We roll a pneumatic drill rig up to the site, mount a Shelby tube sampler, and pull undisturbed cores from the subgrade. In Austin, the soil profile can shift from high-plasticity clay (CH) to silty sand (SM) within 50 feet, so we take samples at 1.5-m intervals. Each core goes straight to the lab for moisture-density curves and CBR swell testing. We run the Modified Proctor (ASTM D1557) on every pavement section to get the dry density target. Before we set the compaction spec, we also run a CBR test to estimate the subgrade modulus for the flexible pavement layers.
For fat clays with PI > 30, lime treatment is standard before placing the base course. We verify with soaked CBR at 4 days.