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SuDS is short for Sustainable Drainage System. SuDS is a storm attenuation system or a means of reducing the rate of water entering the drainage system and preventing it backing up and causing flooding like that shown in these recent images.
The purpose of SuDS is to determine how you manage your sites surface water drainage or disposal. Mostly this is rainwater from roofs, gutters and external areas of hard cover, road surfaces and parking.
Most new building development and extensions are likely to require SuDS
FAQ. “Why can’t I dispose of my surface water to the public sewer?”
Ans. “You can ask to connect your development to the sewer/drainage network (if there is one nearby).However owing to instances of urban surface water flooding, the planning authority may stipulate the use of SuDS instead of disposal to the public drainage network.”
If your site is not near a sewer you will no option but to design a SuDS.
From April 2015 any development of 10+ units (including residential, industrial, commercial and mixed use sites) is required by law to assess the potential use of Sustainable Drainage Systems, often as part of a drainage strategy.
Types of SuDS
There are many different types of SuDS systems that can be used, these include: • Soakaways • Rainwater harvesting • Green Roofs • Infiltration Systems • Filter Strips • Filter Drains • Swales • Bioretention Systems • Pervious Pavements • Attenuation Storage Tanks • Detention Basins • Ponds and Wetlands. Information concerning the design and suitability of these can be found in the Ciria SuDS Manual (2015).
As a starting point in designing a SuDS you are likely to require an in-situ soil permeability test otherwise known as a soakaway test, percolation test or infiltration test. Rock permeability can also be tested in the same way.
Q. What is a soakaway? A Soakaway is an installation where rainwater can infiltrate into the soil. It is one of the most common and simplest types of SuDS used in the UK.
This article describes the soakaway testing services Geoinvestigate can provide for your development, project or site.
Some ground conditions have much higher permeability and show more favourable soakaway behaviour than others. While on the one hand the permeability of gravel and sand can be high, the permeability of clay is likely to be extremely poor. That is why clay pot has been used since ancient times for storing and carrying water. Even today clay is used for water proofing ponds and reservoirs.
It’s hardly surprising then that in general gravely and sandy soil provides the best ground conditions for a soakaway and clay does not. However in reality in the UK there is a very wide range of soil composition, soil type and permeability determined by various combinations of clay, silt, sand and gravel. How the soil is formed and whether it was by glaciation, rivers or weathering also affects its permeability and soakaway behaviour.
Therefore many sites are not straightforward because the ground shows a wide range of permeability and soakaway suitability. For this reason a specialist engineering geological/geo-environmental company like Geoinvestigate is best placed to investigate, test and identify the ground conditions which are suited to soakaway installation.
Soakaway Test Procedure
A soakaway or percolation test measures how quickly water drains away through soil or rock. The results from this test allow you to determine if a soakaway is suitable and if so what size of installation is needed.
A mini digger, water bowser/water container, tape measure and stop watch are required to carry out a soakaway test. The following image shows 2 percolation tests carried out simultaneously by Geoinvestigate to save time and our client money.
On this occasion the client supplied the excavator and water in 1000 litre IBCs helping to reduce the cost further. However where required Geoinvestigate can also provide our own mini-digger and 1000 litre road bowser.
The test pit/s should be located in the area/s where the soakaway/s is likely to be placed. Where possible pits should extend to a slightly greater depth than the final installation. The soil exposed in each test pit should be logged by a competent person (prefereably a geologist or geo-environmental engineer) and the test section adjusted accordingly. A photographic record of each test should be made.
While the gravel underlying this site proved to be very favourable for a SuDS soakaway outcome this is NOT always the case and many sites in the UK are UNSUITABLE for soakaways.
Soakaways require a relatively large space and this is not always available within the confines of a small site.
The soakaway must be a minimum of 5m away from your building, 5m from a road and 2.5m away from any boundary. Alternatively a vertical deep borehole soakaway may provide a solution where space is tight.
Areas subject to prolonged seasonal flooding or a high water table are unlikely to be suitable for soakaways. For this reason sites located on river flood plains and river terraces, marshy ground and within flood risk zones need to be assessed carefully.
Soakaways are unlikely to be appropriate if ground water enters the test pit during excavation or the water table is encountered at the base of the pit.
Soakaways should be avoided on the higher side of a retaining wall where there is likely to be a risk of de-stabilising the wall and/or flooding the neighbouring property on the lower side.
Soakaways are unlikely to be successful where hard rock is present at very shallow depth.
Do not locate a soakaway close to the basement or cellar of a building.
Do not locate a soakaway on an unstable slope.
Soakaways may be prohibited in certain parts of the country subject to gypsum or salt dissolution ground hazards such as occurs in Ripon, North Yorkshire and Cheshire.
Because much of Britain is covered by clay soil many sites are not be suitable for soakaways. While Geoinvestigate can help clients by providing them with a Preliminary Soakaway Suitability Assessment or PSSA a soakaway test may be unavoidable even where impermeable clay has been identified. This is because proof may be needed that the site has failed before other SuDS options can be considered or consent is given to discharge into the sewer network. The following images show failed soakaway tests the 1st in a heavy clay soil and the 2nd in low lying ground next to a river with the water table lying more or less at surface.
As part of a preliminary investigation for a barn redevelopment Geoinvestigate were asked to assess whether the site would be suitable for a SuDS soakaway installation. The following images speak for themselves. Geoinvestigate were able to report that the ground comprised sand and gravel and that the conditions were almost perfect for the installation of soakaways. This was confirmed by subsequent soakaway testing.
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