Soakaway Design

A conventional shallow soakaway comprises a partially perforated cylindrical chamber. The chamber will need to be large enough to store the run-off before the water is able to be drained into the surrounding soil. If the soakage is required at a greater depth a deep bored perforated plastic or steel pipe will need to installed in order to discharge the water from the sealed storage chamber.

Soakage or percolation testing will need to be carried out in order to determine the soil infiltration rate and the drainage capacity of the soil. There are a number of tests used to determine whether the soil conditions are suitable to consider soakaway drainage as an option. The testing is normally carried out in accordance with either BRE Digest 365 ‘Soakaway design’ or BS 5930 ‘Code of practice for site investigation’

The BRE Digest 365 soakage test is normally carried out in trial pits. The pits should be filled with gravel to ensure the sides of the trial pit remain stable during charging and draining. The soil infiltration rate is determined by filling the pit with clean water from a bowser or tanker and determining the length of time for 75% of the volume of water to drain away.

BS 5930 field permeability or percolation tests are normally carried out in boreholes using either the falling head or constant head test method. Borehole tests are scheduled in order to establish the infiltration rate for soils that may be too deep to safely measure using the trial pit method. The borehole test is also often carried out as part of the exploratory site investigation to provide a preliminary indication of the suitability of the soils for soakage drainage.

Conventional trial pit soakage tests are relatively expensive and will cause some disruption to the site. The borehole tests are similar to the method given in BRE Digest 365 and are generally less accurate than the trial pit test; however for most soil types they still give reasonable estimates of infiltration rates.