Swell And Shrinkage

Swell and Shrinkage – Not What You Think it Means

We call the properties that define the volume of dirt swell and shrinkage. Yes, take a moment to giggle like a third grader. Ok now back to it. Hey! I said a moment, not an hour of unnecessary giggling. You know who you are… where was I? Oh yeah, the reason dirt can’t always have the same properties is based on numerous factors like soil type, moisture content, etc.

Here’s something to remember: A cubic yard of dirt measured as it lies in the earth prior to any earthwork being performed will measure out greater than a cubic foot of dirt when it is excavated. If this soil is compacted once placed, the volume could be less than a cubic foot of dirt.

This makes sense. Consider digging in your backyard to plant a bush or tree. Isn’t it always harder to put the dirt you’ve just dug up back into the hole from which it came? Isn’t there usually a mound rather than the flat surface you began with? I like to think of this as an overstuffed suitcase after a long vacation. Unless you squish it back pretty good, or own one of those crazy vacuum sealing machines from the TV infomercials, the clothes have an innate desire to expand and generally give you a hard time when packing.

We have three main volumes to consider in the following equations:

Vb is bank-measure or measure of the earth in its natural state
Vl is loose-measure or measure of the earth in transport
Vc is compacted-measure or measure of the earth after it’s been compacted

Swell Calculations

Swell is the increase in volume of earth once it’s been excavated from its natural state. Here’s what the calculation for swell looks like:

Vl=100%100%+swell%Vb=LVb 

Where 100% represents the original volume, and swell % represents the increase in volume once excavated. If you know the soils load factor L, you could calculate this as a ratio of bank measure volume to soil load factor, L.

Shrinkage Calculations

Shrinkage is the decrease in volume of earth once it’s been replaced and compacted compared to the volume of dirt in its natural state (it’s not compared with the swell volume).

Vc=100%100%−shrinkage%Vb 

Where 100% represents the original volume, and shrinkage % represents the decrease in volume if compacted when placed.

Common Swell and Load Factors

You can use these swell and load factors to plug into the equations above. These are just general calculations to give you an idea to help push you in the right direction. In real life scenarios, you can only estimate, so there is no perfect answer. These methods are meant to get you as close to the real thing as possible.