Soil Liquefaction Analysis in Madison, Dane County

Madison sits at an elevation of roughly 870 feet on the isthmus between Lakes Mendota and Monona, a geography that shapes everything from the city's views to the soil beneath its streets. The downtown area and much of the isthmus are underlain by glacial lake deposits that few property owners think about until a structural engineer asks for a soil liquefaction analysis. With the last significant seismic event in the region—the 2012 M4.1 quake centered near Clintonville—still referenced by local engineers, the memory that Wisconsin is not immune to ground shaking persists. Our team has worked on sites from the rapidly developing East Washington Avenue corridor to the historic neighborhoods near Camp Randall, and what the borings reveal is a layered profile of sand, silt, and clay that demands site-specific evaluation under ASCE 7 Chapter 20. For deeper stratigraphy where SPT refusal is shallow, we often recommend pairing the analysis with a CPT test to obtain continuous tip resistance and friction sleeve data without the disturbance that split-spoon sampling introduces.

Liquefaction is not just a coastal problem—the glacial lakebed sands of Dane County can lose shear strength when pore pressures rise, and the simplified procedure remains our most reliable field-validated tool for quantifying that risk.

Our approach and scope

A mid-rise residential project on Williamson Street encountered a classic Madison scenario: loose fine sands from 4 to 12 feet below grade, sitting directly above a thick clay unit typical of glacial Lake Yahara deposits. The structural team needed to know whether these sands would lose strength under the design earthquake ground motion, and a standard soil liquefaction analysis was the only way to answer that question. The evaluation follows ASTM D1586 to correct field SPT N-values for energy, overburden, and fines content, then applies the simplified procedure developed by Seed and Idriss—still the backbone of modern practice—to calculate the factor of safety against triggering. In zones where the factor dips below 1.1, the report quantifies settlement potential and lateral spreading risk so the design team can decide whether to densify the sand, replace it, or switch to a deep foundation system that bypasses the liquefiable layer. When the site geometry demands a broader seismic picture, we incorporate findings from a seismic refraction survey to map the depth to bedrock and identify velocity contrasts that influence wave amplification.

Local geotechnical context

The contrast between two Madison neighborhoods illustrates why soil liquefaction analysis cannot be reduced to a single design value. On the near-west side, where the University Avenue corridor sits on thicker sand lenses within the glacial outwash, borings often show higher relative density and correspondingly higher SPT blow counts. Head south toward the Monona shoreline and the post-glacial lake sediments become finer, looser, and more prone to both static settlement and cyclic mobility. A developer who assumes the same ground conditions apply across both zones is setting the project up for costly redesigns. The IBC requires liquefaction evaluation for sites assigned to Seismic Design Category D and above, and while Dane County falls into a moderate seismicity bracket, the combination of loose saturated sands and a shallow water table—frequently within 6 feet of grade near the lakes—elevates the risk profile enough that skipping the analysis is a regulatory and structural gamble. Our reports map the factor of safety against liquefaction for each boring, giving the structural engineer a clear picture of where mitigation is needed and where the soil can be relied upon as-is.

Reference standards

ASCE/SEI 7-22 Minimum Design Loads and Associated Criteria for Buildings and Other Structures, 2021 International Building Code (IBC) — adopted by City of Madison, ASTM D1586 Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils, ASTM D2487 Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), ASTM D6066 Standard Practice for Determining the Normalized Penetration Resistance of Sands for Evaluation of Liquefaction Potential, Youd et al. (2001) — Summary Report, NCEER/NSF Workshops on Evaluation of Liquefaction Resistance of Soils

Reference parameters

Parameter	Typical value
Corrected SPT blow count (N1)60cs	Site-specific from field data
Design earthquake magnitude (Mw)	Per USGS deaggregation for Madison
Peak ground acceleration at site	From ASCE 7-22 ground motion maps
Factor of safety against liquefaction	Reported per boring depth interval
Post-liquefaction volumetric strain	Calculated per Zhang et al. (2002)
Lateral spreading displacement estimate	Per Youd et al. (2002) empirical model
Fines content classification	Per ASTM D2487 (Unified Soil Classification)
Groundwater table depth at time of exploration	Measured in field borings

Frequently asked questions

Is liquefaction really a concern in Wisconsin, so far from active plate boundaries?

It is a common misconception that liquefaction only matters on the West Coast. Wisconsin sits within the stable continental interior, but historical seismicity—including a magnitude 5.1 event near Milwaukee in 1947—demonstrates that moderate earthquakes do occur here. When loose, saturated sands exist at shallow depth, as they do across much of Madison's isthmus and lakeshore areas, even modest ground shaking can generate excess pore water pressure. The IBC and ASCE 7 require evaluation based on the mapped seismic hazard, not on perceived risk, and our analyses have identified liquefaction-susceptible layers on multiple Dane County project sites.

What does a soil liquefaction analysis typically cost for a single-family home or small commercial site in the Madison area?

For a typical residential or small commercial parcel in Dane County, the analysis falls in the range of US$2,200 to US$3,770. This assumes one to two borings with SPT sampling, laboratory fines content testing on selected samples, and the engineering calculations required to determine the factor of safety against liquefaction. Sites with deeper liquefiable layers, more borings, or the need for CPT correlation may push toward the upper end of that range. We always provide a written proposal specific to the site before starting work.

How do you account for the glacial lake clays that dominate Madison's subsurface when running a liquefaction evaluation?

The Lake Yahara clays are generally not liquefiable because cohesive soils do not exhibit the rapid pore pressure buildup that saturated granular soils do. Our analysis screens each layer for liquefaction susceptibility using the Chinese criteria and the modified compliance approach that considers plasticity index and moisture content. The clays are modeled separately for cyclic softening where warranted, but the primary liquefaction focus is on the sand lenses and silty sand interbeds deposited during glacial retreat. The interaction between these liquefiable sands and the surrounding cohesive matrix is what makes Madison's stratigraphy particularly interesting from a geotechnical standpoint.

Soil Liquefaction Analysis in Madison, Dane County

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Reference standards

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