Drainage Installation
The following article outlines the requirements for effective drainage fields and drainage mounds
Suitable soils for a soakaway are ones that are well drained such as well aerated subsoils which are usually brown, yellow or reddish in colour. Examples of subsoils with good percolation characteristics are sand, gravel, chalk, sandy loam and clay loam. It is important that the percolation characteristics are suitable in both summer and winter conditions. Poorly drained or saturated subsoils are often grey or blue in colour. Brown and grey mottling usually indicates periodic saturation. Examples of subsoils with poor percolation characteristics are sandy clay, silty clay and clay.
If you are considering installing a soakaway a preliminary assessment should be carried out including consultation with the
Environment Agency, and local authority to determine the suitability of the site. The natural vegetation on the site should also give an indication of its suitability for a drainage field.
A trial hole should be dug to determine the position of the standing ground water table. The trial hole should be a minimum of 1m2 in area and 2m deep, or a minimum of 1.5m below the invert of the proposed drainage field pipework. The ground water table should not rise to within 1m of the invert level of the proposed effluent distribution pipes. If the test is carried out in summer, the likely winter groundwater levels should be considered. A percolation test should then be carried out to assess the further suitability of proposed area.
Drainage fields or mounds should be designed and constructed to ensure aerobic contact between the liquid effluent and the subsoil. Drainage fields should be constructed using perforated pipe, laid in trenches of a uniform gradient which should be not steeper than 1/200. Pipes should be laid on a 300mm layer of clean shingle or broken stone graded between 20mm and 50mm. Trenches should be filled to a level 50mm above the pipe and covered with a layer of geotextile to prevent the entry of silt. The remainder of the trench can be filled with soil; the distribution pipes should be laid at a minimum depth of 500mm below the surface. Drainage trenches should be from 300mm to 900mm wide, with areas of undisturbed ground 2m wide being maintained between parallel trenches (see Diagram 1).
An inspection chamber should be installed between the septic tank and the drainage field. Drainage fields should be set out as a continuous loop fed from the inspection chamber
Surface Water Drainage is governed by the Building Regulations 2000 which state that surface water soakaways should not be built:
- within 5m of a building or road or in areas of unstable land
- in ground where the water table reaches the bottom of the device at any time of the year
- sufficiently far from any drainage fields, drainage mounds or other soakaways so that the overall soakage capacity of the ground is not exceeded and the effectiveness of any drainage field is not impaired
- where the presence of any contamination in the runoff could result in pollution of groundwater source or resource.
Prior to installing the soakaway a percolation test should be carried out to assess the suitability of the proposed area.
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