Boise sits at 2,730 feet above sea level on a broad alluvial plain where the Boise River drops sediment across Ada County. That sediment varies from clean coarse gravels near the foothills to fine silts in the lower terraces, and getting the gradation wrong on a foundation or pavement design can trigger differential settlement or frost heave. Our laboratory runs the full grain size analysis—sieve stack plus hydrometer—on every disturbed sample we receive, following ASTM D6913 for the coarse fraction and ASTM D7928 for the fines. A complete particle-size distribution curve tells the engineer whether the material qualifies as well-graded gravel (GW), silty sand (SM), or lean clay (CL) under the Unified Soil Classification System. For Boise projects that sit on the transition between coarse river deposits and fine lacustrine silts, we often combine the gradation report with an Atterberg limits determination to resolve the dual-symbol classifications that the USCS demands.

A single gradation curve determines the soil’s compaction potential, drainage class, and frost susceptibility—three parameters every Boise foundation needs.

Our approach and scope

The analysis starts on a stack of 8-inch-diameter brass sieves arranged from 3-inch down to No. 200, mounted on a mechanical shaker that runs for a timed cycle determined by the maximum particle size in the sample. After the coarse fraction is weighed and recorded, the minus-No. 200 material goes into a 1,000-mL sedimentation cylinder with a Type 152H hydrometer floating in a sodium-hexametaphosphate dispersion. A technician reads the hydrometer at elapsed times of 15, 30, 60, 120, and 1,440 minutes, while a separate cylinder tracks temperature corrections against a control blank. The two curves—sieve and sedimentation—are merged at the No. 200 breakpoint to produce a continuous gradation plot from 75 mm down to 0.001 mm. For Boise soils that contain volcanic ash lenses from the Glenns Ferry Formation, the hydrometer session is extended to capture the full clay fraction, because those fine particles control both the drained shear strength and the permeability that governs sub-drain design.

Grain Size Analysis (Sieve + Hydrometer) for Boise Construction Projects

Local ground factors

Boise lies in Seismic Design Category D per the current IBC, and the USGS hazard maps assign a peak ground acceleration of roughly 0.25 g for the downtown area. Fine-grained alluvial soils with more than 35 percent passing the No. 200 sieve—common in the lower Boise River terraces near Garden City—become suspect for liquefaction and cyclic softening when the gradation curve lacks a coarse skeleton. A grain size analysis that stops at the No. 200 sieve misses the clay fraction that controls whether the soil behaves as a plastic fines matrix or a non-plastic silt. Without the hydrometer data, the engineer cannot apply the liquefaction susceptibility criteria of Boulanger and Idriss (2004), potentially classifying a liquefiable silty sand as a safe material. The Boise foothills also carry colluvial silts that grade into residual clay derived from weathered basalt; those soils can hold perched water and lose strength during spring snowmelt if the fines content exceeds 50 percent and the D10 falls below 0.005 mm.

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Relevant standards

The grain size analysis, encompassing sieve and hydrometer testing for Boise construction projects, complies with ASTM D6913-22, ASTM D7928-21e1, ASTM D2487-17e1, AASHTO M 145-91 (2021), and IBC 2021 Section 1803.

Other technical services

Sieve analysis to ASTM D6913

Complete mechanical shake-down from 3-inch to No. 200 with retained-weight tabulation and percent-passing curve. Suitable for soil fractions with less than 10 percent fines by dry mass.

Hydrometer analysis to ASTM D7928

Sedimentation test with Type 152H hydrometer capturing the silt and clay fractions from 75 µm to approximately 0.001 mm. Temperature-controlled bath and blank correction applied per ASTM.

Combined gradation report

Merged sieve-plus-hydrometer curve with D10, D30, D60, Cu, Cc, and USCS symbol. Delivered as PDF with raw data tables for direct import into gINT or OpenGround.

AASHTO soil classification

Group classification per AASHTO M 145 for pavement subgrade evaluation, including the group index calculation that Idaho Transportation Department uses for flexible pavement design.

Typical parameters

Parameter	Typical value
Test standard (coarse)	ASTM D6913-22
Test standard (fines)	ASTM D7928-21e1
Sieve diameters	3 in to No. 200 (75 mm to 75 µm)
Hydrometer type	Type 152H, graduated in g/L
Minimum sample mass	500 g (sands) to 5,000 g (gravels)
Dispersion method	Sodium hexametaphosphate, 40 g/L solution
Reported coefficients	Cu, Cc, D10, D30, D60, percent gravel/sand/fines
Classification output	USCS group symbol + AASHTO M 145 class

Common questions

How much does a grain size analysis with hydrometer cost in Boise?

How long does the lab take to deliver a gradation report?

The mechanical sieve portion finishes within one working day. The hydrometer sedimentation run requires a minimum of 24 hours plus oven-drying and calculation time, so the standard turnaround for a combined report is two to three business days after sample receipt.

What sample mass do you need for the complete analysis?

ASTM D6913 specifies a minimum dry mass based on the maximum particle size. For Boise sands and gravels with particles up to 19 mm, we ask for at least 1,000 grams. If cobbles larger than 75 mm are present, the required mass increases to 5,000 grams or more to maintain statistical validity of the coarse fraction.