In-Situ Permeability Testing (Lefranc & Lugeon) in Ashford

A few months ago we were called out to a site just north of the M20 where a developer was struggling with persistent groundwater in a cut-and-cover basement. Two trial pits had filled overnight, and the contractor’s sump pump estimate was off by a factor of three. That scenario—underestimating mass permeability in the Ashford Beds—is common across the borough. The variable interface between the Hythe Formation limestone and the overlying Atherfield Clay creates preferential flow paths that only a properly executed in-situ test can quantify. Whether you need a Lefranc test in a single borehole interval for a soakaway design under BRE Digest 365, or a full Lugeon packer sequence in fractured ragstone for a tunnel alignment beneath the Stour valley, the field data drives every subsequent geotechnical decision. Complement the permeability profile with a CPT investigation to map the stratigraphic boundaries before setting packer positions.

A single lab permeability test on a remoulded sample can miss the fracture flow that governs site drainage in Ashford's Hythe Beds—field testing is the only way to capture it.

Scope of work

Ashford’s geology runs in roughly east-west bands: the Lower Greensand Group outcrops north of the town, the Gault and Folkestone Beds sit beneath the urban centre, and the Weald Clay dominates the southern parishes toward Hamstreet and Woodchurch. In the Greensand, hydraulic conductivities measured by Lugeon tests in open boreholes frequently exceed 1×10⁻⁵ m/s where the sandstone is poorly cemented, while the Weald Clay matrix often returns Lefranc values below 1×10⁻⁸ m/s but with secondary permeability through silt partings and desiccation cracks that only a multi-stage test reveals. The shallow water table across the Stour floodplain—typically 1.2 to 2.8 metres below ground level in winter—means constant-head Lefranc tests in gravels require careful flow-rate measurement to avoid under-reading when the borehole filter zone is not adequately developed.
Our test rigs carry calibrated digital flow meters and pressure transducers logged at 1-second intervals, satisfying the data-density requirements of BS 5930:2015+A1:2020 and BS EN ISO 22282-2 for variable-head and constant-head procedures. For Lugeon tests in rock, we run five pressure stages per interval following Houlsby’s method, plotting Q vs. effective pressure to interpret flow regime—laminar, turbulent, dilation, or wash-out—directly from the pressure-flow curve rather than relying on a single-point estimate. The resulting permeability values feed directly into Plaxis or SEEP/W models for dewatering system design, avoiding the common mistake of applying textbook values that ignore Ashford’s local heterogeneity.

Area-specific notes

In Ashford, the biggest field permeability risk we see is misinterpreting a single-interval result as representative of the full profile. The Hythe Beds limestone contains solution-enhanced fractures and occasional karstic voids—particularly near the scarp slope south of Kingsnorth—where a Lugeon test can show apparent permeabilities exceeding 50 Lugeon units in one 3-metre stage and less than 3 in the next. If the designer uses only the high value, the dewatering system is oversized and unnecessarily expensive; if only the low value, excavations flood. The second risk concerns Lefranc tests in the River Stour gravels: fine sediment migration during drilling can smear the borehole wall and reduce measured permeability by an order of magnitude. We mitigate this with careful development—surging and pumping the test interval before measurement—and by running falling-head tests in addition to constant-head to cross-check the result. A properly executed test sequence, with packer seals verified at each stage and groundwater levels monitored in a nearby observation well, eliminates the ambiguity that leads to costly change orders during earthworks.

Standards used

BS 5930:2015+A1:2020 – Code of practice for ground investigations, BS EN ISO 22282-2:2012 – Geotechnical investigation and testing – Geohydraulic testing – Water permeability tests in a borehole using open systems, BRE Digest 365 – Soakaway design, Eurocode 7 – BS EN 1997-2:2007 – Ground investigation and testing

Typical values

Parameter	Typical value
Test methods	Lefranc (variable/constant head); Lugeon (packer, 5-stage Houlsby)
Applicable formations	Lefranc: soil, weathered rock, gravels; Lugeon: fractured ragstone, chalk, sandstone
Standards compliance	BS 5930:2015+A1:2020, BS EN ISO 22282-2:2012, BRE Digest 365
Measurement range	Lefranc: 1×10⁻⁹ to 1×10⁻⁴ m/s; Lugeon: 0.1 to 100 Lugeon units
Borehole diameter requirement	Lefranc: ≥100 mm (cased); Lugeon: ≥76 mm (NX or larger)
Data output	Hydraulic conductivity (k, m/s), Lugeon value, flow regime classification, transmissivity
Typical test depth in Ashford	3–35 m bgl depending on formation and project requirement
Reporting	Graphical Q vs. P curves, time-drawdown plots, interpretive report with dewatering recommendations

Frequently asked questions

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

For a standard Lefranc test at a single depth interval in soil, budget between £520 and £650 including borehole preparation, test execution, and reporting. Lugeon packer tests in rock typically run £680 to £850 per interval due to the additional equipment, packer setup, and multi-stage pressure sequence. The final cost depends on access conditions, depth, and the number of intervals tested—mobilisation across Ashford is straightforward from our Kent base, so travel costs are minimal.

When is a Lugeon test required instead of a Lefranc test?

A Lugeon test is the appropriate method when you are investigating rock mass permeability, particularly in the Hythe Beds limestone, ragstone, or chalk that underlies much of Ashford. The packer system isolates a discrete interval of open borehole, allowing you to measure fracture flow under controlled pressure stages. A Lefranc test, by contrast, is designed for soil, weathered rock, or gravel where the borehole is cased and a filter section is installed. If your borehole log shows RQD above 50% and you need to assess grout take for a tunnel or dam, specify Lugeon. For a soakaway in the Stour gravels or a basement in the Gault Clay, Lefranc is the correct choice.

How many test intervals do you recommend for a site investigation in Ashford?

It depends entirely on the geological profile. In the Weald Clay south of Ashford, one or two Lefranc tests at soakaway depth may suffice if the clay is homogeneous. However, in the Lower Greensand north of the M20, where the Hythe Beds can alternate between tightly cemented and highly fractured zones within a few metres vertically, we typically recommend Lugeon tests at 3-metre intervals through the full rock section. For the mixed soil-over-rock profiles common along the A2070 corridor, a combination approach works best: Lefranc in the overlying gravels and weathered zone, switching to Lugeon once competent rock is encountered. We always review existing borehole logs and the groundwater monitoring record before finalising the test programme.

In-Situ Permeability Testing (Lefranc & Lugeon) in Ashford

Our service areas

Slopes & Walls

Excavations

Roadway

Scope of work

Area-specific notes

Need a geotechnical assessment?

Standards used

Typical values

Frequently asked questions

Location and service area