In Situ Permeability Testing (Lefranc & Lugeon) in Slough

Slough’s transformation from a quiet market town into the UK’s largest trading estate was no accident—the underlying geology had a say in it. The patchwork of London Clay, Langley Silt, and the Taplow Gravels of the Thames Basin creates a subsurface where water doesn’t move in predictable ways. On a site near the Grand Union Canal last winter, standing water in trial pits prompted the structural engineer to request direct measurement of ground permeability before finalising the drainage design. We ran a series of in situ permeability tests using the Lefranc method in the gravel lenses and Lugeon tests in the weathered chalk beneath them. Getting the hydraulic conductivity right meant the difference between a functional SUDS basin and a permanently waterlogged car park. When borehole logs suggest one thing but site conditions tell another story, a test pit investigation often reveals the true variability of the strata before committing to a full permeability testing programme.

A single Lugeon value of 10 Lu in fractured chalk can mean the difference between a straightforward sump pump and a full-scale wellpoint system.

Our service areas

Scope of work

A common scenario in Slough is a mid-rise commercial block where the basement excavation cuts into the interface between the River Terrace Deposits and the underlying London Clay. The contractor needs to know how much groundwater to expect during construction, and whether a dewatering system is viable. A Lefranc test at variable depths within a borehole gives us point measurements of hydraulic conductivity in the silty sand layers, while a Lugeon test in the chalk bedrock quantifies fracture flow under pressure. The data feeds directly into the groundwater control plan. We run these tests in accordance with BS 5930:2015, typically using a constant-head or falling-head setup depending on the soil type. For larger sites, combining the permeability profile with a Cone Penetration Test provides a continuous stratigraphic log that helps correlate the hydraulic boundaries across the footprint, reducing the number of boreholes needed and tightening the geotechnical model.

In Situ Permeability Testing (Lefranc & Lugeon) in Slough — Technical reference — Slough

Local geotechnical context

The Langley Silt complex that underlies much of Slough is notorious for its anisotropy—vertical permeability can be an order of magnitude lower than horizontal permeability. Relying on lab tests on small-diameter samples from a borehole core often misses this, leading to overly optimistic dewatering estimates. BS EN 1997-2 (Eurocode 7) explicitly requires that the number of in situ permeability tests be sufficient to characterise the mass permeability, not just the matrix permeability of a 100 mm specimen. On one industrial plot near the M4, we recorded a Lugeon value exceeding 25 Lu in a chalk fracture zone at 8 metres depth, directly below a proposed attenuation tank. The design had to incorporate an impermeable cut-off to prevent short-circuiting. Skipping the field test would have meant a non-compliant drainage system and a very expensive retrofit after the first wet winter.

Need a geotechnical assessment?

Reply within 24h.

Email: contact@geotechnical-engineering1.com

Relevant standards

BS 5930:2015 + A1:2020 Code of practice for ground investigations, BS EN ISO 22282:2012 Geotechnical investigation and testing – Geohydraulic testing, Eurocode 7: BS EN 1997-2 (Geotechnical design – Ground investigation and testing), CIRIA C750 – Groundwater control: design and practice

Reference parameters

Parameter	Typical value
Test methods	Lefranc (constant/falling head), Lugeon (packer test)
Applicable standard	BS 5930:2015 + A1:2020, BS EN ISO 22282
Soil types tested	Gravels, silts, sands, weathered/fractured chalk
Borehole diameter	76 mm to 150 mm (depending on strata)
Test interval (Lugeon)	Typically 3 m to 5 m in rock
Reporting parameter	Hydraulic conductivity k (m/s), Lugeon units (Lu)
Accreditation	UKAS-accredited laboratory for permeability verification

Common questions

What’s the difference between a Lefranc test and a Lugeon test, and which one do I need for my Slough site?

A Lefranc test measures hydraulic conductivity in soils—gravels, sands, silts—using a simple borehole cavity with or without a filter. It’s the go-to method when you’re dealing with the River Terrace Deposits or Langley Silt around Slough. A Lugeon test is a packer test designed for rock, specifically the chalk bedrock that sits beneath the superficial deposits here. It injects water under pressure into an isolated section of the borehole to quantify fracture flow. If your foundation or basement hits chalk, you’ll likely need both: Lefranc for the overburden, Lugeon for the bedrock.

How much does a Lefranc or Lugeon permeability test cost in Slough?

For a Lefranc test campaign in the Slough area, budget between £440 and £860 per test, depending on borehole depth, the number of test intervals, and whether you need constant-head or falling-head configuration. Lugeon tests typically fall in the upper end of that range due to the extra time for packer setup and the five-stage pressure cycle. The final cost depends on site access, traffic management, and how quickly the drilling crew can progress through the ground conditions on the day.

Why can’t I just use lab permeability tests on core samples instead of going to the trouble of a field test?

Lab tests on a 100 mm core give you the matrix permeability of that tiny piece of soil or rock, but they completely miss fractures, fissures, and macro-scale fabric that control how water actually moves through the ground. The chalk under Slough is heavily fractured in places, and the Langley Silt has a strong horizontal layering that a small vertical specimen can’t capture. Eurocode 7 (BS EN 1997-2) is clear on this: the number of field tests must be enough to characterise the mass permeability. A lab value alone is rarely defensible for a dewatering or drainage design.

Location and service area

We serve projects in Slough and surrounding areas.

View larger map