Compaction of Subgrades and Aggregate Base Courses

Construction compaction is the process by which the weight-per-unit volume of soils and aggregates is increased by the use of applied force. This weight-per-unit volume, or density, is measured and compared to a laboratory-determined maximum density of the same material. This comparison of field to laboratory density values is the measure by which the contractor’s work is evaluated as successful or not.

Several factors come into play when the contractor begins to address work that requires compaction. First, is the condition of the material upon which the project will be constructed. Frequently, the native ground is sufficient to begin the work with little or no attention. However, situations can arise when the in-place condition of the native material will not support the work to be done on top of it. In other words, if the material upon which compaction activity is to take place is not sufficiently compacted itself, achieving compaction on top of it will be difficult or impossible. In extreme instances, such as native soils containing large amounts of organic material, it can be necessary to “bridge” the unsuitable soil with cobbles or to physically remove the unsuitable material and replace it in order to create a firm enough surface upon which to begin construction.

Moisture and Density
Once the condition of the subgrade has been determined to be appropriate, the contractor will begin work with the fill material. The material must have a suitable moisture content in order to meet the density standard for the project.

The purpose of the laboratory-determined maximum density of any material is to establish a density value and to identify the optimum moisture content for the material. Moisture in the material acts as a lubricant. When the moisture content is at an appropriate level, it helps the individual particles that make up the material pack and interlock thus reducing to very small the amount of void space in a compacted fill. Material at optimum moisture content will require less compactive effort and thus support efficiency.

If there is insufficient moisture in the material, the particles will not find their most tightly fitting arrangement and acceptable density will not be achieved even with extensive compactive effort. If there is too much moisture, the material will be subject to too much lubrication. Particles, rather than finding their most tightly fitting arrangement, will instead constantly slip past each other. Again, acceptable density will not be achieved regardless of the amount of compactive effort applied. Of any single factor, moisture content is the most critical in achieving compaction success.

Various types of materials will require different approaches in order to achieve successful compaction. For example, smooth drum rollers can be highly effective when used to compact sandy materials or aggregate base courses. Sheepsfoot rollers with vibratory capability are often used for materials with high clay content. However, sheepsfoot or vibratory equipment is unsuitable for some materials, such as river run aggregates that contain smaller amounts of clays and other fines.

Awareness of material and equipment issues will help the contractor maintain efficiency during the project.

Two methods of density determination are in most use in California. One is the Caltrans method which measures both laboratory and field densities as wet densities. Wet density determination is based upon the density of the material being tested, including the weight of water in the material.

ASTM provides the second method. Laboratory and field densities are measured as dry densities. As implied, this method measures the density of the material being tested without the weight of the water present in the material.

Typically, both wet and dry density testing is done by nuclear gauge. Testing information can be a valuable resource in managing the project. The information will reveal moisture contents of the fill materials and may also provide an indication of appropriate lift thickness to achieve successful compaction.

Compaction can be achieved on almost any type of soil and on any aggregate mixture with the proper moisture content and compaction method.

Success and efficiency can be achieved by knowledge of the materials in use on the project, properly matching the compaction equipment to the material, and utilizing testing information developed by the project inspector.