Compaction is an engineering term used to describe the ability of a soil type to be treated with mechanical energy and compressed such that air voids are removed. With individual grains compressed to remove air voids, it becomes more difficult for the soil being compressed to ‘settle’ further on its own. The strength of the soil in loads other than compression can be increased because the individual particles within the soil become interlocked and friction can become a more important function of the soil behavior. Compacted soil, because air spaces between the particles are reduced has lower hydraulic conductivity (passes water less easily under a given pressure).

Why do a Compaction Test?

Compaction can be important when high loads such as building foundations may cause a soil to settle over time causing shifting or even collapse. It can be valuable for soil that you want to retain in place, such as along an embankment or behind a retaining wall to be compacted. The compaction process, by increasing the friction in the compacted soil helps to maintain against horizontal slippage which can either result in a landslide off from an embankment or in higher pressure behind a retaining wall, causing it to bow outwards. Because compaction lowers hydraulic conductivity, it can be useful, or even essential in the functioning of earthen dams, drainage ditches and levees.

A measurement of compaction is the change in density, or weight per unit volume increase after the soil in question is compacted. That’s why sometimes ‘compaction’ is also called ‘densification’. This is actually not a correct designation as ‘densification’ actually includes both ‘compaction’ which is described above as well as ‘consolidation’. Consolidation involves fluid flow out of the soil being densified, such as when you are treating clay heavy soils. Water is squeezed out from between pores in ‘consolidation’, so that the net effect in addition to compression is less pore space available to refill hydrologically.

To recap, soil densification includes both compaction and consolidation. One nuance to keep in mind, is that with compaction, the presence of water will affect the level of compaction possible. Dry sands will not compact as efficiently as wet soils. This is in part because water acts as a lubricant, allowing grains of soil to squeeze more tightly together, further eliminating air voids. Therefore, adding water during compaction can increase the density that is obtainable in the compaction process. Because water is essentially incompressible, once the air voids are full and the water begins to carry the load, further densification is prevented. As water is added beyond this point, the level of density that can be achieved actually decreases. Water can also soften organic soils, making them more pliable and therefore able to be compacted more densely.


This is why it is important to understand the % moisture of a sample being tested. As in other types of CPT testing, when evaluating sub-surface compaction potential, it is important to ‘ground-truth’ a sample from the same type of deposit that you expect to be distributed throughout a site.

The standard test for compaction is the Standard Proctor Compaction Test. The Standard Proctor Compaction Test measures the degree of compaction in terms of its dry unit weight when a given amount of mechanical energy is applied. The standard was originally developed to simulate field compaction in the lab and is performed under ASTM D698.

There are 4 factors affecting the extent of compaction using this test. The soil type and gradation, the moisture content, the dry density and the compaction effort. One goal of this test is to establish the optimum moisture content that will enable the highest density, or dry unit weight when the soil being tested is left to it’s native conditions.

In order for a site to meet design requirements or the various standards that safety demands, a site will have to be compacted to a specific level of density. This is achieved through means such as a steamroller of a given weight or a mechanical vibrator of a given size. A CPT test can established the types of soil conditions that will require compaction and can have a significant impact on a projects success. Knowing how deep excavation must go in order to establish a suitable footing both increases safety and helps to manage construction estimate scopes. Understanding how a CPT sounding and the Standard Proctor Compaction Test, or what may be referred to as an ASTM D698 test are related can be essential to delivering the right information to your customers.