What is a Infiltration / Soakaway Test?

In order to determine if your ground conditions are suitable for the discharge of surface water using soakaways, site specific infiltration tests will be required. The tests will measure the rate of water infiltration into the ground (the infiltration rate). We provide infiltration testing in accordance with the BRE 365 specification in order to determine if discharge of surface water to the ground is / is not feasible. From our on-site investigations, we can determine your sites infiltration rate, enabling the accurate design of soakaways / other infiltration device.

In order to satisfy planning authorities, it is required to prove if surface water discharge through infiltration is / is not feasible on your site.

Our tests are BRE365 approved.

Why do I require a soakaway?

In order to satisfy planning authorities, it is required for new developments to manage surface water that enters the site, ensuring the proposed development will not increase the risk of flooding elsewhere. Sustainable drainage systems (SuDS) are required in order to replicate the natural surface water discharge rate into the nearest watercourse. Different solutions can be advised depending upon the site characteristics.

Where ground conditions are suitable, planning authorities will demand the first form of surface water discharge is via infiltration, replicating natural surface water discharge conditions. According to the BRE365 guidance, soakaways should discharge from full to half-volume within 24 hours, and this is to ensure that the soakaway or other proposed infiltration device has enough capacity to deal with another storm event within that 24 hours.

How do soakaway tests work?

On site locations where soakaways will be proposed investigated through the excavation of a trial pit. Soakaways should be located at least 5m from a habitable dwelling. If there’s any risk of contamination in the runoff polluting the groundwater, then a soakaway cannot be installed. For ‘New Dwellings and Commercial / Industrial buildings’ the trial pit size is required to be:

Excavated to the same depth of the anticipated full-size soakaway below the invert level of the drain.

The trial pit should be 0.3 to 1m wide and 1 to 3m long.

It should have vertical sides trimmed square and, if necessary for stability, should be filled with granular material.

The trial pit should be filled, and allowed to drain three times to near empty.

When granular material is used, a full-height, perforated, vertical observation tube should be positioned in the pit so that water levels can be monitored with a dip tape.

The trial pit is filled on three occasions to give a true reading on the ground conditions. The soil infiltration rate will be taken as the worst of these three readings.