Our team runs a portable CPT rig and a trailer-mounted drill rig for field work across Boise. We push cone rods to refusal or 30 meters, collecting continuous soil behavior type logs. In the lab, we combine that data with resonant column tests to measure shear modulus and damping at small strains. The whole workflow feeds directly into seismic foundation design parameters for each project. We also run cyclic triaxial tests on undisturbed samples to quantify pore pressure buildup under earthquake loading. That gives structural engineers real numbers, not textbook assumptions, for their models. Before field work, we always cross-check with MASW-Vs30 surveys to confirm the shear-wave velocity profile.
A measured Vs30 profile in Boise's alluvial valley can shift the site class from D to C, reducing seismic demand by up to 25 percent.
Methodology and scope
Local considerations
Boise experienced a magnitude 5.0 earthquake in 1894, and the nearby Sawtooth Fault can generate M6.5 events. For a four-story apartment building on the Bench with 4 meters of loose sand over basalt, the design spectra from ASCE 7 default maps may overestimate short-period motion. We run site-specific response analyses using DEEPSOIL or equivalent linear models to capture the true amplification. That work is part of every seismic foundation design package we deliver. Without it, the foundation could be overdesigned by 30 percent or, worse, under-designed for the actual soil column.
Applicable standards
ASCE 7-22 (Minimum Design Loads, Site Class), IBC 2021 (Chapter 18, Seismic Requirements), ASTM D4015-21 (Resonant Column Test), NCEER 1997 (Liquefaction Triggering Criteria), ASTM D1586-18 (Standard Penetration Test)
Associated technical services
Site-Specific Response Spectra
We develop acceleration spectra using Vs30 profiles from MASW or downhole surveys, combined with ground motion prediction equations. The output includes design spectra for 0.1 to 10 seconds period.
Liquefaction Hazard Assessment
We evaluate cyclic resistance using SPT N-values, fines content, and cyclic triaxial tests. The report maps liquefaction potential at 1-meter intervals and recommends mitigation if FS < 1.1.
Seismic Bearing Capacity
We compute ultimate and allowable bearing capacities under seismic loading using Meyerhof and Brinch Hansen methods with dynamic reduction factors. Results consider both vertical and horizontal accelerations.
Foundation Type Recommendation
Based on the site-specific seismic parameters, we recommend shallow (spread footings, mats) or deep (drilled shafts, driven piles) foundations. We also evaluate tie beams and grade beam details for lateral loads.
Typical parameters
Frequently asked questions
When is seismic foundation design required in Boise?
It is required for all Risk Category II and higher buildings per IBC 2021. Most commercial and multi-family projects in Boise fall under Category II (occupant load > 300) and need site-specific seismic analysis if site class is F or if PGA exceeds 0.15 g.
What is the typical cost range for a seismic foundation design study in Boise?
The cost typically ranges between US$1.240 and US$3.650 depending on the number of borings, lab tests (resonant column, cyclic triaxial), and complexity of the response analysis. A smaller residential project on the Bench may fall near the lower end.
How long does a seismic foundation design report take to complete?
Field work and lab testing usually take 2 to 3 weeks. The analysis and report writing add another week. Total turnaround is 3 to 4 weeks from the start of drilling. Expedited schedules are possible with prior coordination.
What is the difference between ASCE 7 default site class and a measured Vs30 profile?
The default site class uses generalized geologic maps and may assign Class D (stiff soil) to Boise's alluvium. A measured Vs30 profile can show higher velocities (Class C) if the basalt is shallow, reducing the spectral acceleration demand. That difference directly affects foundation cost and safety.
Can you design foundations for both new construction and retrofits?
Yes. For new construction we provide design parameters for the structural engineer. For retrofits we evaluate existing foundation capacity under current seismic codes and recommend strengthening or replacement. Both follow the same ASCE 7 and IBC framework.