Soakaway drainage is the traditional way of disposing of surface water for areas remote from a suitable public sewer or watercourse. The soakaway chamber must have sufficient capacity to store the immediate water run-off from roofs and hard surfaces and the water must be able to disperse into the surrounding soil quickly enough for the soakaway to be able to cope with the next storm.
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’
BRE Digest 365 Soakage Tests
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 Permeability Tests
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.