Percolation testing and soakaway design for drainage

A percolation test measures how fast water soaks away into the ground at the depth your soakaway will sit, and the result is the single number that sizes the structure. Do the percolation test and soakaway design properly and the drainage strategy writes itself. Guess the infiltration rate from a soil map and the lead local flood authority will send the application back, usually after validation and usually at the worst possible moment.

What the test measures and where the method comes from

Infiltration rate is expressed in metres per second, and it describes how quickly water moves out of a pit into the surrounding soil. It is a property of the soil at that depth, on that day, in those groundwater conditions.

Two methods are in common use and they answer different questions. Soakaway design for surface water follows the method in the relevant BRE digest on soakaway design, which is the standard referenced by lead local flood authorities and by the SuDS Manual. Drainage fields for foul effluent from a septic tank follow the method set out in Approved Document H, which produces a different value used in a different formula.

Using the foul method to size a surface water soakaway, or the other way round, is a common and avoidable error. State in the report which method was used and why.

The test method in practice

The surface water method runs as follows.

1. Excavate a trial pit to the depth at which the soakaway will operate, so the test measures the right soil horizon rather than the topsoil.
2. Fill the pit with water and allow it to drain completely, which conditions the soil and removes the false speed of a dry first fill.
3. Refill and record the time taken for the water level to fall from 75 per cent to 25 per cent of the effective storage depth.
4. Repeat the cycle three times and take the slowest of the three results.
5. Log the pit dimensions, the depth of each horizon, the water levels and the times, and photograph the pit.

Taking the slowest result is the point that gets dropped. It is not conservatism for its own sake. Soil that has been wetted behaves differently from soil that has not, and a soakaway is sized for the condition it will be in after a wet fortnight, not after a dry August.

Three pits rather than one is also standard on anything larger than a single plot. Ground varies across a site and a single test in the one sandy corner is not evidence.

Pit geometry is worth getting right as well. The method assumes water leaves mainly through the sides rather than the base, so a deep narrow pit and a shallow wide one give different answers for the same soil. Record the dimensions and use them in the calculation rather than assuming a standard shape.

Seasonal variation and groundwater

This is where infiltration testing goes wrong most often. A test carried out in a dry summer produces a rate that will not be repeated in February.

Groundwater level matters twice. If the water table rises above the base of the soakaway in winter, the structure does not work at all. And the accepted design position is that a soakaway needs a margin of unsaturated ground beneath it, commonly taken as a metre, so the winter groundwater level is a design constraint rather than a detail.

So establish the seasonal high groundwater level, not just the level on the day. Monitoring in standpipes over a period is the reliable route. A desk study using published borehole and hydrogeology records gives you the expectation, and the Geotechnical Assessment should set out both.

Where the site sits in a source protection zone, over a principal aquifer or on contaminated ground, infiltration may be resisted entirely on water quality grounds regardless of how well the ground drains.

Turning a rate into a soakaway size

The design calculation balances inflow against outflow. Inflow is the rainfall volume from the contributing impermeable area for a given storm duration and return period. Outflow is the infiltration rate multiplied by the effective side area of the soakaway below the water level, with the base commonly discounted because it silts up.

Several storm durations have to be tested, because the critical duration for a soakaway is often long and low intensity rather than short and intense. Add the climate change allowance and the urban creep allowance the authority applies.

Half drain time is the check that catches undersized designs. The structure has to empty enough to be available for the next storm, and authorities usually apply a limit measured in hours.

Where infiltration is not the answer

Clay does not soak away. Neither does made ground over a compacted formation, nor a site with a high water table, nor a contaminated site where infiltration would mobilise pollutants.

When the test fails, the drainage hierarchy moves you on to a discharge to a watercourse, then to a surface water sewer, then to a combined sewer, each with its own consent and its own discharge rate limit. The Drainage Strategy has to show the hierarchy was followed in order, with the failed infiltration test as the evidence for moving down it.

A failed test is useful evidence. Keep the logs and submit them, because an authority that cannot see the test will assume it was not done.

Partial infiltration is the middle ground and it is underused. A site that drains slowly may still take a proportion of the runoff through permeable paving or a shallow infiltration trench, with the remainder discharged at a restricted rate. That reduces the attenuation volume and often saves more space than it costs.

Timing the test against the application

Do the testing before the layout is fixed, not after permission. The soakaway position, the depth and the required footprint all affect the plot layout, and a soakaway that will not fit within the root protection area of a retained tree or within the adopted highway boundary changes the drawings.

Winter testing is more representative but delays the programme. The workable compromise on most small sites is to test as early as the ground allows, record the date and the antecedent weather, and apply a suitable factor of safety where the test was carried out in dry conditions.

Where the ground investigation is being done anyway for foundations, add the infiltration testing to the same visit. The marginal cost is small and the ground investigation evidence for planning then covers both questions in one report.

And put the raw data in an appendix. Lead local flood authorities read the appendix.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.