Shrink/swell factors for earthwork calculations 1

stoddardvilla,

Shrink/swell factors account for 1) how much will the material swell or "fluff" upon excavation (for use in computing how many bucketfulls/truckloads it will take to excavate and transport the material) and 2) how much will the swelled material shrink when compacted in-place.

I can't imagine that the program's help file doesn't include rough numbers to use for these quantities based on the soil type.

For cohesive soils, I would crudely estimate a shrink factor of 20-30%, with the swell factor maybe 5% larger. You should ask a professional estimator or conduct data gathering over several projects to refine the numbers.

Jeff

You could also have the geotechnical engineer perform some tests to determine the in-place density, loose density, and compacted density of the soil. You can then use these densities to get an estimate of shrink and/or swell.

another factor often used around here is ground compaction. When preparing the subgrade to accept fill - how much does the ground compact once the heavy equipment runs over it? This can range anywhere from a few inches to a foot and can amount to a significant amount of earthwork. However you estimate, always make sure the contractor who bids is made aware of the factors you used, even if you don't use a factor.

If there is no borrow, export or overhaul, do you really need the factor?

I've used those factors in Terramodel and can't remember what we did exactly. We did used to drop our DTM by 1' to account for topsoil, however. (Or was that 6" because they only replaced 6" of topsoil but in our area there was on average 1' in situ.) If you're moving dirt from one end of your site to the other, you still need the shrinkage factor. You can ignore the swell, but you need the shrinkage factor to make sure you don't come up short on dirt.

Thanks, all very helpful replies..

I have pondered this myself. As an engineer I don't care about swell factor. How many truck loads it takes to complete my design is of no concern to me. Seems to me that shinkage factor would only come into play when the in situ soil is considerably looser than the compacted embackment. I understand the industry usually uses a 10% shrinkage but I cannot quatify that. Sounds to me like it added just to be conservative. I expect that in most situations the in situ and compacted densities are so similar that it is not waranted.

From my experience in residential subdivision design, it's a lot more expensive to import dirt than to have an excess. We were dealing with a lot of sinkholes (not good fill dirt) and a variable topsoil and of course the size of the project did not warrant a detailed soils analysis. A generous shrinkage factor went a long way to making sure that the dirt balanced there was a small amount of excess fill.

We never had a contractor complain about too much dirt, but in one particular area, where we'd allowed for a foot of topsoil (6" was the norm), we did get complaints about too little dirt. The upshot was that the retention basins (for once) were all more than adequately sized because that was where the contractor had got his dirt from.

Often I don't bother with them, neither does anyone in our firm. Recently, however, I did a site with 187550 CY of earthwork. I was in fairly cohesive soils and a good bit of rock. I'm not a soils experts, so I inquired externally about shrink:swell. This is what the Geotechnical Group Manager of an ENR 500 firm said, verbatim.

When excavating the rock you can get a pretty high expansion between 30% to 40% swell from in-place volume to loose volume. This will largely depend on excavation method. Blasting will result in less swell unless the blasting pattern is very tight.

The silt/clay overburden soils will swell from around 15% to 20% from in-place volume to loose volume.

Shrinkage factors will be on the order of 5% to 10% going from loose volume to in-place for clay and 15 to 20% for rock. These are negative values.

These are just ranges and I would not hold me to these numbers.


Engineering is the practice of the art of science - Steve

Loess in the midwest has shrink of the order of 30-35%, (insitu density to 90% density Modified Proctor). Diatamacious soils in California shrink 40-50%.

Francesca hit it on the head. And remember when the change order for re-design of the grading plan or for double haul or for imported material / delay of the contractor is being approved by the owner, the responsibility / blame for the extra cost will usually fall on the engineer. This can sometimes get very expensive...

I am concerned only with the in situ to compacted factor. Are you telling me Civilperson that you are dealing with soils that shrink from in situ to compacted 30% on the low end?

Shrink factor is a comparison of volumes. If 1.3 cubic yards of cut are required to make 1.0 cubic yard of fill then the shrink factor is 30%.