Determining a site’s seismic shear-wave (S-wave) velocity profile is essential for designing sensitive structures and those exposed to large dynamic loads. To comply with the International Building Code (IBC), Uniform Building Code, or National Earthquake Hazards Reduction Program standards, it is necessary to know the seismic site classification, which ranges from A (the best – hard rock), down to F (the worst – soft or organic clay).
Proper site classification is crucial to a project. Choosing a site class that is too low creates safety and liability risks. Using an overly conservative class leads to unnecessary additional costs. Classifying the site correctly during design can save tens of thousands to millions of dollars, depending on the project.
RETTEW estimates S-wave velocity and classifies sites precisely using non-invasive geophysical methods, providing an efficient alternative to traditional intrusive approaches. RETTEW also performs numerical liquefaction potential analysis (LPA) for sites with materials that may lose bearing capacity during earthquakes, ensuring a comprehensive site evaluation.
Shaking Out the Truth: Turning a Former Coal Ash Basin into Build-Ready Ground
A former coal ash basin became available for development. However, the owners needed to confirm that the subsurface could support construction. To comply with earthquake-resistant standards like the IBC, RETTEW performed several analyses: seismic site classification (categorizes soil for seismic response), probabilistic seismic hazard analysis (PSHA, estimates earthquake risk statistically), deterministic seismic hazard analysis (DSHA, assesses risk from the largest credible quake), and LPA (evaluates risk that soil loses strength during shaking).
To complete the seismic site classification survey, RETTEW conducted refraction microtremor (ReMi) measurements along ten profiles, forming five cross-hair patterns throughout the site (see Figure 1). Each profile provides the weighted-average S-wave velocity—how quickly earthquake-related waves travel through soils and rocks—to a depth of 100 feet, known as VS100. These VS100 results place the site in Classification Zone CD, which allows for less stringent (and less costly) engineering standards than the default Class D.
RETTEW also completed a PSHA/DSHA using the ReMi results, geologic information, and historical seismic data. This information yields spectral response accelerations for the maximum considered earthquake (MCE). Engineers use these accelerations to calculate the stresses that any structure must withstand.
In the final phase, RETTEW performed an LPA using available geotechnical boring data and historical seismic records. The goal was to determine if any subsurface layers could liquefy during the MCE. Given that liquefaction probability above 20% is a concern, all borings modeled from this site fall within the “safe” zone (see Figure 2).
Site owners can now present this information to land developers and engineers, who can then design and build confidently and at the optimum cost.
Introducing Drew Burford
Senior Geophysicist
From memorable field moments to groundbreaking discoveries, Senior Geophysicist Drew Burford has a seismic story of his own. Go beneath the surface and get to know Drew!
