Soil placed as engineering fill (embankments, foundation pads, road bases) is compacted to a dense state to obtain satisfactory engineering properties such as, shear strength, compressibility, or permeability. In addition, foundation soils are often compacted to improve their engineering properties. Laboratory compaction tests provide the basis for determining the percent compaction and molding water content needed to achieve the required engineering properties, and for controlling construction to assure that the required compaction and water contents are achieved.
During design of an engineered fill, shear, consolidation, permeability, or other tests require preparation of test specimens by compacting at some molding water content to some unit weight. It is common practice to first determine the optimum water content (wopt) and maximum dry unit weight (γd,max) by means of a compaction test. Test specimens are compacted at a selected molding water content (w), either wet or dry of optimum (wopt) or at optimum (wopt), and at a selected dry unit weight related to a percentage of maximum dry unit weight (γd,max). The selection of molding water content (w), either wet or dry of optimum (wopt) or at optimum (wopt) and the dry unit weight (γd,max) may be based on past experience, or a range of values may be investigated to determine the necessary percent of compaction.
Experience indicates that the methods outlined in 5.2 or the construction control aspects discussed in 5.1 are extremely difficult to implement or yield erroneous results when dealing with certain soils. 5.3.1-5.3.3 describe typical problem soils, the problems encountered when dealing with such soils and possible solutions for these problems.
Oversize Fraction8212;Soils containing more than 30 % oversize fraction (material retained on the ¾-in. (19-mm) sieve) are a problem. For such soils, there is no ASTM test method to control their compaction and very few laboratories are equipped to determine the laboratory maximum unit weight (density) of such soils (USDI Bureau of Reclamation, Denver, CO and U.S. Army Corps of Engineers, Vicksburg, MS). Although Test Methods D4914 and D5030 determine the “field” dry unit weight of such soils, they are difficult and expensive to perform.
One method to design and control the compaction of such soils is to use a test fill to determine the required degree of compaction and the method to obtain that compaction, followed by use of a method specification to control the compaction. Components of a method specification typically contain the type and size of compaction equipment to be used, the lift thickness, acceptable range in molding water content, and the number of passes.
Note 38212;Success in executing the compaction control of an earthwork project, especially when a method specification is used, is highly dependent upon the quality and experience of the contractor and inspector.
Another method is to apply the use of density correction factors developed by the USDI Bureau of Reclamation (2,3) and U.S. Corps of Engineers (4). These correction factors ma......
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