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Septic Tank Soakaway

If you are planning on installing a soakaway or drainfield for your septic tank system here are a few things you will need to do:

First you will need a copy of the Approved Document H - Drainage and Waste Disposal which you can obtain here: Section H - Drainage and Waste Disposal. Following the guidelines in the document for percolation tests, then proceed as follows:

Percolation test method (in order to size your soakaway system):

Excavate a hole 300mm square to a depth 300mm below the proposed invert level of the soakaway pipes.
Fill the 300mm square hole to at least the 300mm mark and allow to soakaway over night.
On the next day, fill to the 300mm mark again and observe the time it takes to soakaway in seconds, from 70% full to 25% full.
Divide the time taken by 150.  The answer gives you the average time in seconds (V).
Carry out this test at least 3 times in at least 2 different places.
Drain fields or soakaways should only be installed when the results are between 12 -100.
If the value is outside these parameters you might need to install a sewage treatment plant.

Design and Construction of your Soakaway:

Soakaways should be always constructed with perforated pipe, and laid in trenches with the same gradient not steeper than 1:200
Pipes should be laid on a 300mm layer of shingle or medium of up to 50mm.
The trenches must be filled 50mm above perforated pipe and covered with a membrane and then filled in with soil.
The trenches for the soakaway should be between 300mm and 900mm wide with a distance of 2m between trenches.
A inspection chamber must be installed between the septic tank and the soakaway.
Soakaways should be constructed in a circuit to make a continuous loop.


To calculate the size of the soakaway, use this formula:

Area = P x V x 0.25
P= the number of persons served
V= percolation test value
The calculated Area should be converted to an amount of linear trench based on the width of the trench between 0.3m to 0.9m. The layout of the trench network will depend upon the percolation test.
Example if the result is 108m2 and the trench is 900mm wide you will need 120m of trench
And lastly, you must get planning permission from your local authority.
And there you have it........ a new working septic tank soakaway system!
If you are having problems calculating the soakaway area please give us a call an we will talk you through the process on 01296326111

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