The ground under Bangor varies dramatically from the compact glacial tills of the Penrhosgarnedd side to the fractured Ordovician shales and slates exposed near the Menai Strait, and that contrast defines how water moves through the subsurface. A site on the elevated terraces around Upper Bangor can show low-conductivity stony clay, while a plot closer to the harbour may sit on more permeable weathered bedrock with open joints. When a permanent excavation or basement is proposed, assuming uniform drainage across the city is a costly mistake—our field permeability testing programme quantifies these differences directly. The Lefranc method recovers localised hydraulic conductivity in soil and very weak rock, typically within boreholes advanced by spt-drilling, while the Lugeon test characterises fracture flow in the stronger rock mass that underlies much of central Bangor. The data feeds directly into dewatering system sizing, cut-off wall design, and even foundation drainage compliance with BS EN 1997-2.
A single Lugeon test in fractured Bangor bedrock can reveal a conductivity three orders of magnitude higher than the intact rock core suggests—this is the difference between a dry excavation and a flooded one.
Regional considerations
We reviewed a basement excavation off Holyhead Road where the contractor skipped in-situ permeability testing because the borehole logs showed ‘stiff clay.’ Two metres of that clay were actually a thin lacustrine deposit overlying a rubbly, water-bearing layer—and within 48 hours of cutting into it, the excavation base heaved and flooded. In Bangor, the risk isn’t always a uniform aquifer; it’s the perched water tables within glacial sequences and the unpredictable connectivity of fractures in the Bangor Volcanic Group. A Lefranc test isolates specific horizons, revealing these lenses before they become a problem. For rock socketed piles or deep utilities near the shoreline, the Lugeon test identifies zones where saline intrusion might travel, which has implications for concrete durability and long-term reinforcement integrity. BS EN 1997-1 requires that the ground model account for groundwater conditions—without field permeability data, that model is an assumption, not a design basis.
Q&A
What is the difference between the Lefranc and Lugeon test, and which one does my Bangor site need?
The Lefranc test measures hydraulic conductivity in soil or very weak rock using a simple cavity or screened section within a borehole, typically under a falling or constant head. It is suited to the glacial till and alluvial deposits common across lower Bangor and the coastal fringe. The Lugeon test is a packer-isolated water pressure test in competent rock, used to quantify fracture conductivity in the Ordovician shales, slates, and volcanics that form the ridge through Upper Bangor. If your foundation will bear on rock, or if you are designing a deep excavation with rock faces, the Lugeon test is the appropriate tool. Most projects with variable ground conditions benefit from a combination of both, sequenced as the borehole advances through soil into bedrock.
How long does a field permeability test take on site in Bangor?
A single Lefranc test can often be completed within one to two hours once the borehole is prepared to the target depth. Lugeon testing requires more time—typically three to five hours per test interval—because of the five-stage pressure sequence and the need to observe pressure decay and flow stabilisation at each step. A full day on site usually allows for a mix of both test types across several boreholes, provided the rig and crew are working efficiently. We always schedule an additional contingency for Bangor’s weather, as heavy rain can temporarily alter shallow groundwater levels and influence test interpretation.
What does in-situ permeability testing cost for a typical project in Bangor?
For a standard investigation involving two to three Lefranc tests or a single Lugeon test within an existing borehole programme, costs typically fall between £530 and £900, depending on depth, access constraints, and the number of test stages required. A full day of mixed testing with multiple packer setups and piezometer installations will sit at the upper end of that range or beyond. We provide a fixed-price quotation after reviewing the borehole schedule and site geology so there are no surprises.
Can the Lugeon test tell me if I need to grout the rock beneath my foundation?
Yes, directly. The Lugeon value quantifies the water take of the rock mass under pressure, and a value above 3–5 Lugeon units typically indicates open, interconnected fractures that may require grouting to reduce permeability and improve bearing characteristics. By running the test before and after grouting, you can verify the effectiveness of the treatment. In Bangor’s fractured slates, we often see pre-grouting values exceeding 10 Lugeons that drop to below 1 after systematic injection—this objective evidence is invaluable for approving foundation construction and satisfying building control requirements.
