When the Iowa DOT specifies a soaked CBR value for a new arterial or parking lot in Sioux City, the number has to hold up against freeze-thaw cycles and the silty loess that covers much of Woodbury County. ASTM D1883 and AASHTO T 193 lay out a clear procedure, but the interpretation changes once you account for local moisture — average annual precipitation hovers around 28 inches, and spring thaw saturates the upper subgrade every year. Our laboratory CBR test runs on samples compacted at optimum moisture and then soaked for 96 hours, replicating the worst-case scenario that Sioux City pavements actually see. We correlate the results with grain-size distribution to confirm whether fines content explains a low bearing ratio before the pavement section gets over-designed. The Missouri River bluffs add another variable: loess-derived soils that look stiff in a Shelby tube but lose structure when remolded, which is exactly what the CBR mold simulates during compaction.
A soaked CBR of 3% versus 6% in Sioux City loess can mean the difference between 4 inches and 8 inches of additional aggregate base — small numbers with big cost implications.
Methodology and scope
Sioux City sits at roughly 1,200 feet elevation on the eastern edge of the Loess Hills, a landform that runs narrow along the Missouri River and creates some of the most variable subgrade in the Midwest. The loess thickness can exceed 60 feet in parts of Plymouth County, just north of the city limits, and its collapse potential under load makes the laboratory CBR test essential before writing any pavement specification. We compact three lifts in a 6-inch mold, apply the surcharge ring to mimic overlying base and asphalt, and run the piston at 0.05 inches per minute — exactly as ASTM D1883 describes. The soaked CBR value we report typically falls between 3% and 8% for untreated loess subgrade, which is why Iowa DOT pavement design tables often call for lime stabilization or a thicker aggregate base when the ratio drops below 6. Our lab runs the test alongside Atterberg limits on the same material, because plasticity index is the single best predictor of whether a Sioux City soil will pump fines into the aggregate layer after a few seasons of freeze-thaw.
Common questions
How many sample bags do you need for one laboratory CBR test in Sioux City?
A single CBR mold holds about 5 kilograms of material, but we recommend sending at least 15 kilograms of disturbed sample per soil type. The extra material lets us run a companion Proctor curve and still have enough for Atterberg limits if the soil turns out to be borderline plastic. For Shelby tube samples, one 3-inch diameter tube typically yields enough material for a single-point CBR if the sample is homogeneous.
What does a laboratory CBR test cost for a Sioux City pavement project?
A standard soaked CBR test with Proctor compaction runs between US$140 and US$230 per sample, depending on whether we are testing a single point or three points for a full moisture-density curve. If the package includes Atterberg limits and a sieve analysis, the bundled price is usually more economical than ordering the tests separately. Volume discounts apply when you send more than ten samples from the same project.
Why does the Iowa DOT require a soaked CBR instead of an unsoaked value?
Iowa's climate puts pavements through multiple freeze-thaw cycles each winter, and spring melt saturates the subgrade just when truck traffic increases. A soaked CBR test — 96 hours submerged under a surcharge ring — replicates that worst-case condition. An unsoaked value might look acceptable in October, but after four days of soaking the same loess soil can lose half its bearing capacity. The soaked result is what drives the pavement thickness in the design tables, and skipping it is the most common cause of under-designed sections we see in Woodbury County.
