Plymouth's geology demands respect. The city straddles Middle Devonian limestone and slate, cut through by tidal estuaries like the Plym and Tamar, with water tables rising and falling up to 5 metres between winter and summer. When a developer on the eastern edge of the city, near the 50-metre contour line south of Dartmoor, proposed a three-storey apartment block with an underground car park last year, the borehole logs showed weathered slate with closely spaced joints. That is where a single Lefranc test in a borehole at 6 metres depth revealed a permeability coefficient ten times higher than the desk study assumed. We see this pattern across Plymouth: rockhead variability within 30 metres laterally, and fracture flow dominating the hydrogeological regime. A field permeability test is not a box-ticking exercise here. Without it, dewatering plans fail, cut-off walls leak, and basements flood. Our team runs these tests using variable-head and constant-head methods under BS 5930 and BS EN 1997-2, reporting values in m/s with full field logs. The test pits we excavate nearby often confirm the fracture spacing we measure downhole.
A Lugeon value above 10 in Plymouth's transition zones between limestone and slate means dewatering must be designed for fracture flow, not porous media.
Methodology applied in Plymouth
Typical technical challenges in Plymouth
We were called to a site off Embankment Road last autumn where a contractor had skipped in-situ permeability testing and relied on laboratory permeameter values from remoulded samples. The ground was estuarine alluvium overlying fractured limestone at 4.5 metres depth. The lab data suggested a permeability of 1 × 10⁻⁸ m/s, which the contractor used to size a sump-and-pump system for a 3-metre deep excavation. When they hit the rockhead, inflow exceeded 12 litres per second within two hours, flooding the excavation and delaying the project by six weeks. A Lugeon test in the same rock would have shown values above 25 Lu immediately. The cost of that single missing test was orders of magnitude higher than the test budget. Plymouth's coastal and estuarine settings, combined with karstic features in the Devonian limestone, create precisely these conditions. Dewatering assumptions based on soil classification alone are unreliable here. The CPT testing we perform in the overlying soils gives excellent stratigraphic control, but only a downhole permeability test in the rock provides the hydraulic conductivity data needed for a defensible dewatering and waterproofing strategy.
Our services
Our Plymouth-based field permeability testing is integrated with complementary ground investigation services, ensuring consistent data across the site. We execute all work through our UKAS-accredited laboratory protocols.
Lefranc testing in boreholes
Variable-head and constant-head permeability tests in soil and weathered rock, isolating test sections with pneumatic packers. Data reported as k (m/s) with full time-decay curves.
Lugeon testing in rock
Multi-stage pressure testing in fractured limestone, slate, and granite to assess rock mass permeability and groutability. Standard five-stage cycle with real-time flow vs. pressure plots.
Combined SPT and permeability profiling
Borehole drilling with SPT N-value measurement and targeted permeability tests at depths of hydrogeological interest, reducing mobilisation costs.
Dewatering feasibility assessment
Interpretation of field permeability data to estimate inflow rates, well spacing, and pump sizing for temporary and permanent dewatering systems in Plymouth's variable ground conditions.
Quick answers
What is the difference between a Lefranc test and a Lugeon test?
A Lefranc test measures permeability in soil or weathered rock using a single borehole section isolated by a packer, typically with falling or constant water head. It is suited to unconsolidated materials. A Lugeon test is designed for fractured rock: we inject water under pressure in up to five stages and measure the flow rate in litres per minute per metre of borehole. The Lugeon value indicates both permeability and the hydraulic behaviour of fractures (dilation, wash-out, or clogging). In Plymouth, we use Lefranc in the overlying soils and weathered zone, then switch to Lugeon once competent limestone or slate is encountered.
How much does a field permeability test cost in Plymouth?
The cost for a Lefranc or Lugeon test in Plymouth typically ranges from £480 to £910 per test section, depending on borehole depth, number of pressure stages, and whether the test is combined with an existing drilling programme. A standalone mobilisation for a single test will be at the upper end due to crew and equipment transport. We recommend bundling permeability tests with borehole drilling and SPT sampling to optimise the day rate.
Which British Standards apply to field permeability testing?
Our testing follows BS 5930:2015 + A1:2020 for ground investigation practice and BS EN 1997-2:2007 (Eurocode 7 Part 2) for field testing procedures. For water permeability testing in boreholes specifically, we apply BS EN ISO 22282-2:2012. Reports include the test method, raw data curves, calculated permeability in m/s, and Lugeon values where applicable, all traceable to these standards.