Fill soils in many geotechnical applications must be defined by some material property. Often it's strength (i.e., friction angle), subgrade behavior (i.e., CBR) or compressibility (modulus). The field exploration program shows us the soils on the site and we get some idea what may be available for use in earthwork.
So, we know we want to use the on-site soils and we take bulk samples to the laboratory to evaluate the strength (or some other characteristic). How do we prepare the sample for testing? Well, we'd want to know the properties when compacted and we'd want to determine some specification level for how much compaction.
Proctor tests are done in the laboratory to evaluate the moisture-density relations. That means for some level of moisture content, under prescribed energy, you'll get some dry density for that soil. Under the most favorable moisture content you'll get some, "Maximum" density, which is directly related to the amount of, "Prescribed" energy (refer to ASTM D698 v, D1557. If I know the maximum dry density is 120 pcf (1.9 g/cc), I may want to fall off that in testing, 'cause it may not be likely that the contractor will achieve this level of compaction during production. So, I'll chose 95 percent compaction. I could chose 94 or 97, but I chose 95. I get the lab to prepare a bunch of samples at 95 percent compaction and they do testing. I get the strength at 95 percent and anything else I want. Now I do geotechnical engineering (slope stability, etc.) and specify the contractor use THOSE soils and place them at 95 percent compaction. Not just any compaction, but the compaction that we used in the lab - i.e., either Standard or Modified Proctor.
Next up, we can discuss compaction moisture content. There is an entire body of engineering that discusses whether geotechnical properties are influenced by the compaction moisture content in addition to the degree of compaction.
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ípapß gordo ainÆt no madre flaca!