Drainage Soakaways Systems
Soakaways are a traditional way of disposing of surface water from buildings remote from a suitable public sewer or watercourse. A soakaway must have capacity to store immediate run-off from roofs and hard surfaces and the water must then be able to disperse into the surrounding soil quickly enough for the soakaway to be able to cope with the next storm.
Soakaways are probably the most common form of surface water disposal and are usually suitable for areas less than 100m². Soakaways are generally formed from square or circular pits, filled with ether pea shingle, Polybed system Soakaway crates.
Soakaways serving larger areas are generally lined pits, trench type soakaways or constructed from specialist proprietary units. It should be expected that a domestic shingle filled soakaway may need to be renewed about every ten years.
For small soakaways serving 25m² or less, a design rainfall of 10mm in 5 minutes is quoted in the Building Regulation Approved Document H as being an appropriate worst case.
For soakaways serving areas greater than 25m² reference should be made to BS EN 752-4, or BRE Digest 365 Soakaway design. BRE Digest 365 is the most commonly used document.
The Building Regulations dictate an order of priority for the selection of surface water from buildings and these are:
- An adequate soakaway or some other adequate filtration system, or where that is not reasonably practical,
- A watercourse, or where that is not reasonably practical,
- An appropriate sewer
A soakaway must must not be used:
- Within 5m of a building or road, 2.5m of a boundary or in an area of unstable land in ground where the water table reaches the bottom of the soakaway at any time of the year.
- Near any drainage field, drainage mound or other soakaway so that the overall soakage capacity of the ground is exceeded and the effectiveness of any drainage field impaired.
- Where the presence of any contamination in the runoff could result in pollution of groundwater source or resource.
- Percolation tests should be carried out to determine the capacity of the soil
A Septic Tank is a form of holding chamber utilising a form of internal construction to slow down the flow through the tank unit. The outlet from the septic tank leads to a soakaway system below the ground surface.The diagram shows a traditional single septic tank having just one chamber and using inlet and outlet T pipes. This is the simplest form of septic tank and works.
A: Sewage enters via the inlet T pipe and discharges to the lower of the tank.
B: Gravity pulls the solids in the sewage to the base and via anaerobic biological action, a scum layer can form on the surface.
C: Effluent (with a very low solids content ) leaves via the outlet T pipe. This can then enter a second or third chamber and then lead to a soakaway field drainage system.The basic principle is to remove as much of the solids content from the final effluent as possible.
What To Look For If You Suspect A Damaged Or Full Septic Tank
A: Backing up of sewage in the drainage pipe work and manholes.
B: A high level or possible flooding of effluent in the area of the tank access covers.
C: Dark effluent with an unpleasant odors emerging onto the surface near the tank or at some other location from the tank.
D: A combination of all of the above factors.
The common septic tank problem that is encountered with is the failure of the soakaway system to function correctly. If the system is not effectively separating solids from liquids then the effluent leaving the septic tank and entering the soakaway will in time tend to clog the soakaway. As more and more solid material enters the soakaway then less of the sub soil has sufficient permeability to allow dispersion of the effluent into the sub soil. If the rate of sewage entering the tank per day is greater than can be dissipated by the soakaway then an excess of sewage in the system will result. This will build up in the drainage pipes and manholes and/or may rise above the soakaway and breakout onto the surface. To rectify the problem a simple de-sludge of the tank and if possible a jet clean of the soakaway may be all that is necessary. It is more likely however that some repair work to the soakaway will be required.
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