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Natural soils’ permeability testing methods and guidelines vary from county to county, but the most popular method in the UK is BRE Digest 365 ‘Soakaway Design’ BRE, 2016. This method requires at least three fill and drain cycles in order to derive the most accurate and representative infiltrations rates. By doing so, this method allows you to exam natural deposits’ performance in the event wetter conditions. It should be noted that this test needs to be undertaken within the underlying subsoils or natural deposits not Topsoil or Made Ground or the like.
The key rule for a sustainable drainage design is maintaining a maximum attenuation time within the natural deposits or soils. Thus, unsaturated zones above the groundwater table should be maintained to achieve the maximum attenuation capacity within the natural deposits.
Deeper soakaways (e.g. borehole soakaways) could be required at some sites where ‘impermeable underlying strata’ would dictate conventional soakaways’ depths exceeding 2.0m. However, it should be noted that all of the UK’s aquifers (especially chalk aquifers) are designated and protected under the current legislations as well as in accordance with Environment Agency (EA) requirements. Therefore, prior to consideration of deeper soakaway options such as borehole soakaways, all of the other options (e.g. shallow conventional soakaways) MUST BE exhausted. As a result, it is recommended that early consultation with the EA should be undertaken to determine whether they would agree in principle to the use of borehole soakaways on the site and to clarify their requirements for attenuation, filtration and sediment control measures upstream of any discharge point. Deep boreholes may alter groundwater flow paths, or even cut off groundwater flow completely and boreholes can also interconnect aquifers that were previously separate, consequently this could all result in resource and quality problems.
Please contact us for further information and guidance.
Other Natural soils’ permeability testing methods are as follows:
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Vertical tube permeability tests (falling head or constant head) in line with BS5930:2015 and BS EN ISO 22282:2012 where borehole soakaways are only feasible and foreseeable option; and
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Vp percolation test in line with DETR Circular 3/99 and BS6297:2007 where the site has no connection with the existing services/sewage network and a local sewage treatment plant is necessary.
