Liquefaction Potential Assessment in Lexington, Kentucky

Working in Lexington, you quickly learn that the ground beneath your feet changes dramatically from one drainage basin to the next. The deep alluvial deposits along the Kentucky River corridor bear little resemblance to the stiff, often cherty residual clays draped across the Inner Bluegrass plateau. At our lab, we have seen preliminary SPT refusal at 4 feet in the Hamburg area while, less than six miles southwest near the river, sandy silts extend past 30 feet with groundwater at 8 feet. That contrast matters enormously when a seismic event triggers excess pore pressure. Our liquefaction analysis quantifies factor of safety against triggering using site-specific cyclic stress ratios, corrected blow counts from ASTM D1586, and fines content from wash-sieve procedures, giving the design team a clear picture of post-earthquake settlement potential before a single footing is poured.

In Lexington's alluvial corridors, a factor of safety below 1.1 can mean the difference between serviceable settlement and a total loss of bearing capacity within seconds of shaking.

How we work

One project that stays with us involved a four-story medical office building slated for an old karst-floored valley just off New Circle Road. The first three boreholes hit fine sandy silt from 10 to 28 feet, and the groundwater table was perched at 12 feet. We ran a complete cyclic stress evaluation per ASCE 7-22 Chapter 11, pairing the site class with the 2,475-year spectral acceleration for Lexington. The corrected SPT blow counts, normalized for overburden and hammer energy, dropped below the clean-sand liquefaction curve in the upper portion of the profile. By mapping the cumulative thickness of liquefiable layers and computing the post-triggering volumetric strain, we estimated differential settlement on the order of 1.5 to 2.0 inches across the building footprint. That number drove the decision to switch from shallow footings to driven piles bearing into the underlying limestone. When lateral spreading is a concern near creek banks, we often combine this evaluation with a slope stability analysis to check global deformation under seismic loading.

Local geotechnical context

The core of the evaluation runs on a truck-mounted CME-75 drill rig with an automatic SPT hammer, instrumented to log energy transfer ratio on every blow. In Lexington's urban infill sites, rig access is tight and noise ordinances are strict, so we schedule the field work in two-day windows and use a mud-rotary setup to keep the hole open through the sand layers without casing. The biggest risk we see is not the shaking itself; it is the owner or structural engineer who treats Lexington like a uniform low-seismicity zone and skips the site-specific investigation. When we pull a Shelby tube and the sample densifies on the shaking table, the data tells a different story. A few feet of loose, saturated sand beneath a mat foundation can generate enough excess pore pressure to float a slab, crack partition walls, and rack door frames, even under a moderate New Madrid-style event.

Regulatory framework

ASCE/SEI 7-22 Minimum Design Loads and Associated Criteria for Buildings and Other Structures, IBC 2021 Chapter 18 – Soils and Foundations (ICC 2021), ASTM D1586-18 Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils, ASTM D2487-17 Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), Seed & Idriss (1971) Simplified Procedure for Evaluating Soil Liquefaction Potential

Reference parameters

Parameter	Typical value
Corrected SPT N-value, (N1)60	0–30 blows/ft (target interval)
Fines content (ASTM D1140/D2487)	5–95% passing #200 sieve
Peak ground acceleration (PGA)	0.10–0.18g (Lexington 2% in 50 yr)
Groundwater depth during assessment	4–30 ft below grade
Cyclic stress ratio (CSR) demand	0.08–0.35 (site-specific)
Layer thickness evaluated	1.0–5.0 ft increments
Post-liquefaction volumetric strain	0.5–4.0% (Zhang et al. 2002)

Questions and answers

Does Lexington actually have a liquefaction hazard, given its distance from the New Madrid seismic zone?

Yes, the hazard is low to moderate but not zero. The USGS 2023 National Seismic Hazard Model assigns Lexington a 2% probability in 50-year PGA between 0.10g and 0.18g. When saturated loose alluvial sands are present, that acceleration is sufficient to trigger excess pore pressure. The threat is not catastrophic ground failure; it is cumulative differential settlement that damages architectural finishes and utility connections.

At what depth do you typically find liquefiable soils in Lexington?

In the Kentucky River floodplain and tributary valleys, we encounter potentially liquefiable silty sands between 8 and 35 feet below grade. Above that, the profile is often stiff residual clay or compacted fill. Below 35 feet, the limestone bedrock or dense glacial-outwash gravels generally cap the liquefiable sequence, though we verify this with rock coring at each borehole location.

What is the typical cost range for a site-specific liquefaction analysis in Lexington?

For a standard commercial lot with three to four SPT boreholes, a complete liquefaction triggering and settlement report runs between US$2,250 and US$4,090. The final cost depends on the number of borings, the depth of the alluvial package, and whether we need to add downhole shear-wave velocity measurements to refine the site class.

Can you evaluate liquefaction using CPT data if I have it from a previous investigation?

We can, and we often recommend a CPT test as a complementary tool when the client already has SPT logs but wants a continuous profile of tip resistance and sleeve friction. The CPT-based triggering curves (Robertson & Wride 1998) are incorporated directly into our analysis as a parallel check against the SPT-based simplified procedure.

Liquefaction Potential Assessment in Lexington, Kentucky

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Investigation

In-Situ Testing

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How we work

Local geotechnical context

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Explanatory video

Regulatory framework

Reference parameters

Questions and answers

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