A basement excavation in Southsea hit groundwater two metres earlier than the desk study predicted. The contractor had to stop work and rethink the dewatering plan. That situation is common across Portsmouth. The city sits on varied Quaternary deposits—raised beaches, brickearth, and the chalk of Portsdown Hill. Standard lab tests on disturbed samples rarely capture the true mass permeability. We run field tests that measure the ground's actual hydraulic conductivity. Lefranc tests handle the granular soils and weathered chalk near the surface. Lugeon tests apply to the deeper chalk where fractures control the flow. BS 5930 and Eurocode 7 require this data for safe foundation design. We also combine these tests with CPT logging to correlate permeability with tip resistance and sleeve friction, and with grain size analysis to validate the field results against particle-size distributions.
A single Lugeon test in fractured chalk tells you more about groundwater inflow than a hundred lab permeability tests on intact core.
Process overview
Local context
Portsdown Hill and the flat terrain of Portsea Island present two very different groundwater problems. On the hill, the chalk is fractured and highly permeable. Dewatering for a cutting or retaining wall can draw down water levels across a wide area, risking settlement of nearby roads and historic buildings. On the island, the superficial gravels and silts create layered aquifers. An unsealed borehole can connect a shallow perched water table with a deeper confined aquifer, causing internal erosion and ground collapse. We saw this happen near the dockyard, where a single poorly grouted instrumentation hole led to a sinkhole. A correctly executed Lefranc test, followed by proper bentonite sealing, eliminates that risk. Our team grouts every borehole to the surface. We also correlate the results with slope stability assessments when the works involve cuts in the chalk escarpment, ensuring the dewatering strategy does not compromise the long-term stability of the slope.
Reference standards
BS 5930:2015+A1:2020 – Code of practice for ground investigations, BS EN 1997-2:2007 – Eurocode 7: Ground investigation and testing, Houlsby method for Lugeon test interpretation
Additional services
Lefranc variable-head test
A falling-head test in a cased borehole section. Best suited for the River Terrace Deposits and weathered chalk found across the city. We measure the decay of water level with a dipped transducer, giving a direct k-value in metres per second.
Lugeon packer test in chalk
A constant-pressure injection test using a single or double pneumatic packer. Designed for the chalk of the White Chalk Subgroup. The test evaluates fracture flow and detects hydraulic jacking at elevated pressures. Essential data for tunnel and shaft design.
Typical parameters
Quick answers
How long does a single Lefranc or Lugeon test take on site?
A single Lefranc test in shallow granular soils typically takes one to two hours, assuming the borehole is already advanced and the standpipe installed. A full five-step Lugeon test in chalk takes around two to three hours per test interval, including packer inflation, saturation, and the five pressure stages. We can usually complete two to three Lugeon runs in a standard working day.
What is the typical cost of a field permeability test in Portsmouth?
The cost depends on depth, access conditions, and the number of tests. A single Lefranc test typically ranges from £550 to £750. A Lugeon test, which requires core drilling and packer equipment, ranges from £700 to £870 per test interval. Mobilisation and traffic management in restricted areas like Old Portsmouth may add a minor surcharge.
Which test is more appropriate for chalk ground in the Portsmouth area?
The Lugeon test is the definitive method for chalk. It applies pressure to the borehole wall and measures how the fracture network responds. The shape of the pressure-flow curve tells us if the fractures are clean, infilled, or dilating. A Lefranc test can give a bulk permeability value in structureless, putty chalk, but it will not capture the fracture-controlled flow regime that dominates the White Chalk Subgroup beneath Portsdown Hill.
Can you run permeability tests from a floating platform in the harbour?
Yes. We have performed Lefranc and Lugeon tests from jack-up barges and spud-leg pontoons for marine projects in Portsmouth Harbour and Langstone Harbour. The equipment is the same, but the borehole casing must be sealed against tidal fluctuation. We use a stilling well and a pressure transducer to eliminate wave-induced noise from the water level readings.