SOIL SWELL: ACCOUNTING FOR SHRINKING AND BULKAGE IN TAKEOFFS
Excavation site cut and fill
Moving dirt, it's what we do and what we love to do! It is super important to have a firm understanding of how soil shrinks and swells. When soil is moved from one location to another, its volume changes, and this is known as either shrink or swell. Accounting for this change is critical when you're making your calculations because you may need to import or export soil to properly balance the site and complete the project on time and within budget. It comes into play as early as the bidding process.
Three States of Soil
‘Loose’ soil (LCY)
There are three key stages to know about when it comes to soil in our line of work. When soil is in its original state as it was left by nature and has not been touched, it is known as "bank" or "BCY." Once the soil has been moved into a temporary stockpile by a truck, it is referred to as "loose" or "LCY" because it has essentially fluffed up from the excavation, creating air pockets throughout the soil. Finally, once the soil is placed in a fill area by dump trucks and compacted, it is called "compacted" or "CCY." The soil must be adequately packed to be considered CCY. Soil always swells from the BCY to the LCY phase and always shrinks from the BCY and LCY to the CCY phase. Some expansive soils increase in volume between BCY and CCY. If those soils are not found before the bidding process, it can significantly increase your project's cost. Additionally, the expansive soils may increase the number of trucks required to haul the dirt. Soil shrinks when it goes into the CCY phase because of compacting the soil, which pushes it closer together and decreases the amount of void space that existed when it was in the LCY phase. In other words, it loses its fluff due to the applied pressure. For example, if we are talking about roadway excavation, it is typically measured in the BCY state. You must include the location and method of measurement in your specification due to almost all material types changing their volume once excavation begins.
Calculating Your Soil to Account for Shrinking and Swelling
There are many different types of soils with varying contents of moisture and other compactors.
As a result of this, there is no standard amount that soil will either shrink or swell. Soil is measured in cubic yards. When calculating your soil amounts and accounting for shrink or swell, you must factor in the type of material and its density, or your volume totals will be wrong. The swell factor of soil is the amount of volume increase from the undisturbed (BCY) to the excavated (LCY) state due to the air pockets (fluff) created. Swelling pressure is the pressure that the expansive soil exerts if it is not allowed to swell or if the soil volume change is arrested. When it comes to light soil excavation, the shrink may range from 20-40% or more. Moderate soil excavation shrink ranges from 10-25%. Heavy soil excavation with deep cuts and fills shrink approximately 15% and swell around 5%. These averages are helpful to know and use as a guideline.
For all of your earthmoving related questions, contact ECI Technologies and speak with any one of our virtual construction engineers. Their experience and knowledge is unmatched.
For additional reading on soil swell and shrink, check out: