![]() For these purposes typical design infiltration rates have been compiled. Typical Soil Infiltration Rate Valuesįor the purposes of preliminary design and feasibilities it is sometimes required to undertake an outline soakaway design before any detailed site investigation works have been undertaken. Similar to sands, in the UK gravelly layers often include significant quantities of clay which can reduce their infiltration rate significantly. ![]() Gravel layers are also often relatively thin and also often do not extend a great distance horizontally which reduces their effectiveness at infiltrating large volumes of water. Gravel layers are however seldom homogenous and variable particle sizes and sand horizons can significantly reduce the infiltration rate. Gravel layers can be very suitable as their large particle size can lead to a high porosity and corresponding high infiltration rates. Also, in the UK sandy layers often include significant quantities of clay which can reduce their infiltration rate significantly. However care must be taken that the sand layers do not clog up with sediments and the layer will remain stable in saturated conditions as the storm water infiltrates through the medium. The designer may use this fact to justify a smaller factor of safety. As the sediments are likely to be a similar particle size to the silts, the sediment does not have a big impact on the soil infiltration rate. However, due to the smaller particle size of silty soils, they are less prone to becoming blocked by sediments entering the soakaway. Soils with a clay/silt content greater than 40% are likely to be unsuitable. Similarly some silts are also largely unsuitable, though this depends on the particle size. Soils with a clay content of more than 20% are generally unsuitable for infiltration. Generally the more open the macrostructure, the higher the infiltration rate.įor this reason homogenous clays are generally unsuited to soakaways as they can have very low infiltration rates. The infiltration rate is dependent on the physical properties of the soil. Note that even where infiltration rates are too low for soakaways to handle the design storm runoff, they can still be useful for intercepting the first 5mm of rainfall or as part of attenuation storage. However, for very small catchments they could still be used. Soakaways constructed in soils with infiltration rates lower than this value will generally be very slow to empty and therefore will not comply with the requirement for soakaway emptying time. Some guides specify a minimum infiltration rate of around 1x10 -6 m/s. Infiltration rate describes the rate at which water is able to infiltrate through the unsaturated soils surrounding the soakaway. The performance of the soakaway depends largely on the infiltration rate of the surrounding soils.
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