The triaxial test machine we bring to Boise sites is a fully automated load frame with a 100 kN capacity cell, capable of applying confining pressures up to 1700 kPa. For a recent commercial development near the Boise River, we ran a series of consolidated-undrained tests on clay samples extracted at 12 feet depth. The results gave us effective cohesion and friction angle values that directly informed the mat foundation design under IBC 2018 criteria. Before sampling, we also coordinated a compression test on undisturbed samples to validate the unconfined strength trend.
Triaxial test results from Boise Formation clays typically show effective friction angles between 28 and 34 degrees under 100 kPa confinement.
Methodology and scope
Local considerations
Boise sits in a region of moderate seismic activity, and the combination of deep clay layers and a high water table near the river creates a real risk of strength loss during shaking. A triaxial test is the only reliable way to capture the undrained shear strength under cyclic loading conditions. Without it, a foundation designed for static loads may fail under the stress reversal of an earthquake. For critical structures, we recommend supplementing the triaxial data with a site response analysis to evaluate amplification effects specific to the Boise basin.
Applicable standards
ASTM D2850-15 (Unconsolidated-Undrained), ASTM D4767-11 (Consolidated-Undrained), ASTM D7181-11 (Consolidated-Drained)
Associated technical services
Unconsolidated-Undrained (UU) Triaxial
Fast test for immediate undrained strength of clay layers in Boise. No pore pressure measurement. Ideal for embankment stability and short-term bearing capacity checks.
Consolidated-Undrained (CU) Triaxial with Pore Pressure
Full effective stress parameters for long-term foundation design. We measure Skempton's A and B coefficients to evaluate saturation quality.
Consolidated-Drained (CD) Triaxial
Slow drained test for cohesionless soils and pervious materials. Used for slope stability analysis and retaining wall design under steady seepage.
Typical parameters
Frequently asked questions
What is the difference between UU and CU triaxial test in Boise?
A UU test measures undrained strength without pore pressure readings and is faster (1-2 hours per specimen). A CU test includes consolidation and pore pressure measurement, giving effective stress parameters (c' and phi') needed for long-term foundation design in clay layers common in Boise.
How much does a triaxial test cost in Boise?
For a standard CU triaxial test on three specimens, the typical range is between US$2,000 and US$2,410 depending on the number of confining stages and whether pore pressure transducers are required. Volume discounts apply for multiple samples from the same project.
How long does it take to get triaxial test results?
A UU test set (3 specimens) can be completed in 1-2 business days. A CU test with consolidation takes 4-7 days depending on soil permeability. We deliver a full report with stress-strain curves and Mohr envelope.
Do I need a triaxial test or a direct shear test for my Boise project?
Triaxial tests are preferred when you need undrained strength parameters or when the soil is saturated and loaded quickly. Direct shear is simpler and cheaper for drained parameters on granular soils, but triaxial gives more reliable c' and phi' for clay layers found in the Boise Formation.
What sample quality is required for a triaxial test?
We recommend undisturbed tube samples (Shelby tubes) with a diameter of at least 3 inches. Disturbed or remolded samples are not acceptable for strength parameters. Samples must be sealed and kept moist before delivery to our lab.