another site such as a pit. As you dig out the dirt from a site it is going to swell. In its existing condition it is compacted, but once you excavate that dirt it loses compaction and that is what is called swelling. The factor is 35%. That simply means that the volume of the dirt will “swell” to be 35% larger when it is no longer compacted. If you have a 15-yard dump truck and fill it with 15 yards of material the truck is full. However, the dirt on the dump truck is obviously not compacted. When you dump it on the site and compact it in the foundation you will only be able to fill an area of 9 or 10 yards. Previously excavated dirt that is placed on a truck swelled to 15 yards to fill the truck. That is called Loose Vehicular Measurement.
So, the two terms go hand in hand. When dirt is excavated it “swells” because it is no longer compacted. When dirt is being placed it “shrinks” as it is