Standard Proctor Vs Modified Proctor
What is a Proctor test? Proctor test is a test method used to measure the relationship between the dry density and moisture content of a soil. From this relationship (a plotted curve) we can estimate the maximum dry density and optimum moisture content of the soil. An American engineer named Ralph R Proctor (1894-1962) invented the this test sometime between 1932 and 1934.
How is this test undertaken? Proctor test is undertaken in the lab by compacting the soil in a cylindrical mold in layers and by applying constant compaction effort. Several such cylindrical samples at varying moisture content are prepared and their volume and moisture content are measured. The results are then plotted as follows - dry density on Y-axis and moisture content on X-axis. The plotted curve is usually bell-shaped.
What is modified Proctor test? When standard Proctor was developed in the 30’s it worked well for construction equipment at that time. However, over time with the development of heavy rollers and compactors, the compaction effort induced by these heavy machines increased greatly. Therefore, standard Proctor test was no longer representing the field conditions. Hence a revised version of this test, called as modified Proctor test was developed to better represent the field conditions.
How do the tests compare? The biggest difference between the two tests is their compaction energy. In order to produce more compaction energy, a heavier hammer is used and the drop height is also increased (see graphic attached to this article). The number of soil layers in the cylindrical mold is also increased.
Which one should I use? The trend has been such that in the Eastern United States standard Proctor test is more generally used. In the Western United States modified Proctor test is more generally used. Ideally you’d want to use whatever represents your equipment and field conditions better.
A common mistake people often do is prescribing 98% or 100% of maximum dry density based on modified Proctor. This is not required in most cases. Because the compaction effort is significantly higher for modified Proctor prescribing 90% would be sufficient in most cases. If you are using standard Proctor test results as your base then typically you need to recommend 95% or more depending on the purpose.
Is there a correlation between the two? Hard to say. Several studies have been carried out however nothing concrete has come out. It is understandable because several factors are at play during compaction. The most important is soil type. Fine grained soils behave differently than coarse grained soils. If you absolutely need to correlate both then you might say 90 to 92% modified Proctor density is approximately equal to 95% standard Proctor density for coarse grained soils. However be careful with this correlation. It should not be the basis for your engineering judgment or recommendations.