We still see foundation reports in Sunnyvale that rely on visual-manual classifications for fill soils near Moffett Park. ASTM D2487 exists for a reason — the Unified Soil Classification System requires quantitative gradation data, not field estimates. When a mixed soil contains 12% fines instead of the assumed 5%, the drained shear strength and permeability assumptions change. On the alluvial plains between Stevens Creek and the Guadalupe River, these differences trigger unexpected settlement profiles. Running a full sieve stack plus hydrometer analysis — per ASTM D6913 and D7928 — eliminates the guesswork before the structural engineer sizes the footings. For projects near the South Bay salt ponds, we often pair grain size curves with Atterberg limits to confirm whether the fines are silty or clayey: two materials with very different behavior under cyclic loading. A CPT sounding alone will not give you the PSD curve, which is why the lab work remains essential.
A soil with 22% passing the No. 200 sieve does not drain like a clean sand — and Sunnyvale’s high water table turns that into a dewatering cost difference.
Site-specific factors
Sunnyvale sits on Quaternary alluvial fan and basin deposits — interbedded clays, silts, sands, and gravels deposited by the ancestral Guadalupe River system. The layering is notoriously lens-like: a clean sand stratum can pinch out laterally and be replaced by fat clay within 30 feet. That stratigraphic variability means a single misclassified sample distorts the entire geotechnical model. A grain size analysis that skips the hydrometer on a soil with 10–15% fines risks calling a silty sand what is actually a clayey sand — two materials with opposite pore pressure responses during an earthquake. Sunnyvale is in Seismic Design Category D per ASCE 7, and the USGS probabilistic hazard maps show a 0.9g peak ground acceleration at the 2,475-year return period. In that shaking environment, fines-driven liquefaction susceptibility (per Seed & Idriss methodology) changes the foundation design from shallow footings to driven piles. We have seen this exact scenario on industrial redevelopments north of Highway 237, where post-gradation reclassification triggered a liquefaction assessment that the original report had dismissed.
Questions and answers
How many pounds of soil do you need for a full sieve plus hydrometer test?
For a complete ASTM D6913 + D7928 analysis, we typically need about 3 to 5 pounds of representative material in a sealed bag. If the sample is predominantly granular with gravel, we may request up to 10 pounds to meet the minimum mass requirements in the standard. We can pick up samples from your Sunnyvale jobsite or you can drop them at our receiving window.
What does a grain size analysis cost in Sunnyvale?
A combined sieve and hydrometer analysis with full USCS classification runs between $110 and $180 per sample, depending on whether you need the basic PSD curve or the full report with gradation parameters, hydrometer table, and classification. Volume pricing applies at 10+ samples.
How long does the hydrometer portion actually take — can you speed it up?
The sedimentation part is governed by Stokes' Law and cannot be accelerated — the 24-hour reading is required by ASTM D7928. We start the hydrometer run the same day we receive the sample, so the full combined report is typically ready in 3 to 4 business days. For time-sensitive Sunnyvale projects, we offer an expedited 24-hour turnaround on the sieve fraction with a preliminary classification, followed by the final hydrometer data the next day.
