San Bernardino sits in a semi-arid inland valley with hot summers and occasional flash floods. The alluvial soils here range from silty sands to gravelly clays, and their bearing capacity under saturated conditions varies widely. A laboratory CBR test on undisturbed or recompacted samples gives us the California Bearing Ratio needed to design flexible pavements that resist rutting and shear failure. Before running the test, we often perform a densidad cono arena in the field to confirm in-place density correlates with lab compaction. This two-step check ensures the subgrade strength numbers we report match actual site conditions.
A soaked CBR below 3 in silty subgrades means at least 12 inches of base course is required to prevent structural failure under heavy truck loads.
Approach and scope
San Bernardino covers about 62 square miles and sits at an elevation of roughly 1,050 feet above sea level. Its soils reflect both alluvial fan deposits from the San Bernardino Mountains and older terrace materials. The laboratory CBR test follows ASTM D1883 exactly: we soak specimens for four days at controlled moisture, then penetrate them at 0.05 in/min. Key outputs include the CBR value at 0.1 inch and 0.2 inch penetration. We also record swell percentage during soaking, critical for expansive clay zones common east of the I-215 corridor. For projects requiring higher precision, we run the test on three energy levels — 10, 25, and 56 blows per layer — to generate a design CBR curve. This data feeds directly into AASHTO 1993 pavement thickness equations.
Technical reference image — San Bernardino
Site-specific factors
Our lab uses a calibrated 50,000-pound capacity compression machine with a cylindrical penetration piston 1.954 inches in diameter. The mold assembly holds the compacted specimen firmly during soaking and loading. Failure occurs when the piston pushes into the soil at a constant rate — a brittle shear failure in granular soils or a plastic flow in fine-grained ones. In San Bernardino, the main risk is underestimating the drop in CBR after saturation: some silty sands lose 60% of their dry strength when soaked. That is why we always run the soaked variant unless the pavement will never see water — a rare condition in this climate.
Full four-day soaked CBR test with continuous swell measurement using dial gauges. We compact specimens at optimum moisture per ASTM D698, then submerge them for 96 hours. The swell percentage is reported alongside the CBR value at 0.1 and 0.2 inch penetration. Ideal for pavement designs in areas with seasonal rainfall like San Bernardino.
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CBR at Multiple Energy Levels
Three-point CBR curve generated from 10, 25, and 56 blows per layer. This method produces a design CBR for the expected compaction effort in the field. We correlate the results with field density tests to verify construction compliance. Recommended for highways and industrial pavements requiring a safety margin.
Relevant standards
ASTM D1883-21: Standard Test Method for California Bearing Ratio of Laboratory-Compacted Soils, AASHTO T 193-22: The California Bearing Ratio, ASTM D698-12: Standard Proctor Compaction (method used to prepare CBR specimens)
Quick answers
What is the difference between soaked and unsoaked CBR?
The soaked CBR test submerges the compacted specimen for four days before penetration, simulating worst-case saturation after heavy rain. The unsoaked test penetrates the specimen immediately after compaction. Soaked values are typically 30–60% lower than unsoaked, depending on soil type. Most pavement design codes in the U.S. require the soaked value.
How much does a laboratory CBR test cost in San Bernardino?
The typical cost for a single soaked CBR test with swell monitoring ranges from US$130 to US$240. The price varies depending on whether you need the three-energy-level curve, rush turnaround, or multiple specimens. Contact us for a quote specific to your project scope.
How many soil samples are needed for a CBR test?
Each CBR test requires approximately 35 to 45 pounds of representative soil. For the standard three-energy curve, you need three separate specimens, so about 100 to 120 pounds total. The material should be sampled from the subgrade layer at the project site and transported in sealed bags to preserve moisture content.
What CBR value is considered good for pavement design?
A CBR of 10 or higher is generally considered good for flexible pavements. Values between 5 and 10 require thicker base layers. Below 3, the subgrade is considered poor and typically needs stabilization with lime, cement, or geogrids before paving. The design CBR used in AASHTO equations is usually the soaked value at 0.1 inch penetration.
Can you run CBR on disturbed samples from test pits?
Yes, we can run CBR on disturbed bulk samples collected from test pits, backhoe trenches, or auger cuttings. The material must be large enough to represent the subgrade — a minimum of 40 pounds. We recompact the sample at its optimum moisture content per ASTM D698 before testing. This is standard practice when undisturbed cores are not available.
