The alluvial fan deposits beneath Fontana, spreading south from the San Gabriel Mountains, contain a chaotic mix of boulders, gravel, and fine silts that defy simple visual classification. When the Cucamonga fault zone sits just five miles north and the water table fluctuates with Lytle Creek recharge, assuming uniform gradation becomes a project risk that no amount of compaction can fix. We run the full ASTM D6913–04 sieve stack plus ASTM D7928 hydrometer sedimentation on every sample taken below five feet, because the shift from coarse to fine fraction in these young Quaternary sediments determines everything from permeability to liquefaction susceptibility under a design earthquake. A split-spoon grab from SPT drilling often misses the interbedded clay seams that drive differential settlement here, which is why we link grain size data directly to the Atterberg limits on the minus No. 40 fraction before any foundation recommendation leaves the lab.
A soil is not one material — it is a distribution. The D10, D30, and D60 tell you more about field behavior than any single index property.
How we work
A mistake we see repeatedly in Fontana is contractors treating the near-surface silty sand as representative of the entire column, then finding perched groundwater or fat clay lenses at footing depth that completely change the Unified Soil Classification. The sieve stack alone will not catch the 15 to 30 percent fines that turn a well-graded sand into a silty sand with borderline drainage characteristics. Our procedure weighs every retained fraction on the No. 4, No. 10, No. 40, and No. 200 sieves, then runs a 24-hour hydrometer sedimentation on the minus No. 200 material using sodium hexametaphosphate as dispersant. The resulting particle-size distribution curve — plotted from 75 mm cobbles down to 0.001 mm colloids — feeds directly into the USCS group symbol, which is the starting point for every bearing capacity and settlement calculation on the project. For sites within the Fontana Water Company service area where infiltration rates matter, we also cross-check the D10, D30, and D60 values against Hazen's formula and the City of Fontana grading ordinance drainage requirements.
