A recent I-93 widening project near South Bay encountered silty subgrade with CBR values below 3%. The contractor faced potential pavement failure before the first winter freeze-thaw cycle. Boston's glacial till and marine clay deposits rarely behave as the boring logs predict. Our lab runs the California Bearing Ratio test under controlled moisture and density conditions—soaked for worst-case spring thaw scenarios, unsoaked for construction-phase verification. The piston penetration curve tells you exactly how the compacted fill will perform under wheel loads. We pair this with Proctor compaction tests to lock in the moisture-density relationship before CBR specimen preparation, and with grain-size analysis when fines content may control drainage behavior under the pavement section.
A CBR of 6% means the subgrade supports one-tenth the load of crushed stone—get this number wrong and the asphalt cracks in two seasons.
Questions and answers
What CBR value does MassDOT require for subgrade?
MassDOT specifications typically require a minimum soaked CBR of 6% for subgrade under flexible pavements. Values below 3% mandate subgrade stabilization or removal and replacement with select borrow. The exact threshold depends on traffic loading and the pavement design catalog section being used.
How long does a laboratory CBR test take?
An unsoaked CBR test can be reported in 24-48 hours. A soaked CBR test requires 96 hours of immersion plus specimen preparation and testing time, typically delivering results in 5-6 working days from sample receipt.
Do you need undisturbed or remolded samples for CBR?
The laboratory CBR test uses remolded specimens compacted at a specified moisture content and density. We need approximately 50 lb of disturbed soil per material type. The sample should be representative of the subgrade or borrow source.
How much does a CBR test cost in the Boston area?
Laboratory CBR testing typically ranges from US$140 to US$200 per specimen, depending on whether soaked or unsoaked conditions are required and the number of compaction points specified. Volume pricing applies for projects requiring five or more specimens.
