The first thing that goes onto the truck for a seismic microzonation campaign in Orlando is a set of triaxial geophones and a long spread cable—enough to lay out 115 or 230 meters for a solid MASW array. Central Florida is not known for strong shaking, but the combination of shallow limestone, variable sand thickness, and dozens of unmapped sinkhole features means the site response can shift abruptly over a few hundred feet. A borehole sited a block away tells you little about the lot you are actually building on. We map Vs profiles every 20 to 30 meters across the parcel, then tie those to CPT soundings and mud-rotary borings where we encounter the Ocala Limestone. The goal is a ground model that captures the lateral variability, not a single smoothed profile that averages out the problem spots. In Orlando, the karst is the variable that dominates everything else.
In Orlando, the difference between Site Class C and D can be less than 30 meters laterally. A single-average Vs30 misses that.
Top questions
Does Orlando really need seismic microzonation? The shaking here is low.
The shaking is lower than in California, but the site amplification can still be significant. ASCE 7-22 maps show Orlando with Ss around 0.15–0.20g at the 2475-year return period. The bigger issue is site class variability. A Site Class D profile can amplify short-period motion by 30–40 percent relative to Class C, and in karst geology the class can change within a single building footprint. For Risk Category III and IV structures, the Florida Building Code requires a site-specific study when Site Class D or E is present, and you cannot reliably determine the class from sparse borings alone.
How long does a microzonation survey take on a typical Orlando site?
A standard MASW grid for a 2- to 5-acre parcel takes one to two field days, plus another two to three days for the calibration SPT borings with downhole seismic. The site response analysis and reporting add about two weeks. If we encounter unexpected limestone at shallow depth, the field time actually shortens because refusal stops the boring, but the analysis may take longer because the impedance contrast requires careful modeling of the bedrock interface.
What is the typical cost range for seismic microzonation in Orlando?
For a standard commercial lot in the Orlando area, seismic microzonation including MASW grid, calibration borings, and a site response report generally runs between US$4,380 and US$16,130 depending on parcel size, number of grid lines, and whether time-history analysis is required. A small single-structure site with a basic Vs30 map falls at the lower end; a multi-acre campus with a full DEEPSOIL analysis and multiple design spectra falls at the higher end.
How do you handle the limestone interface in the Vs30 calculation?
The Ocala Limestone typically has a Vs of 800 to 1,200 m/s, which is effectively rock per ASCE 7. The challenge is that the depth to limestone can vary from 5 to 30 meters across a single Orlando site. We pick the limestone top from the MASW dispersion curve inversion and confirm it with the SPT refusal depth and downhole velocity logs. Where the limestone is shallower than 30 meters, we use the two-layer Vs30 procedure from ASCE 7 Section 20.4.2, which weights the rock velocity and the overlying soil velocity by the travel-time fractions. This often moves a site from Class D to Class C, and we document the picking uncertainty explicitly in the report so the structural engineer can run bounding cases.
Can you use the microzonation results directly in the structural model?
Yes. We deliver the site-specific design spectrum as a table of Sₐ values at 0.01-second intervals, plus three pairs of spectrum-matched acceleration time histories. These are formatted for direct import into ETABS, SAP2000, or PERFORM-3D. We also provide a site-class boundary drawing in CAD or PDF so the structural team knows exactly which foundation elements sit on which class. For projects where the class changes under the footprint, we recommend running the lateral analysis twice—once for each class—and enveloping the results.
