Soakaway Testing & Infiltration Testing

Most soakaway tests can be completed within a single site visit, although the duration depends on soil permeability and ground conditions. Low-permeability soils such as clay can require longer monitoring periods to accurately determine the infiltration rate.

The cost of soakaway testing depends on factors such as site access, excavation requirements, ground conditions, and the number of tests required. In most cases testing is a relatively small cost compared with the potential delays and redesign costs that can arise if infiltration feasibility is not established early.

Soakaway testing is usually required during the drainage design stage of development where infiltration drainage or soakaways are proposed. Local planning authorities and Lead Local Flood Authorities (LLFAs) often require infiltration testing to demonstrate that surface water can safely discharge to ground.

If infiltration testing shows that the ground drains too slowly, soakaways may not be suitable for the site. In these situations alternative drainage solutions may be required, such as attenuation systems, discharge to a watercourse, or connection to a surface water sewer, depending on local drainage hierarchy requirements.

The number of soakaway tests required depends on the size and complexity of the site. Drainage engineers typically require at least one test in the location of each proposed soakaway or infiltration feature to ensure the results are representative of ground conditions across the development.

Soakaways work best in permeable soils such as sands and gravels where water can drain freely through the ground. Clay soils and heavily compacted ground often have very low permeability, which can limit or prevent the use of infiltration drainage systems.