Our advice is based on the Link team’s collective experience, which includes involvement in hundreds of projects of varying scales where infiltration testing has been required to be undertaken.

On a broader spectrum, infiltration can be defined as the speed at which water passes into, or through soil. The rate at which infiltration occurs depends on the properties of the soil and the underlying geology through which the water is discharged. The capacity of the soil to infiltrate water is given by the infiltration coefficient. This is the long-term infiltration rate into the soil divided by the area of infiltration. The infiltration rate is related to a soil’s permeability, which is expressed as k.

Infiltration testing is commonly undertaken in geotechnical and hydrological assessments to review the permeability and drainage characteristics of soil.

There are several methods that are regularly used to conduct infiltration testing, two of which are falling head permeability tests and constant head permeability tests; these are usually undertaken within laboratories.

More regularly, infiltration testing is undertaken on site to potentially aid with drainage and to review the possible use of soakaways. This can be undertaken through soakaway tests.   

Any soakaway test undertaken for the purposes of drainage must follow the BRE365 requirements.

The importance of using infiltration on a site is highlighted through the SuDS Hierarchy. This shows that infiltration is the ideal method of discharging surface water from a site and therefore is usually requested to be explored first.  

How to undertake soakaway tests? The BRE365 test sheets state that three tests should be undertaken. The infiltration rate is measured by assessing the rate at which water infiltrates into the surrounding ground.

The below table shows typical infiltration coefficients for different types of soil, taken from the CIRIA report C753 The SuDS Manuel.

Soil type/texture	ISO 14688-1 description (after Blake, 2010)	Typical infiltration coefficients (m/s)
Good infiltration media GravelSandLoamy sandSandy loam	Sandy Gravel Slightly silty slightly clayey SAND Silty slightly clayey SAND Silty clayey SAND	3×10-4 – 3×10-2 1×10-5 – 5×10-5 1×10-4 – 3×10-5 1×10-7 – 1×10-5
Poor infiltration media Loam Silt loam Chalk (structureless)Sandy clay loam	Very silty clayey SAND Very sandy Clayey SILT N/A Very clayey silty SAND	1×10-7 – 5×10-6 1×10-7 – 1×10-5 3×10-8 – 3×10-6 3×10-10 – 3×10-7
Very poor infiltration media silty clay loamclaytill	– – Can be any texture of soil described above	1×10-8 – 1×10-6 <3×10-8 3×10-9 – 3×10-6
Other rock* (infiltration capacity depends on the type of rock, the extent and nature of discontinuities and any infill)	N/A	3×10-9 – 3×10-5

The infiltration coefficient values in the table are only considered to be a guide and detailed site-specific investigations should be undertaken to confirm this.

If you have a concern about if you may require an infiltration test to be undertaken as part of your proposed development, please contact us at: mail@linkeng.co.uk and we would be more than happy to provide you with some site specific advice.