Compaction
Compaction is the mechanical pounding of soil and weathered rock into a dense mass with sufficient bearing strength or impermeability to withstand a load. It is primarily used in construction to provide ground suitable for bearing the weight of any given structure. With the advent of huge earth-moving equipment we are now able to literally move mountains. However, such disturbances loosen and expand the soil. Thus soil must be compacted to provide an adequate breathing surface after it has been disturbed. Inadequate compaction during construction results in design failure or reduced service life of a structure. Compaction, however, is detrimental to crop production because it makes root growth and movement difficult, and deprives the soil of access to life-sustaining oxygen.
With proper compaction we can build enduring roadways, airports, dams, building foundation pads, or clay liners for secure landfills. Because enormous volumes of ground material are involved, it is far less expensive to use on-site or nearby resources wherever feasible. Proper engineering can overcome material deficiencies in most cases, if rigid quality control is maintained.
Successful compaction requires a combination of proper moisture conditioning, the right placement of material, and sufficient pounding with proper equipment. Moisture is important because dry materials seem very hard, but they may settle or become permeable when wet. Because of all the variables involved in the compaction process, standardized laboratory and field testing is essential.
The American Society of State Highway Transportation Officials (ASHTO) and the American Society of Testing Materials (ASTM) have developed specific test standards. The laboratory test involves pounding a representative sample with a drop-hammer in a cylindrical mold. Four uniform samples are tested, varying only in moisture content. The sample is trimmed and weighed, and portions oven-dried to determine moisture content.
The results are then graphed. The resultant curve normally has the shape of an open hairpin, with the high point representing the maximum density at the optimum moisture content for the compactive effort used. This curve reflects the fact that dry soils resist compaction, and overly-moistened soils allow the mechanical energy to dissipate. Field densities must normally meet 95% or higher of this lab result.
Because soils are notoriously diverse, several different "curves" may be needed; varied materials require the field engineer to exercise considerable judgment to determine the proper standard. Thus the field engineer and the earthmoving crew must work closely together to establish the procedures for obtaining the required densities.
In the past, density testing has required laboriously digging a hole, and comparing the volume with the weight of the material removed. Nuclear density gages have greatly accelerated this process and allow much more frequent testing if needed or desired. To take a reading, a stake is driven into the ground to form a hole into which a sealed nuclear probe can be lowered.
It is much easier to properly compact materials during construction than to try to make corrections later. A well-compacted, properly tested structure is an investment in the future, well worth the time and effort expended.
Resources
Books
American Society for Testing & Materials. Compaction of Soils. ASTM Special Technical Publication, No. 377. Philadelphia: ASTM, 1965.
Daniel, D. P. "Summary Review of Construction Quality Control for Compacted Soil Liners." In Waste Containment Systems: Construction, Regulation, and Performance, edited by R. Bonaparte. New York: American Society of Civil Engineers, 1990.
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