A six-story mixed-use building on Monroe Street hit pause last spring when preliminary borings struck loose, saturated sand at 22 feet. The water table in Tallahassee sits high—often less than 10 feet below grade in the southern sections near Lake Munson—and that combination of shallow groundwater and cohesionless soil is exactly what triggers a liquefaction flag under ASCE 7-16. The project team didn't waste time guessing. They ordered a site-specific soil liquefaction analysis that correlated SPT blow counts from six borings with the seismic demand for the site's Site Class D profile. Three weeks later, the structural engineer had the peak ground acceleration (PGA) thresholds and the post-liquefaction settlement estimates needed to switch from a shallow footing scheme to a ground improvement strategy. That's the difference between a project that moves forward and one that stalls in redesign. For deep profiles where blow count resolution matters, we pair the analysis with a CPT test to map the stratigraphy continuously and catch thin liquefiable lenses that SPT intervals can miss.
In Tallahassee, shallow groundwater plus loose sand equals a liquefaction risk that too many site investigations overlook. A site-specific analysis is not optional—it is an ASCE 7 requirement for Site Class D and above.
Our approach and scope
Local ground factors
Tallahassee's urban footprint grew fast in the 1970s and 1980s, pushing development southward into drainage basins that were historically marsh and pine flatwoods. Many of those areas—around Capital Circle Southeast and the Apalachee Parkway corridor—sit on Holocene alluvium and loose reworked sands that never experienced significant preloading. The soil liquefaction analysis for these zones often reveals LPI values in the moderate-to-high range, even under the moderate seismicity of the Big Bend region. The consequences of ignoring that data are severe. Differential settlement from liquefaction-induced sand boils can tear apart slab-on-grade foundations, rupture underground utilities, and tilt columns in moment-frame buildings. A 2019 site investigation near the Southwood development found clean sand lenses at 18 feet with SPT N-values below 8. The factor of safety under the design earthquake was 0.9—below the 1.1 minimum required by the IBC. The owner opted for vibrocompaction treatment before foundation construction. That decision saved hundreds of thousands in potential structural retrofit costs down the line.
Reference standards
ASCE 7-16 – Minimum Design Loads and Associated Criteria for Buildings and Other Structures, IBC 2021 – International Building Code (Seismic Provisions), ASTM D1586 / D1586M – Standard Test Method for Standard Penetration Test (SPT), ASTM D2487 – Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), NCEER 1997 Workshop – Seed & Idriss simplified procedure with NCEER modifications
Additional services
SPT-Based Liquefaction Triggering
Compute CSR and CRR at each sample depth using corrected N-values, fines content, and overburden stress. Produce factor of safety profiles for each boring location.
LPI Spatial Mapping
Generate Liquefaction Potential Index contour maps across the site footprint, identifying zones of negligible, moderate, and high risk for foundation planning.
Post-Liquefaction Settlement Estimation
Calculate volumetric strain and vertical settlement using the Ishihara-Yoshimine approach, providing the structural engineer with expected differential movement.
Lateral Spreading Analysis
Apply empirical displacement models (Youd et al.) for sites near free faces or gently sloping ground, critical for properties along the St. Marks River corridor.
Typical parameters
Questions and answers
Is liquefaction really a concern in Tallahassee, given Florida's low seismicity?
Yes. While Tallahassee is not in a high-seismicity zone like California, the USGS hazard maps assign a low but non-zero PGA. What makes Tallahassee susceptible is the combination of Site Class D and E soils—loose sands and silts with shallow groundwater. Under these conditions, even moderate ground shaking can trigger liquefaction. The IBC requires evaluation for any site classified as Site Class D or worse when the design PGA exceeds 0.05g, which applies to parts of Leon County.
What field data do you need to perform the analysis?
We typically work with SPT N-values from standard penetration tests, borehole logs with Unified Soil Classification System descriptions, Atterberg limits on fines, and grain size distribution curves. If CPT soundings are available, we can refine the cyclic resistance ratio (CRR) profiles significantly. We also need the design groundwater table depth and the site-specific peak ground acceleration from the project's seismic hazard report.
How much does a liquefaction analysis cost for a typical Tallahassee project?
For a standard commercial lot with four to six borings and full liquefaction triggering plus LPI mapping, the analysis typically ranges from US$2,400 to US$4,130. The final cost depends on the number of borings, whether CPT data is included, and whether lateral spreading or post-liquefaction settlement modules are required.
What is the difference between the factor of safety and the Liquefaction Potential Index?
The factor of safety (FS) is calculated at each sample depth and tells you whether that specific soil layer is expected to liquefy under the design earthquake. The Liquefaction Potential Index integrates FS over the entire soil column and weights it by depth, giving a single number that represents the overall severity of liquefaction at that boring location. LPI is more useful for foundation decisions because it correlates with observed damage potential.
If my site shows liquefaction risk, what are the remediation options?
Common ground improvement methods include vibrocompaction for clean sands, stone columns for silty sands, and compaction grouting for localized loose zones. For smaller structures, a mat foundation can help bridge differential settlements. In severe cases, deep foundations such as driven piles that bypass the liquefiable layer are the safest choice. The liquefaction analysis provides the parameters needed to design any of these solutions.