The semi-arid climate of the Treasure Valley creates a unique soil profile in Boise Idaho. Deep seasonal drying of the ground surface produces high desiccation cracks in the upper clay layers. Below that, the Boise River alluvium shifts abruptly from clean sands to silty gravels. A proper soil classification using the Unified Soil Classification System (USCS) or AASHTO system is essential to interpret these transitions. Without it, foundation designs can miss the change in bearing strata. We follow ASTM D2487-17 to assign group symbols and names. This data feeds directly into bearing capacity calculations and compaction specifications. For projects on the bench areas east of downtown, we combine classification with a granulometry analysis to quantify the gravel fraction in the coarse layers.
A single soil sample from the Boise River floodplain can shift from USCS SM to ML within 200 feet horizontally. Classification catches that.
Method and coverage
- Grain-size distribution by sieve and hydrometer (ASTM D6913/D7928)
- Atterberg limits for fines (ASTM D4318)
- Organic content determination (ASTM D2974)
- AASHTO group index for pavement subgrade rating
Regional considerations
A common mistake in Boise Idaho is classifying the tan silty sand of the alluvial terraces as SP (poorly graded sand) just because it looks clean by eye. In reality, that material contains 8 to 12 percent non-plastic fines that shift it to SM. That changes the compaction curve and the shear strength estimate. If the contractor specs a standard SP compaction curve, the fill never reaches the required 95 percent relative density. The result is post-construction settlement under the slab. We have seen this repeatedly on warehouse projects near the Boise Airport. A proper classification prevents the rework.
Standards that apply
ASTM D2487-17 (Standard Practice for Classification of Soils for Engineering Purposes, USCS), AASHTO M 145-91 (Standard Specification for Classification of Soils, AASHTO System), ASTM D4318-17 (Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils), ASTM D6913/D6913M-17 (Standard Test Methods for Particle-Size Distribution of Soils Using Sieve Analysis)
Related services
Full USCS Classification (ASTM D2487)
Complete sieve, hydrometer, and Atterberg limits analysis. We assign group symbol, group name, and furnish the complete grain-size curve. Includes organic content check if the sample contains dark fines.
AASHTO Pavement Subgrade Classification
Focused on highway and airfield projects. We calculate the group index per AASHTO M 145 and recommend the subgrade modulus (MR) range based on the classified material type.
Field Classification with Visual-Manual Procedure
On-site preliminary identification using ASTM D2488. We log soil types during test pits or borings, assign provisional USCS symbols, and flag samples that need full laboratory confirmation.
Typical parameters
Top questions
What is the difference between USCS and AASHTO soil classification?
USCS groups soils by grain size and plasticity into 15 major categories such as SM (silty sand) or CH (fat clay). AASHTO rates soils from A-1 (best subgrade) to A-7 (poor subgrade) and calculates a group index for pavement design. Both systems use similar lab tests but answer different engineering questions — USCS for foundations and earthworks, AASHTO for roads.
How much does a soil classification test cost in Boise Idaho?
A full USCS classification with Atterberg limits and sieve analysis typically ranges between US$50 and US$90 per sample. The price depends on whether hydrometer analysis is needed for the fines fraction. Volume discounts apply when submitting more than ten samples from the same project.
Why does Boise soil sometimes classify as ML instead of CL?
The silt-rich alluvium from the Boise River and its tributaries often has low plasticity. When the liquid limit is below 50 and the plasticity index plots below the A-line on the Casagrande chart, the material falls into ML (silt) even if it feels sticky. This is common in the floodplain areas near Garden City and affects the frost susceptibility rating.