Infiltration testing appendices in a drainage strategy
The infiltration rate decides whether the rest of the drainage design is possible, and the appendix has to show enough raw data for a reviewer to recalculate it.
The infiltration testing appendix is the part of a drainage strategy a lead local flood authority reads first, because the infiltration rate decides whether the rest of the design is possible. Soil infiltration testing drainage work has to follow a recognised method, and the appendix has to show enough of the raw data for a reviewer to recalculate the rate themselves. A single number with no test record behind it gets the whole strategy sent back.
Three repeat runs, the pit dimensions, the water levels against time, and the groundwater observation. Those are the four things that have to be in there.
What the appendix has to show
An appendix that passes review contains the test locations on a plan, cross-referenced to the proposed infiltration features rather than placed wherever the rig could get to. Then, for each location, the pit or borehole dimensions, the soil description from the excavation with the depth of each stratum, the test records with water level against elapsed time for every run, the calculated rate for each run, the rate adopted, the factor of safety applied, and the groundwater level observed with the date.
The soil log matters as much as the test. A rate measured in a granular layer that sits above clay will not be achieved by a feature founded in the clay, and a reviewer who can see the stratigraphy will spot that immediately.
Include the dates and the preceding weather. A test run after a dry fortnight in July and the same test in February can give materially different answers, and the honest appendix says which it was.
Test method, pit dimensions and repeat runs
The recognised method in England comes from the BRE digest on soakaway design, and the approach in the SuDS manual follows it. The principle is to excavate a trial pit at the location and depth of the proposed feature, fill it with water, and measure the time taken for the water level to fall through a defined portion of the effective storage depth.
The elements that matter:
- The pit is excavated to the depth of the proposed infiltration feature, not to a convenient depth.
- The pit is filled rapidly to at least the effective storage depth.
- The fall of the water level from 75 per cent to 25 per cent of the effective storage depth is timed.
- Three runs are carried out, each started as soon as practicable after the previous one has emptied.
- The slowest of the three rates is normally adopted, because the soil approaches saturation across the sequence.
The third run is the one that counts. A first run in dry soil can be optimistic by a wide margin, and an appendix reporting a single run is reporting the best case.
Borehole or percolation tests are sometimes used where a trial pit is impractical, and they are acceptable where the method is stated and the geometry accounted for. What is not acceptable is a laboratory permeability value from a sample, presented as though it were a field infiltration rate.
Reading the result
The rate comes out in metres per second and the numbers are small, which makes them easy to misread by an order of magnitude. Convert them into something physical before you accept them.
| Indicative rate | Material it suggests | Design consequence |
|---|---|---|
| Faster than 1 x 10⁻⁴ m/s | Clean sand or gravel | Infiltration straightforward, check groundwater and contamination |
| 1 x 10⁻⁵ to 1 x 10⁻⁴ m/s | Sand, sandy gravel | Infiltration usually workable with sensible footprint |
| 1 x 10⁻⁶ to 1 x 10⁻⁵ m/s | Silty or sandy soils | Infiltration possible but features become large |
| Slower than 1 x 10⁻⁶ m/s | Clay and silty clay | Infiltration generally not viable |
Those bands are indicative and the design is done on the measured value, not on the band. But if the test returns a rate that does not match the soil description, something is wrong with the test rather than with the soil.
Check the half-drain time as well as the rate. A feature that takes days to empty is not available for the next storm, and most authorities expect a stated emptying time.
Factors of safety
The measured rate is not the design rate. A factor of safety is applied to allow for siltation, variability across the site, and the consequence of the feature failing.
The SuDS manual approach scales the factor to what happens if the device does not work: a modest factor where failure causes a nuisance, a much larger one where failure would flood a building or a highway. Factors in the range of one and a half up to ten appear in practice, and the appendix has to state which was used and why.
State it explicitly, with the reasoning. Reviewers accept a well-argued factor of two far more readily than an unexplained factor of ten, and they reject a factor of one almost automatically.
Groundwater and where infiltration is ruled out
Infiltration needs unsaturated ground beneath the feature. The usual expectation is a clear vertical separation, commonly taken as at least one metre, between the base of the device and the highest likely groundwater level, which means the highest level, not the level on the day of the test.
That often requires monitoring rather than a single observation. Where groundwater is close, a standpipe monitored through the winter months gives the seasonal high, and a drainage strategy that relies on a summer dip reading will be questioned.
Infiltration is also ruled out or restricted in several situations: within a source protection zone or over a principal aquifer where the discharge could carry pollutants, through made ground or contaminated soil where it would mobilise contaminants, on unstable ground or in areas of soluble rock, close to foundations or retaining structures, and on sloping sites where the flow would re-emerge downslope.
Where contamination is present the two assessments have to agree. An infiltration design that discharges through a layer identified as contaminated in the ground investigation will be refused, and the Geotechnical Assessment and the drainage evidence should be read together.
How the result drives the hierarchy
National policy and the technical standards for sustainable drainage set a hierarchy: discharge to ground first, then to a surface water body, then to a surface water sewer, then to a combined sewer as a last resort.
The infiltration appendix is the evidence that allows you to move down that hierarchy. A sewerage undertaker asked to accept a surface water connection will want to see that infiltration was tested and found unviable, with the test records, not simply asserted to be unsuitable because the site is clay.
So the test has value even when it fails. A properly documented negative result is what opens up the next option in the Drainage Strategy, and it is also what supports the discharge rate agreed with the lead local flood authority.
Two practical points to finish. Do the testing before the layout is fixed, because infiltration features need space and the footprint depends on the rate. And test at the location of the features, since infiltration rates vary across a site by more than most designers expect.
Where the site also sits in a flood zone, the infiltration evidence sits alongside the Flood Risk Assessment rather than inside it, and the two should use the same levels datum.
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This is general guidance, not legal advice; check the position with your lead local flood authority, sewerage undertaker or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
How many infiltration test runs are needed?
Three at each location, each started as soon as practicable after the previous run has emptied. The slowest rate is normally adopted, because the soil approaches saturation across the sequence. A single run in dry ground can be optimistic by a wide margin, and an appendix reporting one run is reporting the best case.
Can a laboratory permeability value be used instead?
No. A permeability measured on a sample in a laboratory is not a field infiltration rate and reviewers reject it. The test has to be carried out in the ground, at the location and depth of the proposed feature, with the water level recorded against elapsed time so the rate can be recalculated from the data.
What factor of safety should be applied?
One scaled to the consequence of the device failing. A modest factor suits a feature whose failure causes a nuisance, while a much larger one applies where failure would flood a building or a highway. Factors between about one and a half and ten are used in practice, and the appendix must state which and why.
How much clearance is needed above groundwater?
The usual expectation is at least a metre of unsaturated ground between the base of the infiltration device and the highest likely groundwater level. That means the seasonal high, not the level on the day of testing, which often requires a standpipe monitored through the winter rather than a single observation.
Is the test worth doing if the site is clay?
Yes, because a documented negative result is what allows you to move down the drainage hierarchy. A sewerage undertaker asked to accept a surface water connection will want the test records showing infiltration was examined and found unviable, rather than an assertion that the ground is unsuitable.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.