The soils beneath Irving change fast when you cross from the old terrace deposits near downtown toward the Trinity River floodplain. Over by Las Colinas, you might hit stiff, sandy clay that drains well; drive ten minutes south toward the floodplain and you are in fat clays that swell the moment they see water. That difference is why we run the grain-size analysis early—if the fines content jumps past 50 percent in a single block, the soaked CBR can drop from 15 down to 3 in a week. Our lab on the west side of the Metroplex runs the full ASTM D1883 procedure, compacting samples at optimum moisture and soaking them four days before the piston ever touches the surface. For Irving projects that sit on the Eagle Ford shale, we also pair CBR data with atterberg-limits to flag plasticity indices above 25, which almost always signal trouble for a flexible pavement section.
In expansive Irving clays, the four-day soaked CBR value often falls to less than half the unsoaked number—and that soaked value is what the pavement thickness design actually needs.
