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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 soakaway) failure increase. Year after year the pattern continues and quiet often we get reoccurring 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 but find themselves right back to where they started the following winter!

If I had to condense and simplify my top five reasons for drainage field failure, the list would look like this:

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 clayey 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 I 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, 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 traveling 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.