Laboratory CBR Test in St. Albert: What It Tells You Before You Pave

Soaked CBR simulates the subgrade after prolonged saturation—think spring melt or a wet fall. Unsoaked CBR reflects conditions right after compaction. The City of St. Albert’s engineering standards follow Alberta Transportation practice, which typically bases pavement design on the soaked CBR value at 95% standard Proctor density. We always recommend running both if your project spans wet-season construction, because the difference between soaked and unsoaked can be 50% or more in the local clay.

It depends on the soil variability. In older neighborhoods on the glacial till side, one test per 500 m of road alignment is often enough. In the newer subdivisions north of the river where lacustrine clay dominates, we recommend one per 200 m or at every change in visual soil classification. The Alberta Transportation field sampling guide suggests a minimum of three representative samples per soil unit, but in St. Albert’s interlayered deposits we’ve found that five or six gives a much more reliable design value.

A single-point soaked CBR test with Proctor compaction and swell measurement typically runs between CA$180 and CA$290, depending on whether it’s standard or modified effort and how many moisture points you need. A full CBR-moisture curve with four or five points is at the upper end of that range. Turnaround is usually five to seven business days from sample drop-off.

We run CBR on both. For subgrade soils we follow ASTM D1883 with the standard or modified Proctor compaction, soaked 96 hours. For granular base course—like the 20 mm crushed gravel used in St. Albert’s road structure—we compact at the specified density and test unsoaked, since free-draining granular won’t hold water for the soak period. The procedure is the same but the sample prep and moisture conditioning differ.