Improving the resilient modulus of a gypsum sand roadbed soil by increased compaction

A gypsum sand roadbed soil with about 28% gypsum content was tested in the laboratory for the resilient modulus (MR) and California bearing ratio (CBR) under four different compaction efforts. Twenty-eight pairs of CBR specimens were compacted at the optimum moisture content of modified AASHTO compaction. The first seven pairs were compacted using 10 blows/layer, while the second, third and fourth seven pairs were compacted using 30, 50 and 70 blows/layer, respectively. Each pair of the seven pairs was soaked for a certain period of time namely, 0 (unsoaked conditions), 4, 7, 15, 30, 60 and 120 days under the effect of 40 lbs (178 N) surcharge load. The nondestructive test for measuring MR using New Sonic Viewer was conducted before carrying out the CBR test. The travel times of compression and shear waves were measured to allow the calculation of the resilient modulus before and after each soaking period. The results show that the MR increases with increasing compaction effort but decreases significantly with increasing soaking period. The drops in the MR after 120 days of soaking were 38.7%, 36.84%, 33.53% and 29.15% for 10, 30, 50 and 70 blows/layer, respectively. The paper recommends the strong non-linear correlation developed between MR and CBR for soaked gypsum-rich roadbed sand.