1 The Oxford College of Garden Design Illustrated Hard Landscape Seminars - Student Notes Copyright 1995 John Heather Rev Curriculum No. GD x 1.25hrs This part of the seminar deals with disposal of storm water. There is a strong preference for disposal on site where ever possible in accordance with the principles of Sustainable Drainage Systems (SUDS). In future schemes may be refused Planning Permission if they fail to meet national requirements in this respect. Students should refer to the bibliography for further discussion of SUDS. Subject: GD1110 Piped Drainage Systems 1 Sustainable Drainage Systems SUDS principles are that rainwater should be controlled at source ie on site as far as possible. Control includes on-site pollution control, disposal by infiltration into the ground or flow attenuation. These objectives are designed to reduce flooding and pollution and replenish the underground aquifers Start your underground piped drainage design by checking what options are available on your particular site for disposing of the rain water you collect from the paved areas, i.e.:- Permeable ground (soakaways possible); Boreholes and ponds; Water courses and ditches; Sewers. 2 Soakaways - design and construction Soakaways are usually pits or trenches which may be filled with rubble, lined with open jointed brickwork, pierced precast concrete rings or plastic milk-crate cubes. Their function is to provide capacity to store storm water and to allow it to percolate slowly into the soil over a period of 24 hours - ie a storm water safety valve! Soakaways are the preferred method of disposing of rain water. In order to calculate the capacity required in England, a storm is assumed to last 2 hours at a rainfall intensity of 15mm of rainfall per hour ( = 30mm total depth of water which equates to 30 litres per square metre of area to be drained. For small works the plastic storm-crate or a hardcore (rubble) filled soakaway is economic but the calculated volume of the rubble soakaway is taken as only 30% of the pit size; the rest of the volume being taken up by the rubble. The effective volume of the stormcrate filled soakaway is 95%. For paving areas over 100m² it may be economic to use a brick lined or concrete lined excavation. For rubble filled soakaways the conservative rule of thumb for calculating volume given in BRE Digest 365 is to divide the area of paving in m² by 10 and the resultant figure will be the required size of the soakaway excavation in cubic metres. If a lined soakaway is used (ie without rubble fill), the rule of thumb is to divide the paved area by 30. These two rules are suitable for areas where flooding would be unacceptable. Inspection eye Access cover Perforated distributor pipe Rubble fill Inlet drain Geotextile surrounding the rubble Brickwork Inlet drain Granular backfill all the way round Rubble filled trench soakaway (typical long section) Concrete foundation Precast concrete ring soakaway Figure 2. Granular (rubble) filled and concrete ring soakaways 1
2 Where occasional flooding during a very heavy rain storm is not going to do any great harm, and buildings will not be flooded, the rule of thumb volume of the soakaway may be halved provided the infiltration rate is good. Your client should understand the risk. The effective capacity of a soakaway is measured between the bottom of the pit (or the natural water table if higher) and the lowest inlet pipe. For major schemes it may be necessary to establish soil permeability by digging or boring a trial hole, filling it 3 times from a water bowser over a 2 day period and observing the rate at which the water level drops. For small schemes a chat with a friendly Local Authority Building Control Officer should produce helpful local knowledge and advice. If the water level in trial pit drops to half height in 24 hours, the rate of absorption is satisfactory. If it takes much longer (or if the local Building Control Officer advises) the, surface area for absorption can be increased by changing the soakaway from a circular or square pit to a long narrow trench. Alternatively the bottom of the pit can be taken deeper into more absorbent strata where this exists and provided the seasonal maximum water table remains below the excavation. The normal maximum practical depth of a domestic soakaway is 3m. Soakaways should be at least 5m from any building and preferably on the downhill side to avoid water pressure on the foundations and the floor slab. Soakaways should also be well away from any drainage field serving a septic tank system. If you are in an area of very high water table or in heavy clay which prevents the water seeping away, then a soakaway is not possible. 3 Balancing ponds and boreholes In the case where the soil is impervious (eg heavy clay) and there is no storm water sewer available, it may be necessary to consider specialist drilling of a lined borehole into permeable strata. The borehole would take water stored in a cistern like a lined soakaway or a balancing (detention) pond. Storage of water in a pond can have aesthetic advantages and the pond allows for some disposal of the water by evaporation. In England approximately 500mm of water is evaporated annually from 2 a pond. There is a strong environmental case for disposing of rainwater on-site into soakaways, ponds, storage tanks, infiltration basins (normally dry grassed depressions) and/or boreholes where ever practical. This helps to avoid the storm surges which cause flooding, by delaying the run-off into the streams and rivers. Storage and disposal on site potentially makes water available for irrigation in summer droughts or replenishes underground aquifers. 4 Draining to ditches and watercourses. Consent is required before you can discharge water to many rivers and streams. If you are proposing to discharge water to a watercourse, especially where there may be pollution involved, the regional office of the Environment Agency (EA) may give consent subject to a limit on the rate of discharge, etc. Holding tanks or balancing ponds may then be needed. Minor discharge during construction works will probably not require formal consent Pollution can come from agricultural uses like stables, from vehicles and machinery (petrol and oil) and from silt carried by storm water or ground water. Outfalls from land drainage may carry quantities of silt and the EA may also call for a sump or silt trap - a chamber where the silt can settle to the bottom and be cleaned out periodically. Easy access is required for periodic cleaning out any silt traps and your Client should be warned of this necessity! Areas used for vehicle maintenance may also need a petrol interceptor, similar to a silt trap but with 2 or 3 chambers and ventilation to disperse explosive vapour. Climate change and other factors are raising the underground water table in parts of England which may cause springs to appear in places long thought to be permanently dry. Think very carefully before filling in any old ditches. They may be vital long term drainage channels for long forgotten streams or sudden storms. Drain pipes discharging into water courses should be at least 150mm above the normal water level. If the stream is likely to flood and back up the pipe then the EA may ask for an anti flood valve to be fitted.
3 5 Connection to a sewer There are three types of sewer; stormwater, combined and foul. Combined sewers take both foul water and stormwater and are rarely found outside London. Foul sewers carry trade effluent, foul and waste water including domestic drainage. Storm water should never be drained into foul sewers because the storm surge would overwhelm the sewage works which are designed to cope with a steady flow. Storm surges into sewage works can result in untreated sewage overflowing into rivers with consequent serious pollution. If storm water is drained into a combined sewer there must be a water trap on each inlet to prevent foul air backing up. Drains shared by two or more neighbours on private land are called Private Sewers. Public sewers are the main drains owned by the Water Companies. Establish ownership of drains and get the necessary permission if you wish to discharge water into sewers. Check the invert levels are low enough to allow your proposed system to drain into them. 6 Rainfall intensity and pipe capacities As a rule of thumb for small English domestic gardens up to 0.1 hectares, just one 100mm diameter drain laid at a gradient of 1 : 40 will be sufficient carry away all the rainwater. For large gardens and public schemes with extensive hard paving areas or car parking, it will be necessary to calculate the maximum likely quantity of water that will fall under storm conditions and the size of pipes required to carry it (See pipe sizing chart.) Paved area in square metres mm diameter 100mm diameter 75mm diameter 1:10 1:20 1:40 1:60 1:100 1:200 Pipe gradient Paved areas which may be drained by the specified diameter of pipe during a storm of 50mm/hr. intensity Pipe sizing chart. The chart is based on the following assumptions. In England a rainfall intensity of 50mm per hour is used for purposes of the calculation. This is equivalent to 50 litres per hour for each m² of paved area. Such a heavy storm might occur once in 10 years or so and last for only a few minutes. In other parts of the world the local rainfall intensity would need to be checked. It is always worth bearing in mind that assumptions on rainfall intensity are based on probability. At longer intervals, say once in 50 years, a much more intense storm may occur. The capacity of a drain varies with its diameter and the gradient to which it is laid. The chart above shows the area of paving which can be drained using different sized pipes laid at various gradients. 7 Design of underground drainage Your system will need a minimum fall of 1:100 The preferred fall in a 100mm size pipe is between 1 : 40 and 1 : 60. Plan your pipe runs in straight lines with access for inspection and rodding at each change of direction and change of gradient. Short side branches may join the drain without access for inspection provided the junction is at 45 degrees or less in the direction of flow. Access should be designed at the head of each branch drain and on long runs at 22m intervals (45m intervals if a full inspection chamber or manhole is provided). Full details are given in the Building Regulations Approved Document H. All gullies should be covered with gratings to discourage the entry of vermin and debris. Yard or pot gullies have the benefit of collecting debris before they get out of reach in the pipe. Some gullies have a removable container which can collect the silt. The Client will need to be warned of the need to check gullies for blockages and clean them out periodically as necessary. Covers for inspection chambers, and other access points must be suitable to carry the loads (eg light duty for foot traffic (grade C), medium duty for cars and light vans (grade B) and heavy duty for lorries (grade A). For larger drainage schemes the pipe manufacturers can be asked to give help with the design. Ask for a a representative to call. 3
4 Gully 1:80 Average fall about 1 : 60 Access for rodding 1:20 Soakaway Vertical scale exaggerated 1:100 Section of drain to show economic gradient and depth Figure 1. Section of drainage trench following the contours The gradient in different sections of pipe can be varied to keep the pipe at the optimum depth (eg mm below ground) but the gradient should be constant in any one length of pipe between access points. The use of different gradients means that the drain can follow more closely the contours of the site and go some way to avoiding unnecessarily deep, costly excavation or conversely avoiding pipes appearing above ground(!) 8 Laying and protecting pipes in trenches Traditionally drains were made of clay with the spigot and socket joints sealed with cement mortar which was rigid and brittle. Modern drains are either PVC-U or clay with flexible joints made from elastomeric rings or gaskets. The following requirements apply to PVC-U (plastic) drain pipes. Clay drain pipes are equally acceptable; stronger but less easily handled because of their weight. Clay pipes may need slightly modified details. The trench should be kept as narrow as practical - usually 600 wide or less. The pipes are laid on an accurately leveled 100mm bed of granular material (pea shingle) so that they slope to the constant fall you have already calculated, without dips or humps. After testing for leakage, the pipes are covered with further granular material up to the crown of the pipe taking care not to displace the pipe. The trench is then backfilled first with earth free of stones over 40mm and then in 200mm layers consolidating each layer to minimise future sinking of the surface. (See diagram below) In most cases even plastic PVC-U drains can take the weight of soil and traffic loads without distortion, but where the crown of the pipe is less than 600 below gardens or 900 below drives carrying commercial vehicles, the pipe must be protected against crushing by additional granular material and reinforced concrete slabs or by surrounding the pipe in a minimum of 100mm of concrete. Student worked exercise 4 - drainage lines to sewer or soakaway 300mm or more under drives Normal drain Backfilled with excavated material or topsoil 50mm pcc slab Backfilled with excavated material or topsoil 900mm or less under drives or less under gardens 100mm diameter drain pipe laid on 100mm of pea shingle and backfilled with shingle to crown of pipe. Selected backfill without stones Shallow drain 100mm diameter drain pipe laid on 100mm of pea shingle and backfilled with shingle to 75mm above the crown of pipe. Typical sections of pipe trenches. 4