Boise grew rapidly after the 1950s as irrigation projects transformed the sagebrush desert into farmland and subdivisions. Much of the city sits on the Boise River floodplain, where alluvial sands and silts dominate the subsurface. These deposits amplify ground motions during earthquakes because they are relatively loose and saturated. A site response analysis captures how each soil layer modifies the bedrock motion. Without this study, a foundation design may underestimate the forces a building will face. We combine downhole geophysics with advanced numerical modeling to deliver reliable results for Boise Idaho. Before fieldwork begins, we often recommend a [MASW survey](/masw-vs30/) to map shear-wave velocities across the site.
A standard VS30 measurement alone cannot capture the resonant behavior of deep soil columns in the Boise River Valley.
Method and coverage
Regional considerations
In Boise Idaho we frequently see projects where the geotechnical report only provides a site class letter without a full site response analysis. That shortcut can be dangerous. A building on 40 meters of soft silt over basalt will resonate differently than one on 10 meters of stiff gravel. The design spectra from the code may be unconservative for deep soil profiles. We have documented cases where the computed spectral acceleration at 1.0 second was 40% higher than the code-mandated value. Our analysis uses equivalent-linear and nonlinear methods to capture strain-dependent modulus reduction and damping. This is the only way to get realistic seismic demand for tall or long-period structures.
Process video
Standards that apply
ASCE 7-22 (site classification and response spectra), IBC 2021 (Chapter 16 seismic provisions), ASTM D4428 (crosshole seismic testing), NEHRP Recommended Seismic Provisions (FEMA P-1050)
Related services
Equivalent-Linear Site Response (SHAKE-type)
Uses 1D wave propagation with strain-compatible soil properties. Ideal for preliminary design and code compliance. Output includes acceleration time histories and response spectra at the ground surface.
Nonlinear Site Response (DeepSoil / FLAC)
Captures pore-pressure buildup and modulus degradation in soft soils. Recommended for critical structures like hospitals, schools, and high-rise towers. Includes liquefaction triggering assessment per Youd-Idriss 2001.
Typical parameters
Top questions
What is the difference between site class and site response analysis?
Site class (A through F) is a simplified code-based category based on average VS30. Site response analysis computes site-specific amplification using the actual soil profile, shear-wave velocities, and modulus reduction curves. It captures resonance periods and nonlinear effects that the code table misses.
How much does a site response analysis cost in Boise?
The typical cost ranges between US$1,090 and US$4,260 depending on the number of profiles, depth of borings, and whether nonlinear methods are required. A basic equivalent-linear study for a single column is at the lower end.
When is site response analysis required by code in Idaho?
IBC 2021 requires site response analysis for structures on Site Class F soils, or when the building period exceeds 0.5 seconds and the site is near a known fault. Many local jurisdictions in Boise also ask for it on projects taller than 4 stories.
What field data do you need to run the analysis?
We need a soil profile from borings or CPT soundings, shear-wave velocity measurements (MASW, crosshole, or downhole), and cyclic laboratory tests on representative samples. For equivalent-linear runs, at least one VS30 measurement per profile is essential.