In line with the drainage hierarchy, Sustainable Urban Drainage Systems (SuDS) should first consider the discharge of surface water runoff from impermeable areas to ground. This is also known as infiltration testing or percolation to ground. GeoSmart’s SuDSmart Pro report is a desktop based assessment of the Site conditions, which can provide a preliminary opinion on whether infiltration to ground is likely to be feasible at the Site.
Many developers and architects include a soakaway as part of the surface water drainage system, however, if infiltration to ground is recommended in the GeoSmart SuDSmart Pro report, it is imperative to undertake infiltration testing before plans are finalised as the results could significantly impact the size and/or design of the drainage system. If infiltration to ground is not considered feasible within the SuDSmart Pro report, then infiltration testing is unlikely to be requested by the council.
The calculated soil infiltration rate is used to determine the feasibility of soakaway drainage, the design storage volume and the number of infiltration SuDS features required, which will be used in construction drawings also known as Detailed Drainage Design.
There are many types of soils across the country which absorb (infiltrate) water at different rates depending on their permeability and porosity. The speed at which water enters the soil is known as an infiltration rate and a test can be done to obtain this rate.
Infiltration testing should be undertaken in the area where focused infiltration features such as soakaways are proposed.
It is important to remember that other seasonal factors such as groundwater levels and soil moisture content may affect infiltration results. Infiltration testing should be under a worst case basis.
Most planning conditions recommend that infiltration testing should be undertaken in accordance with BRE Digest 365. There are several key stages when undertaking an infiltration test:
An example of a good infiltration rate, which would sufficiently discharge water to ground and where focused infiltration features would be suitable would be x 10 -4 or -5.
A poor infiltration rate, which may still support infiltration to ground via focused infiltration, if this was the only discharge method feasible, but would need to be larger in size to accommodate for a slower emptying time would be x 10 -6.
An example of an infiltration rate where discharge to ground would not be feasible would be x 10 -7. In this case, other discharge options with the drainage hierarchy would have to be explored.
If you wish to discuss infiltration testing or other drainage related matters on your Site, please get in contact with us.