The upper 18 to 24 inches of Mesa's native soil are often dominated by Stage II to Stage III caliche — a calcium-carbonate hardpan that can fool a grader into thinking the subgrade is stronger than it really is. Once that crust gets wet during monsoon season, the bearing value drops fast. A laboratory CBR test measures exactly how much strength the soil retains after a 96-hour soak, following AASHTO T 193 method. We run the test on samples pulled from the site, compacted to Proctor density inside 6-inch molds. For projects east of the Loop 202 where the soil transitions to finer alluvial silts, the soaked CBR often falls below 6 percent. That number drives the pavement structural section the City of Mesa requires. Before coring for a plate load test on a stiff caliche pad, we always check CBR first — it tells you whether the hard layer is just a crust or the real deal.
A caliche crust that reads CBR 80 dry can drop to CBR 8 soaked — the soaked number is the only one that counts for pavement life in the Sonoran Desert.
Common questions
How much does a laboratory CBR test cost in Mesa?
A standard 3-point soaked CBR test on a single sample runs between US$140 and US$230, depending on whether we need to run a companion Proctor compaction curve or if that data already exists from the same boring.
How long does the soaking phase last?
The specimen stays fully submerged for 96 hours — four full days — with a surcharge weight simulating the pavement load. We read swell every 24 hours so you can see how the soil expands over time, not just the final number.
What's the smallest CBR value Mesa allows for residential streets?
City of Mesa Standard Details require a minimum soaked CBR of 10 percent for local residential streets. If the subgrade falls below that, the typical fix is lime treatment or an increased aggregate base thickness until the structural number meets the design traffic loading.
Can you run the CBR test on a sample we already collected?
Yes, as long as the sample is undisturbed or bag-sealed and still at or near its natural moisture content. We'll need roughly 40 to 50 pounds of material to run the full 3-point compaction series. If the sample has dried out, we may need to recondition it before running the Proctor.
