Exeter’s built environment reflects a long negotiation with its underlying geology, from the Roman walls that skirt the city’s red sandstone bluffs to the Victorian sewers tunnelled through the Heavitree Breccia. The Permian strata that define the city’s topography also present a permeability puzzle that every foundation engineer working in the Exe Valley eventually confronts: fracture-dominated flow in the sandstone, interbedded lenses of sand and clay within the breccia, and a water table that rises rapidly during winter months along the floodplain. Our field permeability testing programme applies the Lefranc and Lugeon procedures to quantify this behaviour directly, providing the hydraulic conductivity values that desk studies can only estimate. When combined with a site investigation borehole that delivers intact core recovery through the breccia, the resulting dataset allows groundwater control measures and cut-off requirements to be designed with confidence rather than a generous factor of safety.
A Lugeon value of less than 3 in the Dawlish Sandstone below 20 metres depth often means grout takes will be negligible — an insight that saves weeks of speculative drilling on Exeter sites.
Scope of work in Exeter
Demonstration video
Critical ground factors in Exeter
A residential block excavation off Topsham Road reached formation level in February and stood in 400 mm of water for three days because the pre-construction SI relied on falling-head tests in a weathered breccia zone that yielded a k of 2 × 10⁻⁶ m/s, while the underlying fractured sandstone — never tested — was delivering 12 litres per minute through open joints. The contractor had to mobilise an emergency dewatering system, losing two weeks on programme and incurring costs that doubled the original groundworks budget. A Lefranc test in a CPT hole pushed through to the sandstone would have flagged the contrast in permeability across the lithological boundary. The second risk, less dramatic but equally expensive, arises when Lugeon tests are omitted ahead of grouting campaigns for shafts or retaining structures: without a baseline permeability profile, it is impossible to verify whether the injected grout has actually travelled into the fractures that need sealing, or simply filled the borehole and travelled along the packer annulus. BS 5930 recommends a minimum of three test intervals per borehole in heterogeneous rock, a threshold that becomes essential in Exeter’s breccia where matrix-supported and clast-supported fabrics can alternate over less than a metre of depth.
Our services
Our Exeter permeability testing programme covers the full range of in situ hydraulic conductivity measurement required under BS 5930 and Eurocode 7, configured for the specific challenges of the Permian sandstone and breccia formations:
Lugeon packer testing in rock
Five-stage pressure tests in NQ to PQ boreholes through the Dawlish Sandstone and fractured breccia units, with real-time pressure-flow monitoring and calculation of lugeon values and equivalent hydraulic conductivity for each isolated interval.
Lefranc permeability in soils and weathered rock
Constant-head and variable-head Lefranc tests performed in boreholes, trial pits or pushed CPT rods within the weathered breccia, alluvial gravels and terrace deposits of the Exe Valley, providing k values for temporary works dewatering design.
Permeability profiling for grouting verification
Pre-grouting baseline and post-grouting verification Lugeon testing for cut-off curtains, shaft seals and retaining wall underpinning, with direct comparison of lugeon values to assess grout penetration effectiveness in fracture networks.
Common questions
What is the difference between a Lefranc test and a Lugeon test?
The Lefranc test measures hydraulic conductivity in soil and weathered rock using a short test section (typically 0.5 to 2 metres) open to the borehole wall, with water injected under constant or falling head. It is suited to materials with k values above 1 × 10⁻⁷ m/s. The Lugeon test is designed for rock with fracture-controlled permeability, using a pneumatic packer to isolate a section of borehole (usually 3 to 5 metres) and injecting water at up to five pressure stages. The resulting lugeon value (1 Lu = 1 litre per metre per minute at 1 MPa overpressure) characterises the hydraulic conductivity of the rock mass and its sensitivity to pressure-induced fracture dilation.
How much does field permeability testing cost in Exeter?
Field permeability testing in Exeter typically ranges from £440 to £880 per test interval, depending on whether you need a Lefranc test in soil (lower end, simpler setup) or a full five-stage Lugeon test in rock with packer deployment and pressure monitoring (upper end). The total programme cost depends on the number of test intervals, depth, access conditions and whether the boreholes already exist or need to be drilled. We provide a fixed-price proposal once the investigation objectives and geological profile are defined.
How many Lugeon test stages are required per borehole?
BS 5930 recommends a minimum of three test intervals per borehole in heterogeneous rock, but for Exeter's breccia and sandstone formations, we typically recommend testing every 3 to 5 metres of borehole depth, with additional intervals placed at lithological boundaries (breccia-sandstone contact, weathering front, known fracture zones). Each interval undergoes five pressure stages (low-medium-high-medium-low) to assess whether permeability is pressure-dependent, which is critical for evaluating grouting feasibility and hydrofracturing risk.
Can Lefranc tests be performed in CPT holes?
Yes, Lefranc tests can be carried out in CPT holes pushed through the softer weathered breccia and alluvial deposits common across Exeter. The CPT rod is advanced to the test depth, the cone is retracted to expose a slotted section, and water is introduced under constant or falling head. This approach is faster than drilling a dedicated borehole and provides continuous stratigraphic context from the CPT data. However, it is limited to depths where CPT refusal has not yet occurred, typically above the competent sandstone surface.