Field Permeability Testing (Lefranc/Lugeon) in Lexington

In Lexington, the karst topography underlying the city creates highly variable drainage conditions. Limestone bedrock sits beneath a mantle of residual clay, and solution channels can move groundwater fast. Standard lab tests on small samples do not capture this behavior. The test pits we excavate often reveal open fissures that a borehole alone might miss. We run field permeability tests to measure hydraulic conductivity at the formation scale. The Lefranc method works for soil and weathered rock above the water table. The Lugeon test applies to fractured bedrock zones where water pressure is injected in stages. Both methods produce data that local engineers need for dewatering design and infiltration analysis.

Karst limestone in Lexington can show Lugeon values from near zero to over 50 Lu within the same borehole. Stage testing is the only way to map flow paths.

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How we work

IBC Chapter 18 and ASTM D6391 govern water pressure testing procedures, but here in Lexington the fractured Ordovician limestone makes Lugeon interpretation critical. A single packer isolates a test interval while water is pumped at constant pressure. We record flow rates over time. The Lugeon value equals flow in liters per minute per meter of test interval at 1 MPa pressure. Values below 1 Lu mean tight rock. Above 10 Lu suggests open joints or cavities. For the Lefranc test we use a variable head setup in clayey soils. Water level drop is timed. The solution for hydraulic conductivity follows Hvorslev's shape factor equations. We apply these methods daily across Fayette County.

Field Permeability Testing (Lefranc/Lugeon) in Lexington — Technical reference — Lexington

Local geotechnical context

The contrast between the deep clay soils south of Man o' War Boulevard and the shallow rock near downtown Lexington changes the test approach entirely. Downtown, a Lugeon test in limestone must account for artesian conditions that sometimes appear where the confining layer is thin. South of town, thick Grier Clay can seal the surface, trapping water and creating perched water tables. Running a Lefranc test without first identifying the water table position yields misleading data. Misinterpreting a Lugeon test in karst can also cause grouting underestimates. We have seen foundation excavations flood because the permeability profile looked tight on paper but missed a single open conduit at depth.

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Explanatory video

Regulatory framework

ASTM D6391 - Standard Test Method for Field Measurement of Hydraulic Conductivity Using Borehole Infiltration, IBC 2021 Chapter 18 - Soils and Foundations, ASCE 7-22 - Minimum Design Loads for Buildings and Other Structures, Hvorslev (1951) - Time Lag and Soil Permeability in Ground-Water Observations

Reference parameters

Parameter	Typical value
Applicable standard	ASTM D6391 (Lugeon), Hvorslev method (Lefranc)
Test medium	Soil and weathered rock (Lefranc), fractured rock (Lugeon)
Typical test depth	10 to 200 ft below ground surface
Pressure stages	5 to 6 ascending and descending pressure steps
Measurement precision	Flow rate to nearest 0.1 gpm, pressure to 1 psi
Reporting output	Hydraulic conductivity k (cm/s) or Lugeon units

Questions and answers

When does a project need a Lugeon test instead of a Lefranc test?

A Lugeon test is required whenever the investigation encounters rock, which is common across much of Lexington. The Lefranc test applies to soil and completely weathered rock where the borehole wall stays stable. Once you hit competent limestone, a packer test becomes the standard.

How many pressure stages does a Lugeon test include?

Typically five or six stages. We start at low pressure, increase in steps, and then decrease. The pattern of flow versus pressure reveals whether the fractures are dilating, washing out, or filling. This diagnostic step is essential in karst.

What is the typical cost for a field permeability test in Lexington?

For a single Lefranc or Lugeon test performed within a geotechnical investigation, costs generally range from US$610 to US$1.100. The exact figure depends on depth, number of stages, and whether the test is combined with other borehole work on the same day.

How do you interpret a Lugeon value above 30 Lu?

Values above 30 Lu indicate highly permeable rock with open joints or small cavities. In Lexington limestone, this often means a solution channel. We would typically recommend additional investigation with borehole imaging or packer spacing adjustments to isolate the feature.

Location and service area

We serve projects in Lexington and surrounding areas.

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