Percolation test for a foul drainage field

A percolation test drainage field result is one number, called the Vp value, expressed in seconds per millimetre. It tells you how long it takes water to fall through the soil at the depth the distribution pipes will sit, and the drainage field area is calculated directly from it. The test also answers a blunter question before it answers that one: whether a drainage field is acceptable on the site at all. A Vp below 15 or above 100 and the answer is no, whatever the layout.

Do the test before you buy the land if you can. It is the least costly piece of due diligence on an unsewered plot and the one most often skipped.

What the test measures

The Vp value is the average time in seconds for water to drop 1 mm in a trial hole dug to the depth of the proposed drainage field pipes. Low numbers mean free-draining ground. High numbers mean the soil holds water.

Both extremes are a problem, for opposite reasons. In very permeable ground, effluent passes through the soil too quickly for treatment to occur and may reach groundwater, which is why a lower limit exists. In very slow ground, the field cannot dispose of the daily flow and effluent surfaces, which is the smell and the boggy patch every rural planning officer has seen.

Two other site facts carry equal weight and are not measured by the percolation test. The depth to the highest seasonal groundwater level, which has to leave adequate separation below the base of the field. And the setback distances, which can rule out a field on a small plot even where the soil is ideal.

So a complete assessment is three things: a percolation test, a groundwater assessment, and a siting plan with the setbacks drawn.

How the test is carried out

The method in the Approved Document on drainage and waste disposal is prescriptive, and departures from it get the results rejected.

1. Dig at least two trial holes, located where the field is proposed, each 300 mm square and taken to a depth around 300 mm below the proposed invert of the distribution pipe.
2. Fill each hole with water to a depth of at least 300 mm and leave it to seep away overnight. This pre-soak matters, because a dry soil gives an optimistic first reading.
3. The next day, refill to 300 mm and record the time taken for the water to fall from 75 per cent full to 25 per cent full, which is from 225 mm to 75 mm.
4. Divide that time in seconds by 150 to give the Vp for that run.
5. Repeat at least three times in each hole and average the results across all runs and all holes.

Conditions matter. Do not test in frost, during or immediately after heavy rain, or in a prolonged dry spell, because each of those biases the result. The honest approach on a marginal site is to test in the wetter part of the year, since that is the condition the field has to work in.

Record everything: hole positions on a plan with dimensions from a fixed point, depths, the date and weather, and the individual timings for each run rather than only the average. A set of raw timings is what makes a report credible.

Calculating the field area from the result

The area of the drainage field follows from the Vp value and the population the system serves. The Approved Document gives a formula in which the area in square metres is the population multiplied by the Vp value multiplied by a coefficient, and the coefficient differs according to what is discharged.

Septic tank effluent, which is settled but untreated, uses the higher coefficient of 0.25. Treated effluent from a package treatment plant uses a lower coefficient of 0.20, because the effluent places less load on the soil. That difference is one of the practical arguments for a treatment plant on a tight site, alongside the discharge options it opens up. Take the current coefficients from the Approved Document rather than from an older note.

Population is derived from the number of bedrooms using the method the guidance sets out, with a minimum figure for a single dwelling. Do not use the number of people who will live there on completion, because the system has to serve the house rather than the household.

The resulting area is the area of the trench base, not the footprint of the garden it sits in. Trenches need spacing between them, a distribution arrangement that loads each one evenly, and space for access, so the land required is appreciably larger than the calculated figure. Where the shape of the plot forces narrow trenches, the arithmetic can work on paper and fail on the drawing.

Setback distances that constrain siting

These are what rule out most small plots, and they are easier to check than the soil.

Draw them as circles and arcs on the site plan rather than listing them, because the overlap is what tells the story. On a half acre plot with a stream along one boundary, a house in the middle and a neighbour's borehole beyond the hedge, the available area for a field is often a single awkward strip.

Source Protection Zones add a further layer. A discharge to ground within the innermost zone is generally not acceptable under the general binding rules and needs a permit from the Environment Agency, which may be refused. Check the position on the published groundwater data before designing anything, and treat a zone 1 location as a reason to look at a different disposal route entirely.

The general binding rules also set limits on the volume discharged and require the system to meet the relevant product standard and be installed as the manufacturer specifies. Where you cannot satisfy every rule, a permit is required rather than optional, as the note on foul drainage on unsewered sites explains.

When a field is unacceptable

Four situations where the answer is no, and the sooner the better.

Vp below 15, meaning the ground drains too fast for treatment to occur in the soil. Vp above 100, meaning the ground cannot accept the flow. Groundwater within a metre of the proposed base of the field at its seasonal high, which on clay sites with perched water is more common than people expect. And a plot where the setbacks leave no area large enough once the calculated field size is laid out.

The alternatives, in rough order of how often they work: a package treatment plant discharging to a watercourse where one is available and the general binding rules can be met, which removes the drainage field question but introduces a discharge point to negotiate. A treatment plant with a smaller field, taking advantage of the lower coefficient. A mound or raised system where groundwater is the constraint, which needs specific design. Or a sealed cesspool, which is lawful but expensive to empty and generally resisted by authorities as a last resort.

The comparison between a septic tank, a package plant and the rules each has to satisfy is set out in the note on package treatment plants and the general binding rules.

An illustrative calculation for a single dwelling

The following is illustrative only and not taken from a real site. Take a new four bedroom house on an unsewered plot, with a population figure of six derived from the bedroom count in the way the guidance sets out.

Two trial holes are dug to 900 mm, pre-soaked overnight, and three runs in each give timings that average 6,000 seconds for the water to fall from 225 mm to 75 mm. Divide by 150 and the Vp is 40 seconds per millimetre, which sits comfortably within the acceptable range.

With a septic tank, the field area is 6 multiplied by 40 multiplied by 0.25, which is 60 m². With a package treatment plant, the coefficient drops to 0.20 and the area becomes 48 m².

Twelve square metres does not sound like much until you lay it out. At a trench base width of 500 mm, 60 m² needs 120 m of trench and 48 m² needs 96 m, and with the spacing required between trenches that is the difference between five runs and four across the available strip. On a constrained plot, that is the difference between a scheme that works and one that does not.

Evidence to submit with an application

Planning and Building Regulations want different things, and it pays to prepare for both at once.

For the planning application, a foul drainage assessment setting out the proposed method, why mains connection is not available, the percolation test results, the field area calculation, the siting plan with setbacks drawn, and confirmation of how the general binding rules are met or which permit is being sought. A Foul Sewage Assessment is the document that holds this, and where surface water is also being dealt with on site the two belong alongside a Drainage Strategy using the same levels and the same datum.

Then the supporting material: trial hole logs with photographs, raw timings for every run, the date and weather conditions, a groundwater assessment covering the seasonal high, and the product standard and model of the proposed tank or plant.

The omission that causes the most resubmissions is the groundwater assessment. A percolation test carried out in August on a clay site tells you nothing about where the water table sits in February, and an authority that has seen a failed field on the next plot will ask.

This is general guidance, not legal advice; check the position with your local planning authority, the Environment Agency or a qualified professional before relying on it. To see which drainage documents a rural plot is likely to need, enter the postcode on PlanScope.

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