Proctor, Proctor, give me the news: How dense are we really?

It all starts with a relationship:

Leaning tower of Piza - Mudslides - Sink Holes - Pot Holes. These all represent the love-hate relationship between soil and water. Soil composition and density greatly affect the amount of load that can bear on the soil with little to no settlement or movement. Bearing capacity is often measured in pounds per square foot (PSF) or Pascals (Pa). Soils are generally classified by particle sizes and physical properties such as plasticity, hardness, and specific gravity.

Mother nature doesn't build to a specification, so soils as they lay in nature (in-situ) will have every variable you can imagine when it comes to composition and exposure. The deeper you go, typically the more dense the soil because the weight of the soil on top pressing down may compact it a bit. However, that is not a given assumption because soil has a friction within it's matrix and that friction may be able to overcome the weight and thus resist the pressure from above. This is often called bridging, and a soil bridge can collapse with the introduction of water that can provide lubrication to reduce the internal friction and allow the matrix to consolidate even further. In the dirt business we refer to this as shrinkage or subsidence.

The great minds of the geotechnical industry have developed standards and test methods to help designers with assumptions when it comes to the loading of a structure on the soils. One of these test methods addresses the moisture density relationship of the in-situ or processed soils. They essentially take a representative sample, dry it out, then add back exact percentages of water and pound the snot out of the sample in a steel tube. This process is repeated with varying percentage of water and the resulting unit weights are plotted on a graph. The peak of this unit weight is known as the maximum dry density, or proctor.

What's that mean to me?

From structures to roadways, every design needs to assume that it will be built on, or operate on, an adequate foundation. The difference in density between the soil in-situ and the specified dry density after compaction is generally known as shrink. In fine grained soils, this can be 20 to 40 percent difference, meaning that one cubic yard of in-situ soil (27 cubic feet) will only fill about 17 cubic feet of space when placed in an embankment.

For Estimators, this information is essential to determining the balance of soil on a site, the amount of effort needed to compact the embankment, and the amount of water needed to properly lubricate the soil during compaction. Dry clay doesn't readily absorb water, so the amount of processing necessary to fully hydrate the soil is a huge factor in your cost to move that soil. Likewise, drying out soil that is too wet often require significant effort.

What it ultimately means is the estimator needs to spend a significant amount of time evaluating the geotechnical information, and perhaps even conducting additional pre-bid testing. One trap is that geotechnical reports focus on the area where structures are planned, so on a large site, the frequency of testing for the design may not be adequate to develop a well-founded estimate. Many owners try to tender the responsibility of site balance onto the earthwork contractor, so any assumption of a "standard" shrink factor should be consumed with a large degree of caution.