Dorset Drainage


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The principle of a septic tank soakaway is to disperse septic effluent (sewage without a high concentration of solids) into the sub soil at the site.

The size of the soakaway will be dependant on the following factors,

  • The porosity of the sub soil material.
  • The size of the property and the number of residents it is serving.
  • The type of treatment system, i.e. a septic tank or a sewage treatment plant.

Common septic tank soakaways are constructed via long trench lines filled with a clean stone and have a slotted or holed distribution pipe. These trenches can be interconnected (a looped system), a simple long trench, a herring bone layout or a large filled pit.

Any soakaway is very much dependant on the sub soil permeability. This is most important when considering and planning a new soakaway.

A porosity test can be carried out to determine the size of the soakaway required. The lower the sub soil permeability the bigger/longer a soakaway system needs to be.

Another important factor in the operation of a soakaway is the level of the natural water table in the sub soil. If the water table rises above the base of the soakaway the capacity to function is greatly reduced. We can overcome high water table conditions by the use of an effluent pump unit. These units can be accommodated inside some treatment plants or added separately to the outlet side of a septic tank.


Rain Water Soakaways
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A septic tank soakaway is not the same as a rain water soakaway. Septic tank soakaways disperse slow flows over a 24 hour period. Rain water soakaways disperse high flows over short periods i.e. during heavy showers.

If you find your gutter overflowing or water leaking out of a joint on your rain water down pipe from your gutter this may be a simple blockage i.e. leaves, but could indicate that the rain water soakaway is no longer being effective.

Rain water soakaways tend to be large pits filled with open stone. A typical rain water soakaway from one domestic house would be about 1.0m3 in capacity.


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