Drainage pipe depth. Minimum 600mm or is that unnnecessary?

[saveasteading]

I've been designing drains for nearly 50 years. 

Wrongly perhaps.

 

Today I've been told, and referred to document H, that there must be at least 600mm cover.

And that less than that needs a reinforced slab over it. 

 

Its only a reference in a table, and nowhere else is it mentioned. The actual  regulation doesn't mention any minimum.

 

I can see no purpose in that depth in a garden.

 

A foul drain isn't going to freeze at any depth.

 

Elsewhere online, 450 is mentioned.

 

It matters especially on long runs.

I have been avoiding pointless expense. Nobody has ever said it should be deeper.

 

I am allowed to over-rule this but perhaps there is a good reason I've missed.

Any thoughts or better if it's definitive?

 

I've lost my BS on the subject.

[Gus Potter]

1 minute ago, saveasteading said:

I've been designing drains for nearly 50 years. 

Wrongly perhaps.

I doubt that you've been doing it wrong!

 

2 minutes ago, saveasteading said:

Today I've been told, and referred to document H, that there must be at least 600mm cover.

And that less than that needs a reinforced slab over it. 

Aye and no.. depends on where it is and the loads on ground.. vehicle traffic or garden load for example.

 

3 minutes ago, saveasteading said:

I am allowed to over-rule this but perhaps there is a good reason I've missed.

Yes I think so.

 

Is this the kind of thing you are thinking about? Passed by Scottish BC... more than once.

 

The main BS is BS EN 752.. the head code I'll not post it here for copywrite reasons. A good heads up is to look at the old Scottish regs. The detail below is based on these. I'll try and dig it out but a lot of the recent regs are silent on this.. they just refer you to the Eurocodes!

 

 

 

[G and J]

Our main sewer pipe invert level is only 500 mm at the front of our plot and the required falls mean that gets less near the house. 
 

The paver over the drain looks like a bloody good idea regardless - is it just laid there and then covered up or is it cemented in place?

[saveasteading]

8 hours ago, G and J said:

is it just laid there and then

That's sensible if it might ever get some load.   But the regs say a reinforced concrete slab.

Imagine thd maximum load on that footpath...what possible risk is there to that pipe?

I'm looking at garden though. 300mm for a garden fork risk. 450mm in the Cairngorms for frost getting into the ground. 

600mm? Not "sustainable design".

 

I'm still thinking it is crazy overdesign. Maybe the code compilers assume that trenches are never compacted or all have driveways over them. Or maybe they have never been out of their office.

 

A search threw up a previous BH discussion. 450mm quoted by some, and that appears in some unofficial advice elsewhere too.

 

[G and J]

1 hour ago, saveasteading said:

Or maybe they have never been out of their office.

It feels to me like there are more and more instances of regulations being added for the 0.1% of cases, as we don’t, as a society, take a balanced view.  Instead each instance is driven by litigation prevention.  Sigh.  
 

So we pick our way through, complying with everything, but still trying to build as best quality we can despite the regulations.

[Russell griffiths]

With most things in life you need to add in a big chunk of common sense 

400mm below the lawn= pea gravel and backfill

400 below a pedestrian path = pea gravel + compacted type 1 then path

400below vehicle traffic = pea gravel, then a concrete covering spanning the width of the trench and continue onto solid ground. 
most shallow pipes are near the house, they obviously get deeper the further they travel, so being only 400 deep by the time you get around the front of the house is rare. 
normally very shallow near back of house, I’ve seen plenty with only 100mm concrete cover and then paving. 

[saveasteading]

2 minutes ago, Russell griffiths said:

 a big chunk of common sense 

400mm below the lawn= pea gravel and backfill

 

That is my view entirely. As "the Engineer" that's what I have designed and nobody has ever objected.  And there have been no problems that I know off.

In so doing,  several km of drains have been 200mm less deep and without a concrete slab over. Consequently so has every rodding point, manhole and sewage digester.

 

But someone is pointing out table 10 of Doc H. That is the only point of reference.

There are contradictions too  eg the drawing below and the term "where necessary"

 

I'd rather not respond  " because I'm the Engineer and I know better".

 

I really want to find something definitive. And so would everyone about to start a project.

[saveasteading]

I meant to attach this for anyone interested. It shows depths are to be over 0.6.

 

Anyway I have prevailed  and it will now be " as appropriate".

If I need an easy argument, it will be that "in a field" implies that it can be ploughed. In a garden isn't covered in table 10,  So panic over and 50 years of drains can stay where they are.

[ETC]

Minimum cover is 300mm in a garden 600mm in a vehicular accessible area. If less than this cover with a bit of concrete.

[saveasteading]

1 minute ago, ETC said:

Minimum cover is 300mm in a garden

Agreed that Is sensible. Is that written down formally anywhere?

[ETC]

https://www.buildingcontrol-ni.com/assets/pdf/TechnicalBookletN2012.pdf

[saveasteading]

Thanks for that @ETC. The page and the link.  This NI version I have not seen before. It very sensibly allows the shallower depths of 0.3 and 0.4 according to different bedding standards, on the pages before the one you swiped.

This shall be the reference from now. The English and Scottish versions don't have this, but imply 0.6 whatever. 

 

This table is for rigid pipes but it goes on to discuss flexible pipes and is consistent. I will be reading all of this fine document.

 

The week ends well (not just this).  So well that I'm now expecting our home rugby teams to win too.

 

 

this.

[ETC]

In reality on building sites drainage is laid on and back-filled with bedding stone - I rarely see purpose made drainage trenches.