Following were observed in a series of field and laboratory tests,: (1) after sand compaction, the primary wave velocity and the degree of saturation of in-situ sandy soil decreased due to the injection of air bubbles into the ground; (2) the liquefaction strength of partially saturated sand before and after sand compaction was much higher than that of fully saturated sand; (3) the effects of the size and roundness of soil particles and relative density on the relationship between degree of saturation and primary wave velocity were minor, but the effect of the confining stress on that relationship was significant; (4) for all tested sand, primary wave velocity increased rapidly with increasing degree of saturation from 90 to 100%. Based on these test results, a method to inject air bubbles into the ground is thought to be an effective method to reduce the disaster due to sand liquefaction.