Curves of dry density versus moisture content, as yielded by the standard ASTM compaction tests, usually show a single maximum, but some soils exhibit compaction curves with irregular shapes. This study examines some of the influences of soil preparation on these irregular compaction curves. When a surface tension reducing agent, such as aerosol-AY, is added to the water in soil specimen preparation, the shape of the irregular compaction curve will be altered. This effect is regarded as a verification of the effects of negative pore water pressure on the irregular compaction curve. In repeated compaction tests, it is economical and convenient to reuse the soil specimens. The results of liquid and plastic limit tests reveal approximately the same values in new and reused specimens. However, the reused soil specimens reveal a predominantly higher density in the zone of lower water content in an irregular compaction curve.