Grain Size Analysis (Sieve + Hydrometer) in Seattle’s Glacial and Alluvial Soils

The glacial history of the Puget Sound region didn't just carve out Lake Washington and Elliott Bay—it laid down a bewildering mix of soil deposits that range from clean outwash sands to sticky lacustrine silts and clays. On one block in Seattle you can hit hardpan till; three blocks downhill you're in soft estuarine mud. This variability makes grain size analysis far more than a routine index test here. It becomes the first honest conversation you have with the ground. By pairing dry sieving for the coarse fraction with a hydrometer analysis for fines passing the No. 200 sieve, we reconstruct the full particle size distribution curve—and that curve guides everything from drainage design to liquefaction screening. In a city where winter rains saturate the glacial drift for months on end, understanding how fine-grained materials will behave under sustained seepage is not optional.

A full particle size curve from a single Seattle sample often tells you more about site hydrology in ten minutes than a dozen borehole logs without it.

Methodology and scope

ASTM D422 provides the backbone for combined sieve and hydrometer testing, and in Seattle we lean on ASTM D2487 (Unified Soil Classification System) to translate the curve into an engineering classification. What we see most often in the field is that a split-spoon sample described as 'silty sand' can hide a clay fraction that dramatically alters permeability assumptions. That's where the hydrometer step earns its keep. For deeper exploration on sites where stratigraphy changes rapidly—think slopes above Alki or hillside lots in Queen Anne—we often recommend pairing the grain size data with a CPT test to build a continuous profile without losing the fines context. Similarly, when the grain size curve suggests gap-graded soils or high uniformity coefficients, we correlate those findings with in-situ permeability testing to validate drainage design assumptions before the civil engineer commits to a retention strategy.

Local ground factors

Seattle sits at roughly 47.60°N, where winter groundwater recharge pushes the water table up into the weathered zone of the Vashon till—a recipe for frost action in silty soils that look stable on a summer log. Miss the silt-clay boundary in a grain size curve, and you risk specifying a subgrade that heaves come February. We've also seen projects in the Duwamish industrial corridor where a sand classified as 'clean' by sieve alone still carried enough plastic fines to trigger a marginal liquefaction rating under the 2021 IBC seismic provisions. The grain size analysis doesn't replace a cyclic triaxial program, but it's the first triage step—and skipping it because 'the gradation looked fine in the field' has cost more than one Seattle foundation contractor a spring of rework.

Technical data

Parameter	Typical value
Sieve range (dry)	No. 4 (4.75 mm) to No. 200 (75 µm)
Hydrometer method	ASTM D422 / ASTM D7928 (Type 152H)
Minimum sample mass	200 g for sands; 115 g for silts/clays
Dispersing agent	Sodium hexametaphosphate (NaHMP)
D10, D30, D60 reporting	Coefficient of uniformity (Cu) and curvature (Cc)
Typical turnaround	3–5 business days, plus hydrometer settlement period

Related services

Combined Sieve & Hydrometer (Full Curve)

Complete particle size distribution from 75 mm down to 1 µm, reported with USCS classification, Cu, Cc, and percent gravel/sand/fines. Standard for foundation reports and infiltration studies.

Wash Sieve Only (Fines Content)

Rapid determination of percent passing the No. 200 sieve for quality control during earthwork or aggregate production. Faster turnaround when the full hydrometer curve isn't required.

Correlative Testing Package

Grain size analysis paired with Atterberg limits on the same sample when site drainage or shrink-swell potential is a concern—common for Seattle hillside projects on glacially overconsolidated clays.

Regulatory framework

ASTM D422 – Standard Test Method for Particle-Size Analysis of Soils (sieve + hydrometer), ASTM D2487 – Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), ASTM D7928 – Standard Test Method for Particle-Size Distribution of Fine-Grained Soils Using the Sedimentation (Hydrometer) Analysis, IBC 2021 – Section 1803 geotechnical investigation requirements

Frequently asked questions

How much does a grain size analysis (sieve + hydrometer) cost in Seattle?

For a single sample with full sieve and hydrometer, the fee runs between $90 and $220 depending on whether we're doing just the wash sieve or the complete sedimentation curve with hydrometer readings over 24 hours. Volume pricing applies for multi-sample projects—reach out with the number of samples and we'll give you a straightforward unit rate.

Why do I need the hydrometer if the soil looks sandy in the split spoon?

Because Seattle's glacial soils are notorious for fooling the eye. A 'silty sand' can easily carry 15–25% clay when you run the hydrometer, and that clay fraction controls permeability, frost susceptibility, and liquefaction resistance. The IBC and ASCE 7 don't let you assume clean sand based on field description alone.

Can you run grain size analysis on samples from our own drill rig?

Absolutely. We work with samples from geotechnical and environmental drillers across King County. Just bring the bagged, labeled samples to our lab or coordinate a pickup—we need about 200 grams for sands and 115 grams for fine-grained material to run the full ASTM D422 suite.

What's the typical turnaround for a combined sieve and hydrometer test?

Most reports go out in three to five business days. The hydrometer side needs a 24-hour sedimentation period per ASTM D422, and we run quality checks on the curve before releasing results. If it's urgent, we can sometimes split the report—sieve data first, hydrometer the next day.