Aclay of CH group according to the unified soil classification system (or CV clay according to BS 5930) with about 33% gypsum contentwas tested in the laboratory for the resilient modulus using the ultrasonic wave propagation technique. Forty-eight California bearing ratio (CBR) soil samples were compacted at the optimum moisture content and 95% of modified AASHTO compaction. For each soaking period of 0, 4, 7, 15, 30, 60, 120 and 180 days, three pairs of samples were loaded at 45, 178 and 312 N surcharge load, respectively. The resilient modulus (MR) of the gypsiferous soil tested was determined using CBR soil samples before and after each soaking period, and the following observations were made: (1) soakingresults in a progressive decrease in MR with increasing soaking period; (2) after 180 days soaking the resilient modulus decreased by 92%, 85% and 80% relative to the unsoaked condition for CBR samples under surcharge loads of 45, 178, and 312 N, respectively. Furthermore, a strong linear correlation between the resilient modulus and soaked CBR was found. An empirical equation has been developed relating MR at any soaking period to the unsoaked condition, taking into account the effect of soaking period and surcharge load.

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