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As winter approaches every year and the wet weather sets in, the expected number of calls reporting drainage field (or more commonly referred to as a soakaway) failure increase. Year after year the pattern continues and quite often, we get re-occurring calls from the same home owner who lives with a seasonally dysfunctional drainage field. They are lulled back into a false sense of security when the weather improves, only to find themselves right back to where they started the following winter!


What is a drainage field (aka a soakaway)?

If we think of a typical septic tank and drainage field system, we can broadly assume that the septic tank is there to act as a large “collection bin”. It collects the raw incoming waste water and retains it, long enough for the sludges to settle to the bottom of the tank and for the scum to float to the surface – leaving a clear effluent which can legally discharge into a drainage field. In waste water engineering we think of a septic tank as removing 30-40% of the ‘total pollutants’ from the incoming wastewater from the system. The rest being removed by the biological action in the drainage field.

Therefore, the drainage field’s main purpose is to further treat the septic tank effluent. It also acts to disperse the cleaned septic tank effluent, via slotted or perforated pipes, into the surrounding subsoils where eventually it will enter the local ground water system.


How do I know my drainage field is failing?

  • Septic tank needs emptying more frequently
  • Backed up drains
  • Overflowing manhole
  • Bubbling toilet which is slow to empty once flushed
  • Boggy, wet or stagnant areas of water where the drainage field is sited


WCI’s top five reasons for drainage field failure:


1.  Connection of the roof water to the foul drains

Under storm conditions, where roof water is connected into the foul drain and enters the septic tank, settled sludges within the septic tank are mixed up and carried out into the drainage field causing blockages.


2.  Geographical reasons including seasonally high-water table and poor porosity

When an engineer designs a drainage field, they should undertake porosity tests and a trial hole to ascertain where the water table sits in the winter months and how good the soil is at draining water through it. Where an area suffers from clay based soils and/or a high-water table the functionality of a drainage field might not be viable or legal. Often short cuts are taken!


3.  Infrequent deludging of the preceding septic tank or sewage treatment plant

How many times do we hear “Oh I’ve never had my septic tank emptied!”. A septic tank acts to settle out raw sewage allowing the clear water to enter the drainage field. The more sludge within a preceding tank, the less clear water there is available to flow through into the drainage field and therefore solids start carrying over into the drainage field and can cause blockages.


4.  Damage to the preceding septic tank outlet dip pipe

Quite often we visit a property with a problematic drainage field only to find that the septic tank dip pipe is no longer present, it has broken off and now resides at the bottom of the tank. The outlet dip pipe acts to stop settled material in the septic tank travelling through to the drainage field. If it is not present, solids can migrate into the drainage field causing blockages.


5.  System design issues including undersized septic tank and/or drainage field

We do just come across private drainage systems that are undersized for the application. Reasons include the age of the system, extending a property over time, or possibly just poor engineering. When a system is undersized, the result is often a failed drainage field.


Reading the list above, you will now understand that your drainage field failure may not be due to acute damage or a ‘breakage’ but actually something simple such as a lack of maintenance, a poor original design or a seasonally high-water table.


Can a drainage field be repaired?

The simple answer is no. Jetting a failing or blocked drainage field can prolong its life pushing the problem out of the pipes and into the surrounding aggregates but it will never fix the problem. Such actions only serve to extend the cost of replacement further into the future.


You have a worrying suspicion that your drainage field is failing… now what?

All too often people cut corners with the installation of a new or replacement drainage field. As we say at WCI, do it once and do it right. A drainage field should not be installed without the proper percolation tests being conducted and the drainage field being sized and designed by an expert in accordance with the British Standard BS 6297:2007+A1:2008.

WCI can guide you through undertaking your own percolation tests. The work involves you digging a minimum of three holes, carrying out the tests to a supplied specified method and submitting the data, field sketches/plan and photos for analysis. Unable or uninspired? WCI are able to undertake these works for you! Contact WCI on 01984 623404 and talk through your options.

My own home in Somerset is served by a concrete block built and rendered septic tank with a drainage field. I do understand that foul drainage is a cost that most of us would like to ignore. Well let’s face it, most drainage is underground and isn’t visible. It doesn’t have the same wow factor as new kitchen work tops or a new car! But it does prevent the pollution of your immediate environment and provide you with the ability to flush your toilets, shower and wash your clothes which are pretty crucial necessities.


If you are experiencing any of the aforementioned signs of drainage field failure please call WCI on 01984 623404 and arrange for a FOC survey of your existing foul drainage system. WCI will diagnose the problem(s) and supply you with a written design and quotation covering the solutions available.


Written by Naomi Taylor, Director of WCI and a personal septic tank, and drainage field owner!