Drainage and soakways

[Kelvin]

Our build requires a soakaway for the treatment plant and separate rainwater attenuation. 
 

 We did some percolation tests last year so have identified the ideal location for the soakaway taking into account the burn at the bottom of the field, the neighbour’s borehole and our borehole. We had a  design report completed for planning which states it needs to be at least 18.6m² in size made up of two 9.5m trenches 1m wide entry via a distribution chamber. The warrant has come back and design has been changed to 3x8m trenches. I’ve yet to get an explanation for this change from the architect. 
 

The plot is on sloping land which flattens out as it approaches the burn which will accommodate the soakaway. One of the groundswork companies I spoke with said that, in his experience, he prefers installing single trenches rather than multiple trenches with a distribution chamber especially on sloping land. His view is that often what happens is the liquid ends up biased towards one trench causing early failure. I’d have thought multi-trench soakaways to be a common solution as not every site will have the space for 19m+ long soakaways. However his comment has raised a concern. Any views on this? 
 

The drainage design I had done stated that there was no legal requirement stopping me running directly into the burn so didn’t suggest we do anything but that. The warrant says we must install a rainwater soakaway though which is fine. I was already working on the principle that I don’t want rainwater leaving the plot any faster than it did before we built the house. Plus there is a flooding risk downstream of us so I don’t want to be adding to that. The architect got the SE to do a drainage soakaway design which is a single 14mx1m trench with all rainwater running into it. Would it not be better to have multiple routes for the rainwater to run into? Even if it was a minimum of just separating the garage from the house. 

Edited November 25, 2022 by Kelvin

[Kelvin]

Any thoughts? 

[crooksey]

Usually drainage fields by design are "circular" and feature multiple "trenches".

 

He probably wants a single trench as its easier to dig, but usually a building controller will want a traditional drainage fileld.

 

14m3 is a massive rainwater soakaway, is it staying as an open trench or is it getting filled with soakaway crates? Or even slotted pipe and shinge backfill? If you need a sokaway that big (14m3) I would be seriously worried about your percolation as that is massive for rainwater.

 

Usually for a remote newbuild I would expect rainwater to go into a tank (for harvesting) then the excess/overflow to discharge to a 2m3 soakaway or to watercourse/ditch if you have one nearby and permission.

[Kelvin]

Thanks for replying. 
 

The groundswork company I’ve finally selected specialises in drainage and he’s quite comfortable with what’s been proposed for the foul soakaway. However, he also said the rainwater soakaway is pretty big and he’s recommended multiple routes for the rainwater anyway. Our percolation test was fine so there are no issues with that. He thinks it’s typically just over engineered. 

[crooksey]

At the end of the day, its all down to what building control are happy with. If you have a percolation test, calculated with roof area and pitch, you can easily calculate soakaway sizes with approved calculators. Good luck.