Bedford sits on a tricky geological boundary. The Oxford Clay Formation dominates the urban centre and river valley, but pockets of glacial till and Ouse Valley gravels create sharp contrasts in drainage behaviour. A borehole log might show stiff grey clay at three metres, then hit a water-bearing gravel lens at five metres without warning. That lens changes the dewatering plan overnight. A Lefranc test run inside a borehole at discrete depths catches this before the digger arrives. For fractured limestone or sandstone encountered in the northern parishes near the Colmworth ridge, the Lugeon packer method gives engineers the joint permeability values that a simple falling-head test cannot capture. The team runs both methods under BS 5930 procedures, with no guesswork on hydraulic conductivity.
A single Lefranc test at the right depth saves more time than three weeks of dewatering trial and error on Oxford Clay.
Methodology applied in Bedford
Risks and considerations in Bedford
The packer assembly goes down the borehole on a string of NQ rods. Two inflatable rubber packers seal off a discrete section of the borehole wall, isolating the test interval from the rest of the hole. Water is injected at controlled pressure via a flow meter and pressure gauge at the surface. In Bedford's urban sites, the rig must operate from a fenced compound with limited headroom, so the packer system is set up on a compact tripod winch rather than a full-size drilling rig. If the packer seal fails against an irregular borehole wall, the test is invalid and the interval must be redrilled. This is common in the weathered zone of the Oxford Clay where desiccation cracks create bypass paths. The engineer watches the flow-versus-pressure curve in real time, aborting any stage that shows non-linear response indicative of hydraulic fracturing.
Our services
The permeability test programme for a Bedford site is designed around the specific geological sequence encountered in the borehole log. This is not a one-size-fits-all service.
Lefranc Variable-Head Test
Suitable for silts, fine sands, and soft clays where the water level recovers slowly. We monitor the rise of the water column over time, calculating k from the Hvorslev shape factor for the test cavity geometry.
Lefranc Constant-Head Test
Applied in permeable granular soils such as the Ouse Valley gravels. A steady flow rate is maintained into the test interval while the head is held constant; k is derived from the equilibrium flow equation.
Lugeon Packer Test
For fractured Cornbrash limestone or sandstone stringers north of Bedford. Five pressure stages identify the flow regime (laminar, turbulent, dilation, or wash-out) and provide a Lugeon value for grouting or drainage design.
Quick answers
How long does a Lefranc or Lugeon test take on a Bedford site?
A single Lefranc test in soil typically completes within 90 minutes, including borehole preparation and stabilisation. A full five-stage Lugeon test in rock requires approximately two to three hours per test interval. Site setup and demobilisation add half a day. Multiple intervals can be tested in one day if the borehole is logged and stable.
What does a field permeability test cost in Bedford?
The price range for a Lefranc or Lugeon permeability test in Bedford is £460 to £800 per test interval, depending on depth, access conditions, and whether the test is run inside an existing borehole or requires a new drilling mobilisation. A full day of testing with multiple intervals is priced by programme, not per test.
Which test method is right for Oxford Clay?
Most of Bedford's Oxford Clay is low-permeability stiff clay where a Lefranc variable-head test works well. However, the weathered upper zone often contains silt partings and desiccation fissures that increase permeability locally. We recommend running tests at two depths — one in the weathered crust and one in intact clay — to capture the contrast. If limestone bands are present, a Lugeon test may be more appropriate.