In Pomona, where the alluvial fans from the San Jose Hills meet the older sedimentary formations, we consistently see contractors surprised by how much a soil's behavior can shift within a single jobsite. A material that looks like clean sand during excavation can carry enough silt to trigger differential settlement once loaded. That is exactly why we run a complete grain size analysis combining mechanical sieving and hydrometer sedimentation. This dual approach captures the full spectrum from coarse gravel down to the clay fraction that passes the No. 200 sieve. For any project east of the 57 Freeway where the subsurface transitions from granitic wash to finer basin deposits, having the actual particle size curve is the difference between a foundation designed on assumptions and one backed by data. Without it, you are gambling on compaction specs and drainage coefficients that may not match what lies beneath your footings. We often recommend pairing this test with Atterberg limits to fully define the fine-grained portion, especially when the hydrometer reading indicates plasticity risks that simple gradation curves cannot reveal.
A grain size curve is the soil's fingerprint—skip it, and you are designing blind to how water, load, and vibration will actually interact with the ground.
