Drainage field design

[flanagaj]

Can anyone explain to me why you can design your drainage field in one of two ways.  For example, say you have calculated your drainage field area at 24m2.   You could go with one of the two options below.

1)  40m of trench at 600mm wide and create something like below.


2) Excavate a rectangle of 8m * 3m and fill the whole base with appropriate gravel before laying the pipework.

Now option two will have a lot of gravel in the middle of the drainage area that is some distance from the pipework, where as option 1 does not have that.

Maybe overthinking this, but I suspect that if you did field tests, you would get different results from the two options.   I don't have the space for option 1 and option 2 is going to be difficult as I don't have a lot of space for moving the excavator around.

I did speak with BC regarding infiltration tunnels, but they simply told me to apply for permission from the EA.  Given it's > 2k to apply for a permit and it seems like a lot of work, i think I will just have to go with option 2.  

It's rather frustrating, as Graf told me that some councils are perfectly happy with the infiltration tunnels, so long as you are not in a source protection zone.

Edited Friday at 07:22 by flanagaj

[JohnMo]

Attachment?

[IanR]

38 minutes ago, flanagaj said:

It's rather frustrating, as Graf told me that some councils are perfectly happy with the infiltration tunnels, so long as you are not in a source protection zone.

 

I'm not sure what you mean by "infiltration tunnels" is it a specific product?  However, it's not a Council decision, it's a National requirement. If you are going to have a private sewage system in England then you as the operator have to either meet the General Binding Rules or get a permit from the EA. Building Control should require you to do one or the other as per your local council. As the "Operator" of an STP without a permit, you are criminally liable for the STP meeting the GBR's. The wording as been softened in the 2023 version of GBR's to now say they will only bring a criminal prosecution if your STP causes pollution AND you do not meet the GBR's.

If the "infiltration tunnels" can be used to construct a drainage field and meet BS 6297:2007, there should be no problem.

Edited to add a relevant previous post:

 

Edited Friday at 07:53 by IanR

[flanagaj]

15 minutes ago, IanR said:

If the "infiltration tunnels" can be used to construct a drainage field and meet BS 6297:2007, there should be no problem.

Edited to add a relevant previous post:

 

Yes.  It is the Graf infiltration tunnels as stated in the post you have linked to.  I doubt they are BS 6297:2007 as that is the GBR.  Which is why Graf are saying that some councils are ok with them being installed and others are not.   Maybe said councils should be requiring a permit in all instances.

[flanagaj]

23 hours ago, JohnMo said:

Attachment?

Here are two different ways of installing a drainage field.  The separate trenches requires considerably more space to create the same m2 field, when compared to the excavated rectangle.   Question is.  do they actually perform the same?  I would hazard a guess and say that the separate trenches is a much better design to the other one.

[dpmiller]

which uses more tonnes of stone?

[flanagaj]

12 minutes ago, dpmiller said:

which uses more tonnes of stone?

I am aware of that.  But that is not what I am trying to ascertain. 

[dpmiller]

34 minutes ago, flanagaj said:

I am aware of that.  But that is not what I am trying to ascertain. 

it answers the question tho

[JohnMo]

51 minutes ago, flanagaj said:

Question is.  do they actually perform the same

Does it matter?

 

You have two applicable routes to go down, option 1 or 2. You make your choice and do it, why analyse and question, at the end of day choose one and move on. One being more or less effective changes nothing. You still have options 1 or 2!

 

Sorry to be blunt, to much time thinking and not enough getting on with it.

On 27/06/2025 at 08:14, flanagaj said:

don't have the space for option 1 and option 2 is going to be difficult as I don't have a lot of space for moving the excavator around.

Option 1 is out, so leaves option 2.  Not enough space to move digger about, get some with a digger that knows how to use it. They will step the digger over trenches as they go. You are not going to any more space constrained than our foundation, top of image is a 3m drop to right 6 to 8m drop. All compacted sand. Left of image is a root protection zone digger wasn't allowed to enter.

Edited Saturday at 08:00 by JohnMo

[flanagaj]

2 minutes ago, JohnMo said:

 

Sorry to be blunt, to much time thinking and not enough getting on with it.

I am known to procrastinate!

[Kelvin]

6 minutes ago, flanagaj said:

I am known to procrastinate!

It’s a killer on self-build. You need to impose deadlines on yourself and stick to them. Once decision made you move on to the next thing. As a procrastinator too I need deadlines without them and I get bogged down. 

[saveasteading]

More of the district  trench runs allows the water to find fissures or areas of higher permeability. Eg ancient tree roots. Joiningbtgem all up, in any wat at all, let's it flow all over for temporary storage, thence to soak away.

Remember that gravel is 2/3 stone and 1/3 air or liquid.

Re big areas though, when you flush a toilet and 3 litres goes to digester tank, then 3 litres comes out. That will usually disappear in the first few m of drain and will never reach the middle of a big expanse of gravel.

 

A bath load will reach further of course. 10m?

 

I'd rather like to meet the people who designed these complex drainage field principles. I might establish is it me or them who don't understand the reality.

 

OP: don't worry. It's overdesigned so any will work.