Shrinkage & Expansion

Soil expansion and compaction is one of the most misunderstood aspects of site estimating, and is one of the most common causes of inaccurate take-offs. The following is an in-depth look at how InSite Handles Shrinkage & Expansion.

Shrinkage and Expansion values are entered in the Section Cut & Fill Worksheet.

Bank. The Term Bank is used in InSite for the volume of material in its undisturbed state. In other words, it's the volume of material (either stripping or cut) before excavation.

Expanded. Once the material is disturbed, it will expand. This is the actual volume of material moved around or off the site. On InSite's reports, it is listed in the Expanded column. Let's say we had a cut of 1000 bank cubic yards on a site, and from experience we know that the material expands 20% when it is hauled. Our handled volume will actually be 1000 X 20% or 1200 cubic yards.

Compacted. If you are reusing a material on site, there is the issue of soil compaction. Compaction is the relationship of undisturbed (bank) material to material when compacted and used as fill. In reference to our original 1000 cubic yards, with a shrinkage value of 15%, the actual amount of material available for fill would be 1000 X .85% or 850 cubic yards. The 850 yard figure would be listed in the Compacted column of the Cut & Fill report.

Cuts by Strata. To complicate things further, differing materials can have differing shrinkage values. For Example, topsoil may shrink one amount, sand another, and materials like rock might actually expand (referred to as a negative shrinkage to avoid confusion). InSite lists each stratum’s Bank, Expanded, and Compacted volumes.

Limiting the amount of material used as fill. Experienced estimators have all encountered the site that was so-called "balanced" (Stripping + Cut = Fill + Topsoil Replacement), only to discover that because of structural fill requirements, "unsuitable" material had to be hauled off the site, and "suitable" material had to be trucked in to replace the hauled away material. To determine the actual "balance" of a site, first review the Section Subgrade Report

The Subgrade report lists the fill volumes for each subgrade area of the site. This is the volume of fill required before the subgrade materials are added. Fill requirements for non-structural areas are prime disposal areas for "unsuitable" materials, while structural areas will require "suitable" (engineered or structural) fill.

Next, using the fill volumes required for each subgrade, limit the amounts of materials if necessary on the Section Cut & Fill Worksheet.

The Available from Fill, and Available from Stripping Volumes reflect the compacted amounts. As the Use as Fill Volumes are reduced or zeroed out, the balance will be shown in the To Be Exported column. Once the Worksheet is properly filled out, the Section Cut & Fill Report will actually reflect the work required.

This eliminates the "This is what the report says, but this isn't what it means" explanation required when not using the worksheet.

“I've been expanding the fill, and it has always worked for me". Many estimators have used the technique of expanding the fill when doing manual estimates. Let's figure a fill requirement of 1000 cubic yards. Assuming compaction rate of 30%, 1300 cubic yards of loose material would be needed to "fill". The assumption is made that all of the material on the site will have the same shrinkage. The problem with this method, is that it doesn't work when you deal with materials of differing shrinkage. If one material shrinks 15%, one shrinks 9%, and another 5%, the method of expanding the fill requirement gives inaccurate results.

Software packages that don't allow a differing shrinkage value for differing on-site materials may allow you to expand the fill. This is a limitation that will give you an inaccurate number when handling multiple strata.