The Ro-Tap sieve shaker runs a stacked column of ASTM E11 mesh pans, from the 75 mm opening down to the No. 200, while the hydrometer settles in a 1000 mL graduated cylinder measuring the minus 75-micron fraction. In Jacksonville, where the surficial geology shifts between Pleistocene terrace sands, Holocene marsh clays, and the underlying Hawthorne Group, a single-point classification won't cut it. We run the full mechanical sieve array plus 152H hydrometer sedimentation on every sample pulled from Duval County borings, because the difference between a poorly graded sand (SP) and a fat clay (CH) dictates whether your footing drains freely or traps water against the stem wall. For deep foundations near the St. Johns River, we often pair this with CPT testing to correlate tip resistance against the percent fines curve before selecting pile embedment depth.
If the percent passing the No. 200 sieve exceeds 12 percent and you don't run the hydrometer, you're guessing on drainage and frost behavior. Guessing costs more than the test.
How we work
Jacksonville's consolidation in 1968 merged the urban core with vast unincorporated land, spreading development across barrier island quartz sand, inland pine flatwoods with seasonal high water, and the clay-rich remnants of ancient estuarine deposits. That patchwork means a grain size curve from a site off Butler Boulevard can look nothing like one from a lot near Trout River. Our lab runs the full procedure per ASTM D422 and classifies per the Unified Soil Classification System (ASTM D2487), reporting percent gravel, sand, silt, and clay, plus D10, D30, D60, and the coefficient of uniformity Cu. When the hydrometer curve shows more than 15 percent fines, we cross-check with
Atterberg limits to nail the plasticity index and confirm whether that silty layer is frost-susceptible or just a low-plasticity ML that drains adequately under a pavement section.
Local geotechnical context
Compare a site on Black Hammock Island, where organic silt and fat clay from the salt marsh dominate, against a lot in Oceanway overlying Pleistocene quartz sand with less than 3 percent fines. The Black Hammock sample needs a hydrometer run with full sedimentation correction because the minus 200 fraction controls settlement rate and secondary compression, while the Oceanway sample might only require a sieve stack to confirm clean SP sand suitable for shallow bearing. If you skip the grain size analysis and assume both sites behave the same, the Black Hammock foundation ends up with differential settlement within two wet-dry cycles. The IBC references ASCE 7 for load combinations, but the actual bearing capacity derivation starts with knowing whether your soil is GW, SP, ML, or CH. Without that letter pair, every calculation downstream carries a margin of error nobody should accept on a permanent structure.
Questions and answers
How much does a grain size analysis (sieve + hydrometer) cost in Jacksonville?
A combined sieve and hydrometer test in our Duval County lab typically runs between US$100 and US$170 per sample, depending on whether we need the full sedimentation series or just the mechanical sieve stack. Bulk pricing applies for five or more samples from the same boring program. The fee covers sample preparation, oven drying, wash-through on the No. 200, hydrometer sedimentation with temperature correction, and the signed report with the complete particle size distribution curve.
What sample mass do you need for an accurate grain size test?
For predominantly coarse-grained soils with less than 10 percent fines, we need at least 500 grams of oven-dried material. For fine-grained silts and clays common in Jacksonville's marsh deposits, 150 grams is sufficient. The sample must be representative of the stratum being tested and sealed in a moisture-tight container immediately after extraction to prevent drying during transport.
How do I interpret the coefficient of uniformity (Cu) and coefficient of curvature (Cc) on the lab report?
Cu = D60/D10 tells you the range of particle sizes. A Cu below 4 for gravel or below 6 for sand means the soil is poorly graded (uniform grain size). Cc = (D30)²/(D60 × D10) indicates whether the middle portion of the curve is well-shaped. A Cc between 1 and 3 with a high Cu suggests a well-graded soil that compacts densely; outside that range, you may have gap-graded material with missing intermediate sizes, which can create instability under repeated loading.
Why do I need the hydrometer if I already ran the sieve?
The No. 200 sieve stops at 75 microns, but clay particles extend down to 1 micron and below. The hydrometer uses Stokes' Law to measure the settling velocity of particles in suspension, giving you the silt-versus-clay split. In Jacksonville, where many soils contain 20 to 60 percent fines, skipping the hydrometer means you cannot distinguish an ML silt from a CH fat clay. That distinction controls permeability, shrink-swell potential, and whether the material qualifies as structural fill under the Florida Building Code.
