ASCE 7 and the IBC do not directly address unsaturated soil mechanics, yet the behavior of partially saturated ground governs many failures in Boise. The Treasure Valley sits on a complex sequence of Lake Bonneville silts, fluvial sands, and expansive clays that shift volume with moisture changes. A standard saturated-soil approach misses the real story. We measure matric suction and water retention curves to capture how these soils behave above the water table. That data feeds directly into heave predictions for residential slabs and collapsible settlement in the Foothills. In our experience, pairing unsaturated analysis with a consolidation test on undisturbed samples gives the clearest picture of long-term deformation risks under changing moisture conditions.
Unsaturated soil analysis in Boise distinguishes between a slab that cracks and one that stays level through seasonal moisture swings.
Methodology and scope
- Filter-paper suction tests (ASTM D5298) for total and matric potential
- Pressure plate SWCC curves from 0 to 1500 kPa
- Free-swell and collapse potential under controlled moisture
Local considerations
In Boise, many times we see post-construction damage from a single heavy irrigation cycle hitting dry, collapsible fill. The soil looks stiff when compacted, but once the capillary suction drops below a critical threshold, the structure settles abruptly. That is not captured by standard Proctor or CBR tests. Unsaturated soil analysis captures the metastable state. We flag any lot where the collapse potential exceeds 2% and recommend pre-wetting or deep compaction before the foundation goes in. The Foothills alluvial fans are especially sensitive — we have measured up to 4 inches of differential movement in homes built without suction-based testing.
Applicable standards
ASTM D5298-16 (Filter paper suction measurement), ASTM D6836-16 (SWCC by pressure plate), ASTM D5333-03 (Collapse potential of soils)
Associated technical services
SWCC & Soil Suction Profiling
Complete soil-water characteristic curve determination from undisturbed samples. Includes filter-paper suction at multiple moisture contents and pressure plate testing up to 1.5 MPa. Results are fitted to van Genuchten or Fredlund-Xing models for direct input into unsaturated flow and slope stability software.
Volume Change & Collapse Assessment
Double-oedometer and single-oedometer collapse tests per ASTM D5333, plus free-swell under surcharge. We correlate collapse potential with in-situ suction values so you know the moisture threshold that triggers movement. The final report includes recommended pre-wetting depths and compaction targets for Boise fills.
Typical parameters
Frequently asked questions
When do I need unsaturated soil analysis instead of standard saturated testing in Boise?
Any project above the water table in Boise's expansive clays or collapsible sands benefits from it. Standard testing assumes full saturation, which overestimates strength in dry conditions and underestimates collapse when wetting occurs. If your site has deep groundwater (greater than 15 feet) or uses surface irrigation, unsaturated analysis is critical.
How much does an unsaturated soil analysis cost in Boise?
Typical project costs range from US$1,050 to US$2,690 depending on the number of suction points and SWCC curves required. A basic filter-paper profile with three depths runs on the lower end; full pressure-plate testing with collapse assessment pushes the upper range.
What is the difference between matric suction and total suction?
Matric suction is the capillary tension in the pore water, dominant in unsaturated granular soils. Total suction adds the osmotic component from dissolved salts. In Boise's Lake Bonneville silts, matric suction controls collapse behavior; total suction matters more for expansive clays where osmotic gradients drive swelling.
Can unsaturated soil analysis predict foundation movement from seasonal moisture changes?
Yes. By combining the SWCC with a site-specific moisture fluctuation model, we estimate heave or collapse under wet-season vs. dry-season conditions. In Boise's semi-arid climate, annual moisture swings of 5-8% volumetric water content are common — enough to trigger 1-2 inches of movement in sensitive soils.