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Septic Tank Soakaway

A septic tank soakaway system provides a perfectly functional and legal form of secondary treatment and way of dispersing effluent into the ground after a septic tank or sewage treatment plant.

WCI undertakes septic tank soakaway design and installation in Somerset, Devon, Dorset and Wiltshire


A septic tank drainage field (or more commonly referred to as a soakaway) can be thought of as having two broad functions:

1. To act as secondary treatment after the septic tank. The soils complex biome removes nutrients from the septic tank effluent so the cleaned water can re-enter the local water cycle.

2. To act as a dispersant. The soil disperses the septic tank effluent so waste water is continually draining away from the septic tank.


A correctly designed and installed drainage field, can provide a perfectly functional and legal form of secondary treatment and way of dispersing effluent into the ground after a septic tank or sewage treatment plant.

WCI frequently receives calls from home owners stating that they believe their septic tank has failed or has a problem. 9 times out of 10, when we survey the system, the fault lies with the drainage field and not the septic tank. Common septic tank drainage field problems include:


    • Boggy, wet areas below the septic tank often with lush green and weedy growth.
    • Backed up drains
    • Foul water coming out of the top of the septic tank or preceding manholes.
    • Manholes holding water in the bottom.
    • Toilets are slow to flush, possibly gurgling and noisy.
    • Tide marks within the septic tank.
    • When the septic tank is emptied – water rushes back from the drainage field into the septic tank.
    • Septic tank effluent breaks out to the surface of the land or into a nearby watercourse.

The above symptoms of soakaway failure usually are due to a number of common soakaway problems including:


Roots entering and choking the pipe work

Water thirsty roots will often find their way into soakaways and, over time, roots can block the soakaway pipe work. This can be prevented by only installing soakaways away from shrubs and trees and ensuring the soakaway includes a geotextile covering during installation.


Insufficient porosity

A soakaway is only as good as the ability of the surrounding ground to soak up the water it receives. As stated in Section H of the building regulations: ‘drainage field disposal should only be used when percolation tests indicate average values of Vp of between 12 and 100 and the preliminary site assessment report and trial hole tests have been favourable’. In other words, the soakaway must be designed according to the actual soil characteristics.


Incorrect installation

Soakaways laid at incorrect gradients, incorrect depths and with incorrect layouts will not function correctly. For example, the deeper a soakaway is installed, the less oxygen is available for the aerobic bacteria in the soakaway to break down the septic tank effluent. Often the deeper the dig, the more chance of hitting the water table and impermeable clay subsoils or parent rock which are all detrimental to the soakaway’s performance. Further, if a soakaway is not laid at the correct gradient, water will pool at one end of the soakaway resulting in outcropping or septic effluent rising to the surface.


High water table

WCI usually has more calls to our offices in winter regarding soakaway failure. This is because the water table is much higher in the winter months and can result in the backing up of wastewater towards the property as the septic tank effluent cannot be dispersed through the soakaway. The solids within the septic tank mix and when the water table drops, solids from the tank are distributed in the soakaway adding to its failure. If a watercourse or flowing ditch is available to you, a sewage treatment plant could be installed to prevent such a problem.

When faced with replacing a failed existing soakaway or installing a new one, we recommend that everyone follows the simple design principles:


  • Talk to an expert. We at WCI ONLY deal with wastewater. Why not pick up the phone and arrange a free site visit and quotation today.
  • Do your porosity tests and don’t cut corners. Dig a minimum of 3 holes and repeat each test three times in each hole. Prepare for the task ahead, for example, take a stop watch and get containers of water to hand, at the test holes, before you start. Depending on the clay content of your soil you could be watching water drain from holes for quite a while. A book and a cup of tea could also be a wise investment. If you would prefer WCI to undertake the tests please contact us.
  • Dig a trial pit. Where it is expected that ground water may be high, a pit must be excavated with a mechanical digger to ensure that the bottom of the proposed drainage field will not be within 1.2m of the resting ground water table. WCI can undertake the trial pit when the porosity tests are carried out. Please contact us.
  • Don’t leave the work until the peak of a dry summer. The results will not reflect an average day and you run the risk of under sizing your drainage field or missing a seasonally high-water table.
  • Be honest about the population equivalent (p.e.). Just because there are only 2 of you living in the family 5-bedroom home now, doesn’t mean you won’t move one day. P.e. must be calculated using the British Water Flows and Loads 4 tables.
  • Do your checks. Do you have a public foul main nearby? Are you in a Source Protection Zone 1? Do you meet the Environment Agency’s General Binding Rules? Do you have enough available land area to install a drainage field? How am you going to import tonnes of clean stone to your property? A WCI survey will take all these factors into account.


British Water have compiled a helpful guide to enable owners and users to understand how the discharge from septic tanks and small wastewater treatment systems (package plants) can be discharged to ground.

What is the difference between a septic tank drainage field and a clean water soakaway?


Septic tank soakaway design and installation is subject to building regulations guidelines and British Standard (reference BS6297:2007 + A1: 2008 Code of Practice for the design and installation of drainage fields for use in wastewater treatment).

A septic tank or sewage treatment plant drainage field consists of up to 30m long interconnected parallel trenches, where ridged perforated pipe is installed on a layer of clean stone.

A soakaway for roof water is completely different. Attenuation crates or rubble filled holes are not legal for use with a septic tank or sewage treatment plant.

If you require a septic tank soakaway to be installed, please contact Naomi directly on 01984 623404 (ext 26) or via our contact form below to arrange your free site visit.

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If you have any questions or just need some clarification on the regulations and how they apply to your property, please call us on 01984 623404