Infiltration testing: what a drainage authority expects

A drainage strategy that proposes infiltration without field testing will be rejected, and on a self-build plot that rejection usually arrives at the validation stage rather than at determination. Soakaway infiltration testing for planning is not optional evidence. It is the thing that turns an assumption about the ground into a number the lead local flood authority can check.

The test itself is straightforward enough to commission on a single plot for a modest sum. Skipping it and designing from a soil map is the most common reason a small site drainage strategy comes back.

Why a desk study is not enough

Published soil and geology mapping describes the parent material across a wide area. It does not describe the made ground under the old driveway, the clay lens in the corner of the garden, or the perched water table that sits a metre down through the winter.

Two plots on the same geological unit, 50 metres apart, can behave completely differently. One drains freely, the other holds water for days.

There is also a regulatory dimension. The lead local flood authority is being asked to accept that surface water will disappear into the ground for the design life of the development. Its officer cannot verify that from a map, and will not.

Use the desk study for what it is good at: identifying whether infiltration is plausible, whether groundwater is likely to be shallow, whether the site sits in a source protection zone, and whether contamination might rule out infiltration on quality grounds. Then test.

The recognised test method

The established method for soakaway design in the United Kingdom uses a trial pit filled with water and timed as it empties, repeated three times, with the infiltration rate calculated from the middle portion of the drawdown.

The sequence:

1. Excavate a trial pit at the location and to the depth of the proposed soakaway, with the base at the level the soakaway base will occupy.
2. Record the pit dimensions accurately, because the calculation uses the internal surface area up to the half-depth level.
3. Fill the pit with water to at least three quarters of its effective depth.
4. Record the time taken for the water level to fall from 75 per cent to 25 per cent of the effective storage depth.
5. Allow the pit to empty and repeat, so that three separate fillings are completed, ideally on the same day or on consecutive days.
6. Use the slowest of the three results for design, since the ground wets up progressively and the later tests are more representative of saturated conditions.

The output is an infiltration rate in metres per second. Design then applies a factor of safety, which increases with the size of the contributing area and the consequence of failure, so the rate you measure is not the rate you design to.

A percolation test for a septic tank drainage field is a different test with a different method and a different purpose. Do not substitute one for the other; the drainage authority will notice.

Number and position of trial pits

One pit is rarely enough, and a single pit in the most convenient corner of the site is the classic weak submission.

Provide at least one pit at each proposed infiltration location. On a plot with a single soakaway, that means one pit at the soakaway position, and a second pit is prudent because a result that rules out infiltration at the only tested point leaves you with no fallback.

Position the pits where the soakaway will actually be built, which means outside the building footprint, clear of foundations by the distance the design requires, away from the highway boundary, and downhill of the building rather than above it. Moving a soakaway after testing usually means testing again.

Depth matters as much as position. An infiltration rate measured at 1 metre tells you little about conditions at 2.5 metres, and soakaways are often deeper than people expect once storage volume is calculated.

Seasonal groundwater and the water table

This is where most infiltration proposals actually fail.

The base of an infiltration device needs clearance above the seasonally highest groundwater level, and a metre is the separation commonly required. Testing in August, when the water table is at its annual low, and then designing a soakaway whose base sits below the February water table, produces a system that does not work for half the year.

Deal with it in one of three ways. Test in the winter months, which is the most reliable option and the least convenient. Install a standpipe or dipwell in the trial pit and monitor the level across several months. Or use published groundwater level data and local borehole records to estimate the seasonal high, stating the assumption and applying a margin.

Record the groundwater level observed in the pit at the time of testing, with the date. An infiltration report that says nothing about groundwater is incomplete.

Reporting the results

The report should be short and complete. What a drainage officer looks for:

• A site plan showing pit locations with coordinates or dimensions from fixed points.
• Pit dimensions and the depth of the base relative to existing ground level.
• A log of the ground encountered, with a simple description of each stratum.
• Groundwater observations, with times and dates.
• The raw drawdown readings for all three fillings, not just the derived rate.
• The calculated infiltration rate for each test, with the design rate identified and justified.
• Weather in the preceding days, because testing after prolonged dry weather overstates performance.
• The name and qualification of whoever carried out the test.

Include the raw data. A report giving only a final figure invites the authority to ask for the readings, which adds a fortnight. The same principle applies to the wider drainage submission, and what consultees actually check is set out in what a consultee checks in a drainage model output.

Where infiltration is ruled out

Infiltration is unsuitable, or not permitted, in several situations:

Where infiltration is ruled out, say so clearly with the evidence and move to the next option in the hierarchy: discharge to a watercourse, then to a surface water sewer, then to a combined sewer with the undertaker's written agreement. A report that reaches the right conclusion with good evidence is far stronger than one that strains to keep infiltration alive.

Designing when rates are marginal

Marginal rates are the awkward case, and there are workable responses.

Increase the infiltration area rather than the depth, using shallow, wide devices such as a soakaway trench, a permeable paving sub-base or a rain garden, which present more surface to the soil and avoid the groundwater constraint.

Split the discharge, infiltrating what the ground will take and attenuating the remainder for discharge at a controlled rate. Hybrid solutions are common and accepted.

Apply a higher factor of safety and show the sensitivity, so the authority can see the system still works if the measured rate proves optimistic.

And design the exceedance route regardless. Every infiltration system eventually meets a storm it cannot take, and showing where that water goes, safely away from the building, is what converts a marginal drainage case into an acceptable one. The wider drainage design sits in the Drainage Strategy, with flood risk covered in the Flood Risk Assessment, and the two should quote the same rates.

Ongoing access for cleaning and inspection is a related design requirement, covered in trash screens, headwalls and maintenance access.

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.