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NBS Shortcuts: soakaways

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In this NBS Shortcut, Austin Williams looks at soakaways - and finds holes in the ground to be more complex than they seem

Soakaways are glorified holes in the ground into which stormwater is discharged and from which it is gradually allowed to percolate into the surrounding soil. It is essential that a soakaway be sized accurately to make sure that it doesn’t overflow and to ensure that all the water is dissipated effectively into the surrounding area.

Building Regulations Approved Document H: ‘Drainage and Waste Disposal’ states that methods of drainage ‘other than connection to a public surface water sewer are encouraged where they are technically feasible’. The equivalent Scottish Technical Handbook ‘Domestic
3 Environment’ says pretty much the same sort of thing, with Section 3.6 adding that soakaways in its jurisdiction should be located ‘at least 5m from a building and from a boundary’ to ensure that there is no adverse effect on the foundations of the property served by the soakaway or to any neighbouring land. This rule is also contained in Planning Policy Guidance Note 14 annex 1 and National House-Building Council (NHBC) guidance.

Ideally, soakaways should be situated on land that slopes away from the building and also away from other soakaways in the vicinity, so that there is no percolation from one to another. Northern Ireland’s Technical Booklet N ‘Drainage’ was written way back in 1990 and does not contain any details for soakaway construction other than Section N3, which requires that rainwater drains be tested for watertightness, and have adequate falls, flow rates and diameters. Wherever possible, it is advisable to design to the good-practice guidance contained within BRE Digest 365 (revised March 2007). Soakaway design software is also available from the BRE.

When planning surface-water-drainage layouts, it is important to realise that discharge to a main combined drain is becoming less acceptable and even discharging to a watercourse will require a consent from the Environment Agency. Such approvals may contain onerous provisos for limits on the rate of flow to ensure that the watercourse is not overwhelmed during heavy rainfall. This will necessitate a flow attenuation device, such as a detention basin, in which runoff can be stored temporarily (and which can also facilitate the biological treatment of pollutants if necessary) before discharging into the watercourse. However, where this type of flow intervention is not practicable, and where a sewer connection is inappropriate, soakaways (and other infiltration mechanisms such as filter drains, swales, etc.) provide a possible solution, although a careful environmental impact assessment must be carried out to avoid overwhelming low-lying/ flood plain risk areas.

Of course, it is not always possible to build soakaways. For example, they should not be built in ground where the water table reaches the bottom of the device at any time of the year; or where the presence of contamination within the runoff could result in pollution of naturally occurring groundwater.

To assess whether a soakaway is viable, a site assessment needs to be carried out and a long-winded (but relatively simple) calculation needs to be completed. To this end, a trial hole needs to be dug to test the actual infiltration rate into the soil (see diagram on page 42). The hole should replicate the proportions of the proposed soakaway (although the NHBC is happy to have narrow boreholes provided that the rate of flow is averaged out over various depths and times). The time taken for water to seep into the surrounding ground needs to be taken for several, rapid succession, trial runs. Observations - preferably at different times of the day and year - and negotiations with your friendly Building Control officer are needed to confirm that the water table is suitably low.

These tests should be carried out to determine the capacity of the soil (see Approved Document H2 (AD H2), paragraphs 1.34-1.38). Where the test is carried out in accordance with AD H2, the soil infiltration rate (f) in litres is related to the percolation value (Vp) in seconds/mm) derived from the test by the equation f=10-3/2Vp The storage volume of a soakaway should be calculated so that, over the duration of a heavy storm, it is sufficient to contain the difference between the inflow volume and the outflow volume. The inflow volume is calculated from the rainfall depth and the area drained. The outflow volume (O) is calculated from the equation: O = as50 x f x D, where as50 is the area in m2 of the side of the storage volume when filled to 50 per cent of its effective depth, and D is the duration of the storm in minutes.

For small soakaways serving a catchment area of 25 m² or less, a design rainfall of 10mm overfive minutes may be assumed to be the worst-case scenario.

Soakaways for small catchment areas of less than 100m² are usually 3-4 m deep pits filled with granular material with a particle size of 10-150mm, or lined with dry-jointed masonry. NHBC data suggests that depending on the rate of water infiltration into the ground, smaller pits may be possible (see diagram above right). A geotextile covering should be laid over the top to ensure that no soil is washed down into the soakaway.

Those serving larger areas should be designed in accordance with BS EN 752-4 or the latest BRE Digest 365. Generally, these will be lined pits, trenches or perforated precast-concrete ring units (to a similar depth as the smaller versions depending on soil characteristics) Loadbearing plastics honeycomb units are also produced to serve the same function. Very deep soakaways may use an access shaft to connect the lower perforated chamber to ground level.

Whichever design is chosen, the soakaway should accommodate an above-ground silt trap and be designed for a return period of once in 10 years - that is they need to be designed to suit the most severe decadal weather event, but also take into account storms of differing
For domestic situations, a pre-inspection by the local authority may not be necessary. However, before backfilling, a Building Control inspector together with a representative of the sewerage undertakers and insurer/warranty body should inspect to ensure that the system is functioning correctly. Special care should be taken to ensure that inlet (and outlet pipes or perforations, depending on soakaway design) are free from obstruction and that there is sufficient protection against vehicular access over the top of the pit. A concrete cover may be needed if the chamber is to be concealed beneath the topsoil. These may be either precast concrete components or formed in situ concrete.

In conclusion, the various regional assemblies, in the Highways Agency, Transport Scotland, Welsh Assembly Government, The Department for Regional Development Northern Ireland manual Design Manual For Roads And Bridges: Design of Soakaways, point out that soakaways ‘should be evaluated both with respect to the potential for providing new habitats for protected species (such as water voles and great crested newts) and with respect to the potential for encouraging invasive species’. Make of that what you will.

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