A four-story apartment complex near the Boise River Greenbelt needed foundation design parameters. The developer wanted fast results without mobilizing a drill rig every day. We ran a Flat Dilatometer Test (DMT) profile to 25 meters. The DMT gave us horizontal stress index, modulus, and dilatometer modulus within hours. That data directly fed the shallow foundation design on site. We combined the DMT with a resistivity survey to map deeper gravel layers under the topsoil. The client saved two weeks of field time compared to traditional SPT-only programs. Boise's alluvial soils respond well to the DMT blade insertion. The test is clean, repeatable, and leaves minimal surface disturbance.
A DMT profile to 25 meters provides horizontal stress and modulus data in hours. That speed is critical for Boise's fast-track commercial projects.
Methodology and scope
Local considerations
In Boise, we often see projects where modulus was guessed from SPT blow counts alone. That leads to over-designed footings or unexpected settlement. The DMT directly measures in-situ stiffness, so you avoid those costly conservatism. Another risk is using DMT data without calibrating to local soil behavior. The correlations in ASTM D6635 were developed mainly on Italian and U.S. Gulf Coast clays. Our team has calibrated them against local Boise test fills and plate load tests. That local calibration makes the DMT reliable for foundation design in Ada County. Skipping that step can produce modulus values 30% off for Boise's specific silt-sand mixtures.
Applicable standards
ASTM D6635-15 (Standard Test Method for Performing the Flat Plate Dilatometer), IBC 2018 (Chapter 18, Soils and Foundations), ASCE 7-22 (Minimum Design Loads, Section 11.6 Site Class)
Associated technical services
Standard DMT Sounding
Single push to refusal or target depth, typically 15–30 m. We record A, B, C pressures at 20 cm intervals. Deliverables include raw data plots, ID-KD-ED profiles, and interpreted M and Su values. Ideal for commercial buildings and residential subdivisions in Boise.
DMT with Seismic Module
Adds a downhole seismic receiver to measure shear wave velocity (Vs) at 1 m intervals. We combine DMT modulus with Vs for dynamic site response analysis. Required for IBC Site Class determination on high-importance structures. Used on Boise's larger mixed-use projects near the Bench.
Typical parameters
Frequently asked questions
How deep can the DMT reach in Boise soils?
We routinely push to 25–30 meters in the Treasure Valley alluvium. Refusal typically occurs in the dense gravel layers below 20 m. The blade can penetrate silty sands and low-plasticity clays without issues. We stop if we hit cobbles or cemented layers.
How much does a Flat Dilatometer Test cost in Boise?
A standard DMT sounding in Boise ranges between US$840 and US$970 per test point. That includes mobilization within Ada County, data acquisition, and a basic interpretation report. Volume discounts apply for five or more soundings on a single project.
What parameters does the DMT provide that SPT does not?
The DMT gives horizontal stress index (KD) and dilatometer modulus (ED) directly. SPT only yields blow count (N-value) and empirical correlations. KD lets us estimate overconsolidation ratio and K0. ED provides a small-strain modulus that correlates well with plate load test results. For modulus design, the DMT is far more reliable than SPT.
When should I choose DMT over CPT in Boise?
Choose DMT when you need direct measurement of horizontal stress and stiffness. The CPT gives continuous cone tip resistance and pore pressure, but not horizontal stress. For shallow foundation design on low-to-medium plasticity soils, the DMT's modulus data is more useful. We often recommend both methods on larger projects: CPT for stratigraphy and DMT for modulus.