Effects and observations of surcharge load on the laboratory CBR and resilient modulus values of roadbed soil

A silty sand subgrade soil was tested for soaked CBR in the laboratory under different surcharge loads of 44.5, 89, 178 and 267 N. Four sets of samples (two samples per set) were compacted at 95% of modified AASHTO compaction and tested according to ASTM (D1883–87). A strong correlation between the strength ratio CBR L /CBR 45 (where CBR L refers to the soaked CBR for any surcharge load L, while CBR 45 corresponds to 44.5N surcharge load) for different levels of surcharge load, was found. Using other data for clayey soils, suggests that the correlation between the strength ratio CBR L /CBR 45 and surcharge load is insensitive to soil type. The resilient modulus M R of soil tested was investigated using the ultrasonic wave propagation technique. The four CBR sets were tested for M R before and after soaking. The study revealed that the resilient modulus, similar to CBR, increased with surcharge load and a correlation between M R and corresponding soaked CBR was developed.