Sensitivity of Compaction


Please click through to the source URL above to read the entire context of the snippet below. This is a very broad topic with many different opinions.


When a specification calls for a percent compaction eg 95% what would be the effect if you only achieved 90%. In other words how critical is the the asked for figure and what is considered an acceptable variation from that figure (in terms of +/- %)



The Proctor test is a physical test, brought to the world by, you guessed it, Mr. Proctor (circa 1933). The test proved useful in stabilizing soils for military applications, especially impromptu aircraft landing strips. Proctor (the man) believed that density increased as water, acting as a lubricant, reduced the friction between soil grains, allowing for a more efficient particle arrangement via void filling by the smaller grain sized particles. Additional work by Hogentogler (1937), and Lambe (1958), and the man considered by many to be the master of soil mechanics, Terzaghi, increased the understanding of soil behavior to the point that, under carefully controlled conditions, soil and soil-aggregate composites can be relied on to behave properly under a very wide range of conditions. Over the years, the basic laboratory Proctor test has not changed much. What has changed are the challenges that geotechnical engineers face when evaluating a site. The combination of a good geotechnical engineer and a good structural engineer, working in concert, allow construction to take place at locations that previously were considered ‘unbuildable’, It would take a detailed understanding to ever allow a recommended compaction requirement to be waived, lowered, or increased. The best course of action is to determine why a specified relative density is not occurring. Any deviations from the specified relative densities should be brought to the attention of BOTH the geotechnical engineer and the structural engineer for analysis. It may or may not be acceptable. I’ll leave you with some examples:

  1. Highly compacted, unyielding soil may be required beneath one type of pipe to avoid shear failure of the pipe, but cause it in another type.

  2. Highly compacted expansive soils, especially when compacted on the dry side of optimum, may heave and cause significant damage when lightly loaded and subjected to moisture infiltration.

  3. Highly compacted fine grained, silty soils, may consolidate when placed and compacted at moisture contents below optimum, This is especially problematic in trenches.

  4. Wide variations in density, even though minimum requirements are met, may lead to a non-uniform soil support system for pavements.

I hope you can see that the question you posed is not nearly as easy to answer as it may have seemed. Whenever I hear it said that the “engineer overdesigned” a project, I always reply that the engineer knew instinctively that someone would try to under build it.


There are numerous arguments, pro and con, of the Proctor (moisture density relationship). I’ve dealt with Proctor-responsive soils my entire career and feel that it is a good test with some known limitations.

It is argued that relative density is a better test; however, that is a more difficult test to run and most laboratories don’t have the proper equipment to do so. Essentially all materials testing labs that deal with construction materials have the capability to perform the Proctor in both of its forms, either manually or automatically.

When properly performed in the lab and compared with properly performed field density tests, and properly interpreted by qualified individuals, it is a good method of quality assurance or verification.