Drainage field (soakaway) question

[SelfBuilder101]

New to the forums so hello. I hoped someone in the know could please help me with what is probably a simple question.

I'm working out how much space I need for the drainage field of a new build and wondered what constitutes the drainage field area. Part H helps you work out the area needed but doesn't define where the edge is in relation to the trenches/pipes.

Let's assume I need 48m2 and I have a single loop with the two laterals 2m* apart (regulated minimum gap H/1.42). Can I assume the drainage field extends 1m outside the pipes as in my sketch? If not, what distance should be used?

* 2m dimension would actually be more like 2.5m as needs to be 2m of undisturbed soil between

Reference: https://www.gov.uk/government/uploa...achment_data/file/442889/BR_PDF_AD_H_2015.pdf

 

[Hugh Jaleak]

1 metre should be ok. Are you planning anything adjacent to this area then?

 

[SelfBuilder101]

Thanks Hugh! Nothing adjacent to it other than the boundary. Building regs require a minimum of 15m to the nearest building and I have read 2m to the boundary. Positioning is tight, so I want to be sure I locate it correctly. Also, if it needs to be 48m2, I need to be sure I don't make the loop too narrow as it needs to be 4m equivalent width if I have it 12m equivalent length.

I was sure there had to be some distance to the side of the trench that still counted as in the drainage field and in the absence of anything else guessed it was 1m due to the 2m minimum requirement between adjacent trenches. I had hoped there was some form of guidance or best practice on this. Is that how you'd design the drainage field?

 

[Hugh Jaleak]

If space was limited then I'd have to go with what was available, as you are. I presume you've done a percolation test? Currently you're assuming a need for 48m2, I'd do the calcs and then see what you require.

 

[SelfBuilder101]

Right, big mistake in my assumptions here . Looking at BS6297:2007 section 6.3 you only consider the area of the trenches themselves, not the land in between. Figure 1 in Part H is a bit misleading in that regards.

The process is calculate the required area based on number of people and percolation rate (using a lower factor if a package treatment plant I think TBC). This area is then converted into a linear trench length by dividing that area by the width of the trench (between 300mm and 900mm).

Example: if the required area is 48m2 and the chosen trench width is 500mm then you need 96m linear trench depth. If done in 4 branches that works out as taking up 24m x 8m = 144m2 of ground area! [ 4x500mm +3x2m = 8m]

Section 6.7 of BS6297:2007 contradicts part H and says:

"Trench sides should be separated by undisturbed areas of ground which are greater than 1m wide"​

rather than the 2m part H says. Would be interesting to know what BS 6297:2007+A1:2008 says here.

Please note: I haven't seen the newer BS 6297:2007+A1:2008

 

[SelfBuilder101]

Thanks Hugh. I haven't done the official percolation test yet, but it is reasonably free draining chalk subsoil.

 

[Hugh Jaleak]

May not need to be that big then. This area is clay, have seen them acting in reverse, I've emptied the tank and its filling up again via the outlet as the surrounding soil is waterlogged!