30 January 2015
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Soakaway Construction

Soak away construction

Cotterill Civils have vast knowledge and expertise on soak away design and construction. For a soak-away to work correctly, efficiently and have longevity, its design and construction are very important.

It is a common misconception that a soak away is just a trench with perforated pipe and a gravel backfill. Even worse practise is to dig a deep hole and fill it with hardcore. First of all the depth of a soak away is critical. Ideally no deeper than about 600mm. If it is too deep there is little or no oxygen in the soil to aerobically digest the effluent. Also the chance is that the soak away will be down in the subsoil, often clay, which is impermeable to water. This of course will render the soak away useless.

If it is not possible to achieve a shallow soak away installation by gravity, the need to pump the effluent arises. It is most important not to install deep just because a tank outlet invert is deep and seemingly there is no other option.

Another important consideration is pipe fall. As much as possible the drainage runs should be more or less level. This allows the effluent time to lie in the drain and soak away rather than taking the easiest and fastest route down a falling pipe only to create a wet spot at the end of the pipe run. If the land is falling steeply in the area the soak away is being constructed, it will be necessary to “terrace” the drain runs across the slope as opposed to letting them follow the slope down.

Another problem with soak away fields is the destruction of soils hydraulic properties from prolonged addition of sodium. Modern households discharge large amounts of sodium in the form of salt, sweat and washing powders into the foul drainage system.

When chemical (cationic) exchanges occur from sodium, fines or clay particles may bond together forming a waterproof barrier. This impermeable barrier prevents soak away through blockage of soil passages and biological death of air-dependent cleaning organisms in the soil. It is generally considered that waters containing 50 percent of total cations (sodium, calcium, magnesium and potassium) as sodium are potentially harmful to soil absorptive characteristics. Even in sandy soils, waters of 85 percent or higher are likely to make soils impermeable after prolonged use.

This is not to be confused with “Sodium Binding”, which is a biological term for the cationic or chemical bond that sodium makes with enzymes or proteins, in order to function properly in living organisms. If you are on a septic tank system with a soak away you can help reduce or prolong this problem by using sodium free water softeners, baking powder, hair care products etc.

As you can see there are many influencing factors that contribute towards the effectiveness and longevity of a soak away drainage field. For expert advice or more information on this subject please contact us.



Cotterill Civils Blog 


Author: Andrew Cotterill 

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