Grain Size Analysis (Sieve & Hydrometer) in Tucson

The most common mistake on Tucson jobsites is classifying basin soil by feel alone. A contractor digs a hole, rubs dirt between their fingers, and guesses it's sandy clay. Then the foundation specification is wrong. In a city where alluvial fans mix coarse Catalina foothill sediment with fine playa silts, the gap between visual estimate and actual gradation can be dramatic. A full grain size analysis combining mechanical sieving for the coarse fraction with hydrometer sedimentation for fines removes the guesswork. We run the procedure per ASTM D422 and classify the material per ASTM D2487. The report tells you exactly what you are building on, whether you are placing a mat foundation near Davis-Monthan or excavating for a retaining wall in the Sam Hughes neighborhood.

Gradation controls drainage, compaction, and frost behavior. In Tucson's arid basins, a 5% shift in fines content changes the Unified Soil Classification completely.

How we work

The lab setup for this test uses a stack of ASTM E11 sieves on a mechanical shaker, plus a 152H hydrometer and sedimentation cylinder for the silt and clay fraction. A technician oven-dries the sample, washes it through a No. 200 sieve, and splits the retained material for dry sieving. The minus-200 wash is dispersed in a sodium hexametaphosphate solution and hydrometer readings are taken at timed intervals. The results plot a continuous grain size distribution curve from gravel down to clay colloids. Tucson soils often show a gap-graded signature where coarse sand from ephemeral washes sits in a matrix of windblown fines. Understanding that gradation directly impacts permeability assumptions and compaction specifications. For sites near the Rillito River, we often pair this with in-situ permeability testing to correlate lab gradation with field infiltration rates.

Local ground factors

Soil gradation changes abruptly across the Tucson basin. A site near the Tanque Verde Creek often has clean, well-graded sand with less than 5% fines. Move three miles west toward the Santa Cruz River floodplain and the same depth can be fat clay with 60% passing the No. 200 sieve. The difference in bearing capacity and shrink-swell potential between these two profiles is enormous. Designing a shallow footing system without a grain size curve means assuming a soil type that may not exist on the parcel. For structures on the expansive clays common in central Tucson, the hydrometer fraction tells you whether the soil will heave when wetted. Omitting this test to save a few hundred dollars has led to slab cracks, retaining wall tilts, and stormwater detention failures documented across Pima County. The data is cheap relative to the cost of fixing a foundation later.

Reference parameters

Parameter	Typical value
Test standard	ASTM D422 / ASTM D6913
Sieving range	75 mm to 0.075 mm (No. 200)
Hydrometer method	ASTM D7928 (152H hydrometer)
Sample mass required	500 g for fine soils, up to 5 kg with gravel
Dispersion agent	Sodium hexametaphosphate (NaPO3)6
Reported parameters	D10, D30, D60, Cu, Cc, % gravel/sand/silt/clay
Soil classification	ASTM D2487 Unified Soil Classification System (USCS)

Related services

Standard Sieve Analysis (Coarse Fraction)

Mechanical dry sieving from 75 mm down to the No. 200 sieve. Includes wash loss for accurate fines percentage. Report provides D10, D30, D60, uniformity coefficient, and curvature coefficient.

Hydrometer Analysis (Fine Fraction)

Sedimentation testing for material passing the No. 200 sieve using a 152H hydrometer. Stoke's law calculation for particle diameters from 0.075 mm to 0.001 mm. Combined curve merges sieve and hydrometer data.

Reference standards

ASTM D422 Standard Test Method for Particle-Size Analysis of Soils, 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 Chapter 18 Soils and Foundations

Quick answers

How much sample material do you need for a grain size analysis?

For soils with no gravel, about 500 grams of dry material is sufficient. If gravel is present, we need up to 5 kg to ensure a representative split. The sample must be oven-dried, and we prefer to receive it in a sealed bag to preserve in-situ moisture if we are also running water content.

What is the turnaround time for a sieve and hydrometer test in Tucson?

Standard turnaround is three to four business days. The hydrometer portion requires timed readings over a 24-hour period, which sets the minimum lab time. Expedited two-day service is available when project schedules require it.

How much does a grain size analysis cost?

A combined sieve and hydrometer analysis typically runs between US$110 and US$190, depending on the number of samples and whether we are also performing Atterberg limits or other index tests on the same material.

Why can't I just use a simple sieve test without the hydrometer?

A sieve-only test stops at the No. 200 sieve and tells you the percentage of fines, but nothing about whether those fines are silt or clay. Silt and clay behave completely differently in terms of permeability, frost susceptibility, and shrink-swell potential. In Tucson's expansive clay zones, the hydrometer fraction is the only way to quantify the clay content that drives heave.