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ASTM D4718: Correction of Unit Weight and Water Content for Soils Containing Oversize Particles
The rock correction that adjusts a Proctor maximum density and optimum moisture for the gravel and cobbles scalped out of the laboratory specimen, so field percent compaction on rocky fill is computed against the right target.
| Designation | ASTM D4718 |
| Setting | Laboratory |
| Service line | Construction Materials Testing |
Testing is performed to the current edition of ASTM D4718 referenced by your project specification. This page is a plain-language overview, not a substitute for the published standard.
What ASTM D4718 does
ASTM D4718 is the oversize correction, sometimes called the rock correction. A Proctor test under ASTM D698 or D1557 is run on material with particles larger than a set size scalped off, because they will not fit the mold. The fill actually placed in the field still contains that gravel and cobble. D4718 adjusts the laboratory maximum dry unit weight and optimum moisture content to account for the oversize fraction so that field percent compaction is computed against a target that represents the whole material.
How the correction works, step by step
The percentage of oversize particles in the field sample is measured, along with their bulk specific gravity per ASTM C127 and their moisture content. Assuming the oversize particles are solid inclusions in a matrix of finer soil compacted to the laboratory density, the corrected dry unit weight is computed from the weighted combination of the two. Optimum moisture is corrected the same way using the absorption of the oversize fraction. The practice applies up to a stated oversize percentage, commonly 30 to 40 percent, beyond which the assumption breaks down.
What the numbers mean
A 20 percent gravel content can raise the corrected maximum dry unit weight by 5 to 8 pcf. Without the correction, a nuclear density test on rocky fill compares the in-place density to a target that is too low and reports a passing result on fill that has not been compacted; or, in the other direction, a fill with a high fines content is failed because the target was never achievable with the rock present.
Common questions about ASTM D4718
When does the correction apply?
Whenever the field material contains more than about 5 percent of particles larger than the fraction used in the Proctor. The specification or the engineer usually states the threshold.
Who runs the correction?
The testing laboratory, on every density test where the field oversize content is measured, or as a standing correction when the fill is uniform. The report shows both the uncorrected and corrected target.
How we apply ASTM D4718
Our field technicians measure the oversize fraction during density testing on gravelly and rocky fills and apply the D4718 correction on the report, so percent compaction on the project reflects the material in the ground.
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