The National Building Code of Canada references ASTM D1883 as the benchmark for California Bearing Ratio determination, and in Sarnia—where shallow groundwater and clay-rich tills dominate—this test is not optional. We run the laboratory CBR on both undisturbed Shelby tube samples and remolded specimens compacted to Proctor density, measuring penetration resistance at 0.1-inch intervals with a calibrated loading frame. Sarnia sits on the St. Clair Plain, a glaciolacustrine deposit where moisture sensitivity can drop subgrade modulus by 40% between dry and soaked states. Before locking in a pavement cross-section, we recommend pairing the CBR with a grain size analysis to confirm fines content, because silty clays with PI above 20 respond poorly to saturation and skew the CBR value downward. Our laboratory in Southwestern Ontario processes over 200 CBR samples annually for road widenings, industrial yards, and commercial pads across the Sarnia-Lambton corridor.
A soaked CBR of 3 or less in Sarnia's glaciolacustrine clays means the subgrade cannot support construction traffic without a stabilization plan.
Process and scope
A common mistake we see in Sarnia is contractors accepting CBR values from air-dried samples without running the mandatory 96-hour soak. The difference is dramatic: a sample compacted at optimum moisture may read CBR 18 dry but collapse to CBR 4 or 5 after soaking, which pushes the design from a thin flexible pavement straight into a thick granular base with geogrid reinforcement. We run the full procedure—soaking under a 4.5 kg surcharge, swelling measurement every 24 hours, then penetration at 1.27 mm/min—and report both the unsoaked and soaked curves on the same graph. When the CBR falls below 3, the subgrade typically requires stabilization with lime or cement, and we often cross-reference results with
Atterberg limits to quantify the plasticity index driving the low strength. For clients designing rigid pavements over poor ground, the soaked CBR feeds directly into the modulus of subgrade reaction used in
rigid pavement thickness calculations. We also correlate CBR with
CPT test data when site access allows, building a continuous strength profile that reduces the number of laboratory specimens needed.
Local ground factors
Sarnia's proximity to Lake Huron and the St. Clair River creates a groundwater regime that fluctuates seasonally by 1.5 to 2.5 metres, and this directly impacts the relevance of a laboratory CBR. A sample soaked for 96 hours under surcharge simulates long-term saturation, but if the design groundwater table is misjudged—say, the investigation was drilled in August during seasonal lows—the in-service moisture content may exceed the soaked test condition, producing CBR values even lower than the lab predicted. We have seen parking lots in Sarnia develop alligator cracking within three years because the pavement design used CBR from samples taken above the spring water table, ignoring the capillary fringe that saturates the upper subgrade from March through May. For sites within 800 metres of the river, we recommend running the CBR on specimens conditioned to 2% above optimum moisture content to bracket the worst-case scenario. The cost of an extra CBR point is negligible compared to the cost of excavating failed subgrade under an occupied parking lot.
