Drainage Field Sizing

[bodger53]

Good afternoon, 

Hopefully an easily answered question...

Does the drainage field area include the untouched strips of ground between parallel trenches or is it only the trenches that count towards the area?

Part H just refers to the 'floor area of the drainage field' and doesn't specify.

Thanks in advance!

[ProDave]

When we did ours, we did not dig "strips" we dug a rectangle area of the calculated size, covered the whole area with stones, laid the pipe in "strips" on the stones, covered the whole lot with more stones, then membrane and soil back on top.

[saveasteading]

13 minutes ago, bodger53 said:

floor area of the drainage field'

The stone base, as in PD, allows the water to spread over the whole area. so it all works.

This is a good solution as it avoids any pipe being lower and taking all the waste. Easier to build too, without trenches, muck falling in, fiddly membrane. 

[Drellingore]

I just found this thread in a search, and thought I'd answer for future readers.

 

Approved Document H2 references the infiltration area that you need, and not the overall size of the drainage field. In addition to the area required for infiltration, BS6297:2007+A1:2008 says that there must be at least 1m between trenches, and a 1m margin to the sides of the outermost trenches. It is of minor note that Approved Document H slightly contradicts BS6297:2007+A1:2008 in mandating a minimum of 2m between trenches. 

 

So assuming the widest trench width mention in BS6297, the formula for width is: (num_trenches * 0.9m) + (2m) + (num_trenches - 1m)

 

Depending on the number of trenches in the design I'm working on, the overall area can be 2.3x to 5.2x larger than the infiltration area.