Soakaways
A soakaway is probably as important as the septic tank or sewage treatment plant that precedes
it. Soakaways disperse the outfall from septic tanks and treatment plants into the sub-soil,
to avoid pollution to the surface and water courses alike.
A network of trenches filled with clean gravel, within which a network of rigid perforated
plastic or clay pipes distribute the fluid evenly (level or at a gradient no more than 1:200)
over an area. The trenches are covered by a geo-textile material or plastic prior to backfilling,
to prevent soil from blocking the gravel. The discharges then flow around the pipe work and
filter through to the gravel and into the sub-soil.
The length or capacity of the soakaway has to be able to accommodate the discharge volumes from
the dwelling(s) and this is calculated by a percolation test. Too short and the soakaway will
fail, with fluids reaching the surface and creating a health hazard, killing vegetation, creating
a back-up and failure of the tankage, or worse enter a water course. Too long and you’ve wasted
your money!!
Percolation Test
The percolation test is a requirement of both the Environment Agency (for discharges in excess of
two cubic metres per day) and the local Building Control for all cases.
The percolation test must be carried out at a depth that the fluid is to be discharged at
(no shallower than 500mm and no deeper than 3 metres). If a standard 1 metre invert tankage is
being installed then the outfall invert will be 1.2 metres deep. A raised soakaway is an option
in high water table places and impermeable ground conditions, and the local Building Control
and/or Environment Agency will be able to advise. Alternatively contact S&B Services.
At least three holes must be excavated to the depth of the proposed outfall, in the area selected
for the soakaway and at different positions to afford a true picture of the whole area. At the
bottom of each of these holes a smaller hole measuring circa 300mm square and 250mm deep is
dug.
Each of the three smaller holes is then filled with water and allowed to drain dry overnight.
(If the holes do not drain dry overnight – reconsider the location for the soakaway)
Re-fill each hole with water and note the exact depth of water and the time taken to drain away
completely.
Repeat the exercise twice more, giving you a list of three holes with three readings for depth
and time, a total of nine readings. Convert the time into seconds (60 seconds = 1 minute: 60 minutes = 1 Hour).
Divide the number of seconds for each hole by the depth of water, giving a figure in seconds per
millimetre, known as a Vp
Calculate the average of the nine Vp figures to give one figure to cover the whole area chosen
for the soakaway. Then use the formula below to determine the length of soakaway.
The Vp should ideally be between 10 and 100. If over 100 then other methods of disposal must be
considered.
Vp multiplied by the number of occupants Note 1 – multiplied by the constant 0.25 Note 2 = the
area of soakaway in square metres.
Divide the above figure by 0.6 to arrive at the trench length at 600mm wide. The minimum
soakaway length is 10 metres.
- Note 1 Occupancy is determined by the number of bedrooms and a three bed roomed dwelling is
assumed to have an occupancy of 5. A four bed roomed house to be 6 and so on. For multiple
properties add the figure from each dwelling to arrive at the total occupancy. For other than
domestic properties please contact S&B Services for a detailed calculation.
- Note 2 If calculating for a Package Sewage Treatment Plant a Constant of 0.2 should be used.
When considering a soakaway the bottom must be at least 1 metre above the winter water table and
the location should be at least 50 metres from a bore hole. It is recommended that a soakaway
should be as far from a water course or pond as is practicable and certainly no closer than 10
metres.
For potential discharges over 2 cubic metres a full soil profile is required. This is detailed
on the appication form or annece 4 sheets.
If the percolation test results prove to be unacceptable due to a high water table or clay
content and no other reasonable location can be found, a high level soakaway can be considered.
For further details please contact S&B Services.