In Sunnyvale, many pavement projects overlook the fact that the alluvial fan deposits from the Santa Cruz Mountains create highly variable subgrade conditions within the same street block. We have seen sections of Mathilda Avenue where the CBR value dropped from 15 to below 5 over a 200-foot stretch, which would have caused premature cracking if we had used a single design value. A proper CBR study for road design in Sunnyvale accounts for this lateral variability by testing samples from multiple boreholes along the alignment, not just a single point. Before specifying asphalt thickness, we recommend combining this with a Proctor compaction test to establish the optimum moisture-density relationship for each soil type encountered.
A single CBR value for an entire street is a gamble — we have measured 300% variation within the same block in Sunnyvale alluvium.
Method and coverage
When we work on a collector road project in the Lakewood neighborhood, the typical sequence starts with exploratory borings to recover disturbed and undisturbed samples from each soil stratum. Those samples are transported to our accredited laboratory where we perform the California Bearing Ratio test under soaked and unsoaked conditions to simulate the worst-case spring thaw scenario. The CBR study for road design in Sunnyvale also includes moisture content determination and Atterberg limits on the fraction passing the No. 40 sieve, because expansive clays in the Guadalupe formation can cause subgrade heave. For high-traffic industrial streets near Moffett Park, we often add a georradar survey to map buried utilities and old foundations before drilling, which saves time and prevents damage to underground infrastructure.
Technical reference image — Sunnyvale
Regional considerations
Sunnyvale's rapid post-war suburban expansion turned former orchards and wetlands into residential streets without systematic subgrade testing. Many of those asphalt roads now exhibit alligator cracking and edge failures because the original CBR values were assumed rather than measured. When we evaluate a failed pavement section in the Ponderosa neighborhood, the laboratory CBR often comes back below 3, indicating the subgrade was never stabilized. A targeted CBR study for road design in Sunnyvale helps avoid repeating those mistakes by quantifying the bearing capacity of the existing soil before overlay or reconstruction.
How many CBR tests are needed for a typical Sunnyvale street project?
For a residential street under 1,000 feet, we recommend at least 3 CBR tests representing different soil borings. For arterial roads over 1 mile, the Caltrans Highway Design Manual suggests one test per 500 feet of alignment, plus additional tests where soil changes are visible during excavation.
What does a CBR study for road design include besides the bearing ratio?
A complete study includes moisture content, dry density, swell measurement after soaking, and soil classification (sieve analysis and Atterberg limits). Without those, the CBR number alone cannot predict how the subgrade will behave under seasonal wetting in Sunnyvale's Mediterranean climate.
How long does it take to get CBR results from your Sunnyvale lab?
Standard soaked CBR requires 4 days of soaking plus 1 day for compaction and testing, so you should plan for 5 to 7 business days from sample delivery. Unsoaked CBR can be completed in 2 to 3 days. We offer expedited service for emergency pavement assessments.
Can you perform CBR testing on existing pavement without closing the street?
Yes, we use a Dynamic Cone Penetrometer (DCP) to estimate CBR in situ through small-diameter holes drilled through the asphalt. This method causes minimal traffic disruption and gives real-time results. The DCP correlation is calibrated against laboratory CBR for Sunnyvale soils.
