At 7,165 feet above sea level, Laramie’s subgrade materials behave differently than those in the basins below. The combination of decomposed granite from the Sherman batholith, thin residual soils over bedrock, and winter frost depths reaching 48 inches means that pavement sections designed without local CBR data tend to fail early—often within three to five freeze-thaw cycles. When a contractor calls us after a premature rutting failure on a county road or an industrial yard, the first thing we request is the original laboratory CBR report, and more often than not, it was either skipped or run on air-dried samples that never saw a soaking cycle. In a city where the water table along the Laramie River corridor rises seasonally and clay-rich loess pockets in the western bench areas hold moisture well into June, the soaked CBR value is the only one that counts. Our approach to the laboratory CBR test in Laramie follows ASTM D1883 procedures but adds a conditioning sequence that mimics the localized wet-dry cycling we observe in Albany County subgrades.
A soaked CBR value of 3 on Laramie’s western loess means the difference between 6 inches and 14 inches of base aggregate—and that difference shows up in the first spring thaw.
Questions and answers
What does a laboratory CBR test cost in Laramie?
A single-point soaked CBR test in our Laramie program runs between US$140 and US$230 per sample, depending on whether we need to run a companion Proctor compaction curve or if the optimum moisture content is already established from prior geotechnical work on the site. The three-point CBR curve ranges higher because it requires three separate compaction molds, three soaking baths, and three penetration runs. We provide a written quote before starting any lab work so there are no surprises in the invoice.
How long does the soaking phase really take, and can it be shortened?
The ASTM D1883 standard specifies 96 hours—four full days—of soaking under a surcharge mass that simulates the weight of the pavement structure. Shortening this period is not permitted under the standard, and for good reason: many Laramie subgrade soils, particularly the silty loess and clay-rich paleosol materials, continue absorbing water well past 72 hours. We have measured swell increases of 15 to 20 percent between the 72-hour and 96-hour readings on samples from the west bench. Cutting the soak short produces a CBR value that does not represent saturated conditions and will not hold up under WYDOT review.
Do you need a Proctor compaction curve before running the CBR test?
Yes, and this is non-negotiable if the CBR result will be used for pavement design. ASTM D1883 requires that the sample be compacted at a known percentage of maximum dry density—typically 95 percent of Standard Proctor maximum for residential streets, or 100 percent of Modified Proctor for heavy-duty industrial pavements. Without the Proctor curve, you do not know what moisture content and density you are actually testing, and the resulting CBR number has no reliable relationship to field conditions. We run the Proctor first, then compact the CBR samples at the target moisture and density.
What CBR value does WYDOT require for subgrade acceptance?
WYDOT Standard Specifications generally require a minimum soaked laboratory CBR of 3 to 5 for subgrade soils, depending on the roadway classification and traffic loading, but this is a minimum threshold, not a design target. For flexible pavement sections on collector and arterial streets in Laramie, the structural design typically assumes a soaked CBR between 5 and 10 for native subgrade materials. If the laboratory CBR comes back below 3, the standard approach is to undercut the weak material and replace it with select fill or to increase the aggregate base thickness substantially. We provide the raw data and corrected CBR values so the pavement designer can make that determination based on actual soil behavior rather than assumed values.
