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 June 27, 2025 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 June 27, 2025 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 June 28, 2025 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.

 

[David Cooke]

I'm certain the single rectangular drainage field does not comply with the General Binding Rules or BS 6297. 

Might it be permitted by the EA? 

I'm not sure!

[saveasteading]

@Gus Potter and I have been discussing this in detail.

The resolution? Use the appropriate national building reg's and don't mix them. The other documents are guidelines even if better written and used as the basis for the national regs.

Without  checking, don't the GBRs say to work to the building regs?

 

To my mind, option 1 will work well but not comply. More pipes would make it comply.

Option 2, looks like the book says, except one trench may be too near to the bank.

[David Cooke]

Should we consider BS 6297 as just a guide when applicable to England and Wales?

There are conflicts - for example undisturbed ground between trenches. BS 6297 says 'equal to or greater than 1 metre'. Part H says 2 metres. 

That's a huge difference when it comes to designing drainage fields!

[Gus Potter]

10 hours ago, David Cooke said:

Should we consider BS 6297 as just a guide when applicable to England and Wales?

There are conflicts - for example undisturbed ground between trenches. BS 6297 says 'equal to or greater than 1 metre'. Part H says 2 metres. 

That's a huge difference when it comes to designing drainage fields!

 

@David Cooke great post for spotting this.

 

For the record @saveasteading  CEng asked me for a second opinion on this as he had also spotted conflicts and wanted to sound me out. I can't take any credit for his astute discovery. 

 

@David Cooke both of us are interested in how we address this. I won't say any more just now as the credit has again to go to @saveasteading for spotting this anomaly.

 

It's not just an anomaly, it can potentially lead to a significant increase in size of the drainage field. as you say David. . which is very expensive or if you don't have the space can be a major show stopper. 

 

I'll leave it at that for now and let @saveasteading to flesh this out for you when and if he has time. 

 

 

[saveasteading]

@David Cooke there are other anomalies too.

I'm designing for myself in this instance, and there is lots of ground. I'll make up my mind when I see in the trenches and test the porosity. A balance between the codes and my own opinion.

But if I was designing for someone else I would follow the codes , including taking the more cautious options, because

1. I don't want to have to come back to sort it. 

2. There is professional liability.

3. What if it didn't work and esp if it overflowed into adjacent property?

 

With some clients I might explain, and share the risk/ benefit. With others I'd take no chances.

 

As @Gus Potter and I established, the regs take precedence but as professionals we would see that they had probably miswritten the regs and that the intention is clear.

 

Also as professionals we can decide on what is best. The regulations are actually the little green bit at the start and the rest is suggestions based on research.

 

Our drainage field is into what was once commercial orchard. I'm expecting the percolation to be very variable, depending on old drainage and voids created by roots. An average would be meaningless if the percolation numbers are say 40s/mm and 120s/mm. 

 

What I expect to do is put in the smaller area of trench but have scope to extend it.

 

For the record I have established that all groundwater will eventually end up 800m away downhill in a stream.....which we could pipe into  direct technically.

So we are really doing SUDS in slowing down flow and flooding.

 

BUT if a bco wanted proof they'd expect it to follow the regs. 

 

[David Cooke]

I've not tested this but I do wonder - if you presented the EA with a field design with 1 metre spacings between trenches (ie fully compliant with BS 6297), would it be approved as a bespoke solution (ie do this in response to a stubborn BCO)?

A British water engineer I know often specifies 1 meter between trenches on tight sites.

This part of the regs needs overhauling asap. There are tens of thousands of poorly functioning septic tanks across the country in gardens that have no hope of accommodating a compliant drainage field. All you have to do is go for a cycle around your local area in this weather to smell it!

 

 

[David Cooke]

This document titled 'Advice for local authorities on non-mains drainage from non-major development' is found on the planning portal. It's purpose is to assist BCO's in deciding whether a scheme needs EA involvement or not.

It's flow chart references BS 6297 and the general binding rules (which itself references BS 6297) consistently for determining whether or not EA involvement is required - but not part H. 

 

EA_LPA_advice_non_major_dev_non_mains_drainage_2019.pdf

[David Cooke]

Anyone with any success producing irregular shaped fields for awkward sites?

[saveasteading]

26 minutes ago, David Cooke said:

Anyone with any success

I've never had a design or installation queried for foul or surface water.

I'd have to read the docs again, but I think the proposal of a Chartered Engineer working to the principles of these docs and referring to site specifics will be accepted.

 

Eg. Scotland allows 20% reduction when using a treatment tank. England does not, but it is in the tech papers so apply it if it's necessary.

 

Basically if you use a good treatment tank it's going to be ok, they want it to be OK, and so come up with a justification.