ASTM D2487 and D422 remain the backbone of geotechnical classification in Raleigh, where the transition between Piedmont residual soils and Atlantic Coastal Plain sediments demands precise particle sizing. The Unified Soil Classification System (USCS) requires both coarse and fine fraction distributions to assign proper group symbols—anything less risks mischaracterizing the soil behavior during compaction, drainage, or frost action. In our laboratory, we combine mechanical sieve stacks covering the 75 mm down to 75 µm range with hydrometer analysis based on Stokes' law to capture the silt and clay fraction that often dominates the weathered saprolite common across Wake County. A complete grain size curve, plotted from 2 mm to 0.001 mm, reveals the gap-graded nature of many local soils and directly feeds into seismic liquefaction assessments when the water table sits high in the Neuse River floodplain.
A sieve-only gradation on Piedmont residual soil misses half the story—without hydrometer you're blind to the clay fraction that controls permeability and shrink-swell.
Process and scope
Site-specific factors
Raleigh sits squarely atop the Raleigh Belt metamorphic rocks—predominantly gneiss and schist that weather into silty sands with variable mica content. This mica, particularly biotite, creates a unique challenge for grain size interpretation because the platy particles settle slower than equivalent-mass spherical grains assumed by Stokes' law, leading to systematic underestimation of sand-sized mica if the technician doesn't flag the anomaly. In the eastern reaches of the city toward Knightdale, the Fall Line marks the transition to unconsolidated Cretaceous and younger sediments where clean quartz sands interbed with stiff overconsolidated clays. Here, the grain size distribution directly controls permeability estimates used in dewatering design and filter compatibility for subsurface drainage systems. A poorly graded fine sand with D15 around 0.08 mm adjacent to a silty clay base can trigger internal erosion if the filter criteria aren't verified against the full gradation envelope.
Reference standards
ASTM D2487: Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), ASTM D422: Standard Test Method for Particle-Size Analysis of Soils, ASTM D7928: Standard Test Method for Particle-Size Distribution (Gradation) of Fine-Grained Soils Using the Sedimentation (Hydrometer) Analysis, ASTM D6913: Standard Test Methods for Particle-Size Distribution (Gradation) of Soils Using Sieve Analysis, ASTM D1140: Standard Test Methods for Determining the Amount of Material Finer than 75-μm (No. 200) Sieve in Soils by Washing, IBC Chapter 18: Soils and Foundations
Complementary services
Full Sieve + Hydrometer Package
Combined mechanical sieve analysis and hydrometer sedimentation producing a complete 0.001 mm to 75 mm gradation curve with D10, D30, D60, Cu, and Cc values reported. Suitable for USCS classification, fill quality control, and filter design verification on Raleigh DOT and commercial projects.
Wash No. 200 with Fines Characterization
Determination of percent passing the 75 μm sieve by wet washing, followed by hydrometer analysis on the minus-75 μm fraction. Ideal for contractors needing rapid confirmation of fines content and plasticity correlation before placing structural fill in Wake County subdivisions.
Typical parameters
Common questions
What does a grain size analysis with sieve and hydrometer typically cost for a Raleigh project?
For standard projects in Raleigh, the combined sieve and hydrometer package runs between US$110 and US$210 per sample depending on whether we're doing the full sedimentation series or just the wash-minus-200 with abbreviated hydrometer. Samples with high organic content or requiring special dispersion due to mica-rich residual soils may fall at the upper end of that range because of the extra preparation time needed.
How long does the hydrometer portion take compared to the sieve portion?
The mechanical sieve analysis can usually be completed in one working day, but the hydrometer sedimentation requires a minimum of 24 hours for the full reading sequence—and often we let it run 48 hours for Raleigh clays that settle slowly. Temperature-controlled cylinders and precise timing at each reading interval are what produce a defensible curve; rushing the 1440-minute final reading means you're guessing the clay fraction below 0.002 mm.
Why do Raleigh soils often need hydrometer analysis instead of just a sieve stack?
The weathered Piedmont saprolite that underlies much of Raleigh contains significant silt and clay derived from in-place decomposition of gneiss and schist bedrock. Even soils that look sandy in the field can carry 25-40% fines, and the hydrometer is the only ASTM method that quantifies the clay-sized fraction below 0.002 mm. Without it, you miss the mineralogical signal that governs shrink-swell potential, permeability, and long-term settlement behavior.