Lefranc and Lugeon Permeability Testing in Stoke-on-Trent

The ground conditions across Stoke-on-Trent vary sharply between the Etruria Marl of the southern districts and the glacial sands and gravels that overlie the Middle Coal Measures further north near Tunstall. In Trentham, weathered sandstone can transmit water at rates an order of magnitude faster than the stiff clays found beneath Hanley, making permeability assumptions based solely on borehole logs unreliable. When foundation drainage or dewatering calculations rely on textbook values rather than site-specific data, the margin for error increases substantially. A field permeability test—whether a falling-head Lefranc test in granular material or a Lugeon packer test in fractured bedrock—provides the hydraulic conductivity figure that Eurocode 7 requires for serviceability limit state verification. For sites within the former mining districts of Stoke-on-Trent, where backfilled shafts and collapsed workings disturb natural drainage paths, a single in-situ permeability profile often reveals flow regimes that desk-study predictions would miss entirely.

A single Lugeon value of 5 to 10 units in fractured Coal Measures sandstone can double the dewatering requirement compared with intact mudstone, reshaping the whole earthworks programme.

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Methodology and scope

A recurring mistake on brownfield sites in Stoke-on-Trent is treating all Coal Measures strata as impermeable and omitting dedicated permeability testing from the ground investigation scope. The Middle Coal Measures contain sandstone bands and seatearth horizons that can act as perched aquifers, particularly after prolonged rainfall on the exposed slopes of the Potteries. When a contractor sinks a sump or designs a cut-off drain without measuring mass hydraulic conductivity, pore-water pressures build behind retaining structures and soften the formation beneath access roads. The Lefranc method measures K values between 1×10⁻⁷ and 1×10⁻³ m/s in soil and weathered rock, using a constant or falling-head regime within a cased borehole section, while the Lugeon test applies pressure stages up to 1 MPa in competent rock to quantify fracture flow in Lugeon units. Both procedures follow BS 5930:2015+A1:2020 and BS EN ISO 22282-2/3, requiring careful sealing with a pneumatic packer to prevent short-circuiting along the annulus. Because the glacial till cover in Stoke-on-Trent rarely exceeds 3 to 5 metres, reaching the rockhead for a Lugeon test is straightforward, though the presence of old mine workings demands vigilant core logging before selecting test intervals. The resulting coefficient of permeability feeds directly into groundwater control design, slope stability analysis, and contamination plume modelling.

Lefranc and Lugeon Permeability Testing in Stoke-on-Trent — Technical reference — Stoke-on-Trent

Local geotechnical context

The Coal Authority records indicate over 80 mine entries within the Stoke-on-Trent administrative area, many backfilled with colliery spoil that acts as a preferential pathway for groundwater. Where a site investigation programme does not include in-situ permeability testing, there is a genuine risk of mischaracterising the hydraulic regime beneath proposed foundations. Elevated pore-water pressures migrating through unsealed shafts or fractured sandstone can reduce effective stress beneath a footing by 30 to 40 percent, triggering differential settlement that manifests years after construction. In the Etruria Formation clays, low-permeability lenses create confined conditions where even modest excavation triggers base heave if the upward hydraulic gradient exceeds the critical value. BS EN 1997-2:2007, Section 4.4.1, explicitly requires that permeability be measured under field conditions for Geotechnical Category 2 and 3 structures where groundwater control is safety-critical. A well-planned Lefranc or Lugeon test programme—spaced to capture both vertical and lateral variability—provides the data needed to calibrate a seepage model and size dewatering systems with confidence.

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Reference standards

BS 5930:2015+A1:2020, BS EN ISO 22282-2:2012 (Lefranc), BS EN ISO 22282-3:2012 (Lugeon), Eurocode 7 – BS EN 1997-2:2007

Technical data

Parameter	Typical value
Test methods offered	Lefranc (variable/fixed head), Lugeon (packer test)
Applicable ground conditions	Soil, weathered rock, fractured bedrock
Hydraulic conductivity range	1×10⁻⁹ to 1×10⁻² m/s (combined methods)
Test standard	BS 5930:2015+A1:2020, BS EN ISO 22282-2/3
Typical test depth	2 m to 80 m, depending on borehole diameter
Reporting output	K value (m/s), Lugeon units, transmissivity estimate
Packer type	Single or double pneumatic packer, wireline-deployed

Frequently asked questions

What permeability range can the Lefranc method reliably measure?

The Lefranc method is effective for soils and weak rock with hydraulic conductivities between about 1×10⁻⁷ and 1×10⁻³ m/s. In finer materials below this range, test duration becomes impractical; in coarse gravels above it, the flow rate may exceed the water supply capacity, in which case a Lugeon test or a pumping test may be more appropriate.

How many test intervals are typically needed on a Stoke-on-Trent site?

For a medium-sized development on Coal Measures strata, three to five Lugeon test intervals distributed across the borehole depth are common, supplemented by Lefranc tests in the overlying glacial till if shallow groundwater control is required. The exact number depends on the stratigraphic complexity and the presence of old mine workings, which can create highly localised permeability contrasts that a single test point will miss.

What does a field permeability test cost in the Stoke-on-Trent area?

A single Lefranc or Lugeon test in the Stoke-on-Trent area typically falls between £570 and £760, depending on depth, access conditions, and whether it is combined with an existing borehole programme or requires a standalone mobilisation.

Can a Lugeon test also indicate grout take before ground treatment?

Yes, the Lugeon test is often used as a pre-grouting assessment tool. By recording flow at different pressure stages, the engineer can identify whether the rock mass will accept cementitious grout under low pressure or whether fracture dilation occurs. This information directly informs the grout mix design and injection strategy for shaft stabilisation or cut-off works.

Location and service area

We serve projects in Stoke-on-Trent and surrounding areas.

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