Infiltration

Where possible infiltration must be used in order to prevent increased volumes of water leaving the site.

What the LLFA expect to see 
For outline applications , preliminary ground investigations, or a desk top study highlighting the potential capacity for infiltration should be provided. For full applications or where necessary for discharge of conditions applications, full detailed infiltration testing needs to be provided in line with BRE365 and the infiltration testing methods found in chapter 25.3 of the CIRIA SuDS Manual C753. This should include the locations and results. The lowest found rate should be used as a conservative approach.

After reuse, infiltration is the next option within the drainage hierarchy. As the potential for infiltration can vary across the county due to soil types, ground water levels, and topography, sufficient ground investigations and infiltration testing should be undertaken and supplied to support any application. Any ground investigations should include data from the British Geological Society, intrusive testing such as borehole tests (to determine soil type, depth and the depth of water table), detailed topographic drawings, and infiltration testing in line with the BRE365 testing procedure and the infiltration testing methods found in chapter 25.3 of the CIRIA SuDS Manual C753.This testing requests that three consecutive tests are required within each test pit and if infiltration is found to be viable then the lowest rate should be used. In addition, when conducting detailed infiltration testing, the tests should be carried out at the location, depth and head of water that replicates the proposed design. Groundwater monitoring should preferably be undertaken between November and April. When seasonal groundwater levels are below average levels, consideration should be taken, and onsite monitoring should be adjusted accordingly.

At the outline stage of the planning process it is accepted that intensive infiltration testing is not always achievable, therefore a preliminary investigation on the soil and geology of the site is acceptable. The LLFA would accept borehole testing or a desktop study with the condition that infiltration testing is undertaken at a later stage. Where the capacity to infiltrate is undetermined from a desk study a borehole percolation test could be undertaken to ascertain the likelihood of infiltration. If infiltration is the chosen discharge option within an outline application and detailed testing has not been possible, then an alternative approach should also be provided in case subsequent infiltration testing demonstrates that discharge to the ground will not be viable.

Ground stability should be taken into consideration when infiltration is proposed. The base of the soakaway should be 1 metre or higher than the highest ground water levels. Point infiltration features such as soakaways should not be within 5 metres of a building. If infiltrating into chalk, where it is medium to high density, soakaways should be 5m away from all structures, roads, and railways. Where it is low density soakaways should be a minimum of 10m away, and where dissolution features are known to be prevalent soakaways should be at least 20m away from roads, structures, and railways. In addition, Essex Highways require that point infiltration should not be within 6m of the highway. Blanket infiltration however, (such as permeable paving) can be located as close as possible to buildings as long as there is an impermeable barrier between the two. However, if the permeable paving is to infiltrate additional water from other surfaces e.g. roof areas an offset from the building foundations is needed. In addition, infiltration should not occur on made ground.

The minimum acceptable rate of infiltration is 1x10-6 m/s. Rates found to be slower than this may potentially have to deliver a hybrid drainage solution. If rates are found to be too slow for formal infiltration this does not rule out the possibility of some soakage taking place. Features (for example permeable /porous paving) should be unlined or use permeable lining wherever possible regardless of infiltration rates in order to maximise infiltration capacity. This should be approached with care, as should all infiltrating sites, when dealing with areas that are subject to previous contamination or other issues such as structural stability. Whilst Essex Highways will not adopt permeable paving within roads, if this option is to be used, the features should be lined. This lining can be permeable to encourage infiltration, but there has to be a material separating the storage medium from the formation.


Page updated: 17/02/2020


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