Phoenix’s growth from an agricultural outpost in the Salt River Valley to the fifth-largest U.S. city has left a layered subsurface that challenges geotechnical engineers daily. The valley floor mixes coarse alluvial sands, cobbles, and pockets of silty clay deposited by centuries of flash floods. When a contractor breaks ground near Camelback Mountain or along the Loop 101 corridor, knowing the exact distribution of particle sizes is not a formality—it determines compaction specs, drainage design, and foundation bearing capacity. Our grain size analysis service, combining mechanical sieving with hydrometer sedimentation, delivers the gradation curve that Phoenix projects require. We process samples from test pits excavated in residential subdivisions, from SPT drilling spoils at commercial sites, and from borrow sources in the West Valley. Every curve we produce follows ASTM D2487 classification so that the Unified Soil Classification System (USCS) symbol on your log is defensible under review.
A complete gradation curve is the single most cost-effective index test for predicting how Phoenix soils behave under compaction and water.
Methodology and scope
Local ground factors
The IBC 2021, adopted by Phoenix with local amendments, ties foundation design and seismic site class directly to the USCS classification output from grain size analysis. When a report classifies a soil as SM (silty sand) instead of SC (clayey sand), the difference can shift the site class from D to E under ASCE 7 Chapter 20, altering the design seismic coefficient. We have seen this trigger costly foundation redesigns on projects near the Phoenix Fault zone. Another risk emerges with collapsible silty sands common in older alluvial fans—without an accurate fines content, a standard Proctor curve may overestimate density and leave the pad vulnerable to settlement after the first monsoon soak. Running both sieve and hydrometer fractions in a single, traceable lab batch eliminates the guesswork. Our reports flag borderline classifications and recommend confirmatory testing when the sample plots near the A-line on the plasticity chart, because an incorrect USCS symbol is a liability that no engineer wants to carry into construction.
Applicable standards
ASTM D6913 — Standard Test Methods for Particle-Size Distribution of Soils (Sieve Analysis), ASTM D7928 — Standard Test Method for Particle-Size Distribution of Fine-Grained Soils Using the Sedimentation (Hydrometer) Analysis, ASTM D2487 — Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), IBC 2021 (Phoenix amendment) — Sections 1803 and 1613 for foundation investigation and seismic site classification
Related services
Full Sieve & Hydrometer Package
Combined ASTM D6913 and D7928 testing on a single split sample, yielding a continuous gradation curve from 3-inch gravel down to 1-micron clay. Includes USCS classification per ASTM D2487.
Washed Sieve Analysis
For granular soils with visible fines coatings, we run wash-sieve procedures that separate silt and clay before mechanical shaking, preventing particle agglomeration from skewing the coarse fraction percentages.
Rapid Gradation Screening
When Phoenix earthwork crews need same-day control during fill placement, we run an abbreviated sieve stack on a representative split and report the key gradation parameters within hours.
Typical parameters
Questions and answers
How much does a grain size analysis (sieve plus hydrometer) cost in Phoenix?
The combined test typically runs between US$100 and US$210 per sample, depending on whether it is a full hydrometer with multiple readings or a single-point sedimentation check. Volume pricing applies for projects submitting more than 10 samples.
How long does the hydrometer part of the test take?
The hydrometer sedimentation phase requires a minimum of 24 hours for readings at the specified time intervals, plus an additional 24 hours if the sample contains expansive clays that need extended dispersion. We provide preliminary sieve results ahead of the final hydrometer data when schedules are tight.
Do you pick up samples from our Phoenix jobsite?
We coordinate sample pickup across the metro area, from Buckeye to Apache Junction. Samples should be bagged in sealed, labeled plastic bags with the boring number, depth interval, and date clearly marked. We supply sample bags and chain-of-custody forms at no extra charge.
Can you classify the soil if we only need the USCS symbol?
Yes. Once the complete gradation curve is plotted and the Atterberg limits are determined on the fines fraction, we assign the USCS group symbol and group name in accordance with ASTM D2487. The report includes the gradation table, the 0.075 mm split percentage, and the calculated coefficients of uniformity and curvature.