San Antonio’s geology shifts dramatically from the hard Glen Rose limestone of the Dominion area to the expansive Houston Black clay in Alamo Heights—a 20-mile drive that crosses three distinct soil formations. A single foundation approach cannot serve both, and that’s exactly why grain size analysis (sieve + hydrometer) is not optional here. Where the Olmos Formation outcrops near the Pearl District, poorly graded sands dominate; head toward the Medina River floodplain and fines content jumps above 50 percent. Our lab runs the full ASTM D422 / D6913 suite, combining mechanical sieving with sedimentation hydrometer testing to deliver a continuous particle-size distribution. For earthwork contractors working the Eagle Ford Shale slopes near Government Canyon, this data determines compaction behavior and drainage characteristics long before the first footing is poured. We pair grain size results with Atterberg limits when plasticity classification is needed for expansive clay zones, and with CPT testing to correlate field stratigraphy with lab-measured fines percentages across deep alluvial profiles.
Missing the hydrometer on a San Antonio clay means missing the difference between a well-graded base course and a subgrade that pumps water under load.
Methodology and scope
Local ground factors
IBC Section 1803.5.4 requires soil classification by a qualified geotechnical professional for all bearing materials, and ASTM D2487 explicitly demands particle-size distribution as a primary input to the USCS group symbol. In San Antonio, the consequence of skipping a proper grain size analysis shows up as differential heave where sand lenses and clay layers alternate within the same footing trench. The Eagle Ford and Austin Chalk formations weather into a silty residuum that can misclassify as low-plasticity clay without hydrometer data—leading to overestimated bearing capacity and underestimated swell potential. Foundation repair firms across Bexar County report that the most expensive remediations trace back to mischaracterized fines content. A single grain size analysis costs a fraction of one underpinning pier.
Regulatory framework
ASTM D6913 – Standard Test Methods for Particle-Size Distribution (Gradation) of Soils Using Sieve Analysis, ASTM D7928 – Standard Test Method for Particle-Size Distribution (Gradation) of Fine-Grained Soils Using the Sedimentation (Hydrometer) Analysis, ASTM D2487 – Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), IBC 2021 – Section 1803 Geotechnical Investigations, TxDOT Tex-110-E – Particle Size Analysis of Soils
Associated technical services
Combined Sieve & Hydrometer (Full Gradation)
Complete ASTM D6913/D7928 package from coarse gravel through clay colloids. Includes wash-through-No.200, hydrometer sedimentation, and USCS classification per D2487 with gradation curve and tabulated results.
Wash Sieve Only (Coarse Fraction)
For clean sands and gravels where hydrometer is not required. Material retained on No. 200 sieve is dried and sieved through full stack; percent passing No. 200 reported for quality control on aggregate base course.
Hydrometer-Only (Fine Fraction)
When the minus-No.200 fraction needs detailed characterization—critical for San Antonio's expansive clays and silty residuum. ASTM D7928 sedimentation analysis with dispersant, temperature-corrected readings.
Atterberg + Gradation Package
Combined liquid limit, plastic limit, and full grain size analysis on the same sample. Recommended for all foundation investigations in the Houston Black and Beaumont clay formations of southern Bexar County.
Typical parameters
Frequently asked questions
What does a grain size analysis cost in San Antonio?
For a standard combined sieve and hydrometer analysis (full gradation curve plus USCS classification), fees range from US$90 to US$200 per sample depending on whether the sample requires hydrometer testing and the turnaround time. Wash-only sieve analysis runs at the lower end. Expedited reporting within 48 hours carries a slight premium.
How much sample material is needed for the test?
We require approximately 500 grams of dry material for sandy soils and at least 200 grams for fine-grained soils. The sample should be representative of the stratum in question—bagged and labeled by depth. For gravelly soils with particles larger than 3 inches, a larger bulk sample of 5 to 10 kg is needed so we can run the full coarse-fraction sieving without bias.
Why does San Antonio clay require hydrometer testing instead of just sieving?
San Antonio sits on formations where the fines fraction—silt and clay smaller than 75 microns—controls both swell behavior and drainage. A sieve stack stops at the No. 200; it cannot distinguish between a non-plastic rock flour from the Austin Chalk and a high-plasticity smectitic clay from the Wilcox Group. The hydrometer measures particle sizes down to roughly 1 micron, giving us the true clay percentage that drives foundation design decisions and IBC compliance.