In Situ Permeability Testing (Lefranc & Lugeon) in Belfast

A lab permeameter test measures the hydraulic conductivity of a small, intact soil specimen, typically 75 mm or 100 mm in diameter, under controlled laboratory conditions. A Lefranc test measures the permeability of the ground in situ, over a test section that can span half a metre or more, capturing the influence of fissures, gravel partings, and macrostructure that a small sample will miss. In Belfast's heterogeneous till, the in situ value is routinely one to two orders of magnitude higher than the lab value, which is why BS 5930 recommends field testing for any excavation where groundwater control is critical.

It depends on the site stratigraphy and the size of the development, but as a rule of thumb we recommend at least one test per distinct hydrogeological unit per borehole. For a mid-rise basement excavation on a 0.2-hectare city-centre plot, that typically translates to three to five Lefranc tests at varying depths within the till and one Lugeon test where the borehole penetrates the underlying mudstone. The key is to capture vertical variability—running a single test at the base of a single borehole tells you almost nothing about what is happening 3 metres higher up in a sand lens.

A single Lefranc variable-head test typically takes between 45 and 90 minutes once the borehole has been advanced to the test depth, including packer inflation, the water level change cycle, and the recovery monitoring period. A full five-stage Lugeon test in rock usually runs for about two hours per test interval. We always advise programming these tests into the drilling schedule from the outset rather than treating them as an add-on, because the rig needs to remain over the borehole with the test assembly in place until the data stabilises.

For a Lefranc variable-head test within a soil borehole, budget between £430 and £620 per test interval depending on depth and access conditions. A full five-stage Lugeon packer test in rock typically falls in the £550 to £870 range per interval. These figures cover the on-site testing, data logging, and the engineering report with calculated k values and Lugeon unit plots. The final cost depends on the number of tests, the drilling setup, and whether we are mobilising to a standalone investigation or adding permeability testing to an existing ground investigation programme.