Drainage Field Testing in Farnham & Surrey.
Fitting a septic tank or sewage treatment plant? Before a drainage field can be designed, the ground has to prove it can take the treated effluent. We carry out the BS 6297 percolation test - trial holes, overnight pre-soak and timed runs - and report the Vp figure your installer and building control need.
What you get
- Full BS 6297 method: pre-soak plus three timed runs per hole
- Vp stated per hole, as a site average and worst case
- Weather and ground conditions recorded with the results
- A report your installer and building control can act on
Percolation Testing for Septic Tanks and Treatment Plants
Off the mains sewer, the treated water leaving a septic tank or package treatment plant is normally released into the ground through a drainage field - a network of perforated pipes laid in trenches. Building Regulations (Approved Document H2) and the Environment Agency's general binding rules expect that field to be sized from a measured percolation value, known as Vp, obtained by the BS 6297 method.
Clever Works carries out the full test on site: trial holes dug within the proposed field, a genuine overnight pre-soak, and three timed runs per hole the next day, each recorded against the clock. You receive the Vp for every hole, the site average and the worst case, so your installer or drainage engineer can size the field with confidence - and building control can see exactly how the figure was reached.
What Does the BS 6297 Test Involve?
Each trial hole is 300 mm square, dug to 300 mm below the proposed invert of the distribution pipe - so the test measures the soil the field will actually drain into, not the topsoil above it. The hole is filled to at least 300 mm and left to seep away overnight. That pre-soak matters: it saturates the surrounding ground so the timed runs are not flattered by dry soil.
The next day the hole is refilled and we time the fall from 75% to 25% full - a 150 mm drop at the standard fill. Dividing that time in seconds by the millimetres fallen gives Vp in seconds per millimetre, and the run is repeated twice more so the reported figure is an average, not a one-off. A minimum of two holes is tested; three is common practice on anything other than a small single dwelling.
Planning a rainwater soakaway rather than a foul drainage field? That is a different test with a different result: see our BRE365 percolation testing service.
What Counts as a Pass? The 12-100 s/mm Band
For a drainage field, the accepted percolation value sits between 12 and 100 seconds per millimetre. Faster than 12 and the effluent can pass through to groundwater before the soil has finished treating it; slower than 100 and the field simply will not drain, leading to ponding and backed-up plumbing. Our report states each hole's Vp against that band in plain language.
Timing also matters. Results taken during or immediately after prolonged wet weather, in frost, or in an unusually dry spell can misstate the true Vp - and can be rejected on that basis. We record the recent weather and ground conditions alongside every test, and we will tell you honestly if a visit should wait for the ground to be representative.
The Drainage Field Testing Process
The BS 6297 method runs over two visits. Here is what happens when you book with Clever Works:
Trial Holes Dug to Specification
We set out the holes within the proposed drainage field area and dig each one 300 mm square, to 300 mm below the proposed pipe invert, logging the soil we find.
Overnight Pre-Soak
Each hole is filled to at least 300 mm and left to seep away overnight, saturating the surrounding soil so the timed runs reflect how the field will behave in service.
Three Timed Runs Per Hole
The next day each hole is refilled three times, and the fall from 75% to 25% full is timed on each run. Vp is the time in seconds divided by the millimetres fallen, averaged across the runs.
Certified Report
You receive the hole logs, every timed run, the Vp per hole, the site average and worst case, the conditions record, and a plain statement against the 12-100 s/mm band.
Off-Mains Properties Around Farnham
The villages and rural lanes around Farnham, and across the Surrey and Hampshire border, hold plenty of homes that were never connected to a mains sewer. When an old septic tank is replaced or a treatment plant is installed, the drainage field usually has to be revisited too - and the local ground varies sharply, from free-draining greensand to heavy clay that will not pass a percolation test at all.
Because we test, design and dig, the result is never a dead end. If your ground sits outside the accepted band we can advise on what the site can support instead, and if it passes, our design and installation team can take the project from the measured Vp to a working field in the ground.
Frequently Asked Questions About Drainage Field Testing
How long does a BS6297 drainage field test take?
Two visits over two days. On day one we dig the trial holes and fill each one to at least 300 mm for the overnight pre-soak. On day two we carry out three timed refills per hole and record the fall from 75% to 25% full. The pre-soak cannot be skipped - it saturates the surrounding soil so the timed runs are not flattered by dry ground.
What is the difference between BS6297 and BRE365 testing?
They answer different questions. BRE365 measures how fast the ground can absorb rainwater, for sizing a surface water soakaway. BS6297 measures the percolation value (Vp) for a drainage field taking treated effluent from a septic tank or sewage treatment plant. Building control and the Environment Agency's general binding rules expect the foul-side test to follow the BS 6297 method, so a BRE365 result cannot be substituted.
What happens if my Vp result is outside the 12-100 band?
A Vp between 12 and 100 seconds per millimetre is the accepted range for a drainage field. Faster than 12 means effluent can reach groundwater before the soil has treated it; slower than 100 means the field will not drain. If your ground falls outside the band we set that out plainly in the report and can advise on the alternatives, such as a raised mound system, a different location for the field, or a discharge that needs its own permit.
How many trial holes does a BS6297 test need?
At least two, dug within the area of the proposed drainage field. On anything other than a small single dwelling three holes is common practice, and worth specifying if the field's final location is not yet fixed - it costs little extra on the day and avoids a repeat visit.
Book a BS6297 Test
Tell us about the property and the system you are planning, and we will book the two-visit test at the right moment for the ground conditions.
Why Clever Works?
- Full BS 6297 method, never a shortcut
- Every timed run in the report
- Conditions recorded with the results
- Test, design and install under one roof