Soakaway drainage is the traditional method of disposing of surface water in areas remote from a suitable public sewer system. The use of soakaway drainage in urban areas is becoming more common, to limit the impact on existing positive drainage systems and to provide a sustainable alternative.
The soakaway chamber must have sufficient capacity to store the immediate water run-off from roofs and hard surfaces. The water must be able to disperse into the surrounding soil quickly enough for the soakaway to be able to accommodate the next influx.
For soakaway drainage design the soil infiltration rate (f) is required. This is determined by undertaking soakage tests, in accordance with BRE Digest DG365 ‘Soakaway Design’. The soakage test should be carried out in a trial pit excavated within the proposed soakage medium. Typically the pit would be around 2m deep.
The pit is normally filled with 20mm stone to maintain stability of the sides and avoid operators working around an open excavation. Prior to back-filling with the stone, a slotted pipe of a nominal diameter of 50mm is installed in the pit. The pit is filled with water, and the drop in water level within the pit is measured via the slotted pipe. Three test runs should be carried out in each soakage pit.
The infiltration rate (f) in m/s is determined based on a relationship between the time taken for the pit to empty from 75% to 25% full; the volume of water soaked away over that period; and the size of the pit. The lowest infiltration rate determined from the three runs is used as the design value.
The number of soakage tests undertaken on a site will depend on the size of the site, the proposed future site layout and the geological variation. When it is not practical to excavate a trial pit, for example where ‘deep’ soakaways are proposed or on a congested site, it is possible to undertake a soakage test in a borehole.
For this situation the borehole is drilled into the potential soakage medium. The borehole may be cased to the bottom, or there may be a length of uncased soil below the casing, depending on the nature of the soil. The borehole is then filled or part filled with water and the drop in water level is measured. To maintain stability of the borehole and facilitate measurement of the water level, a perforated pipe with a granular surround can be installed if required. An infiltration rate can then be calculated using the same relationship between time, volume and area of discharge, as defined in BRE Digest DG365.