Compressibility of a compacted sand

A slightly silty quartz sand (nonplastic fines) was compacted according to Modified Proctor at different water contents and then one-dimensionally compressed. Samples compacted dry-of-optimum were found to be stiffer than samples compacted wet-of-optimum at the same relative compaction. This difference in stress-strain behavior is not generally expected for a sand; fabric and/or overconsolidation may explain these results. Regardless of the mechanism, the actual measured modulus on sand backfill at low confining stresses can be significantly less than handbook values. Thus, for the case of shallow depth (such as backfill for a flexible conduit located within a few meters of the ground surface) it is important to consider the water content and the method of compaction, as the degree of compaction by itself will not necessarily achieve the desired modulus.