How shallow can plastic 110mm drain be?

[dnb]

I am getting the last few bits of my building control paperwork sorted out. One of the outstanding parts is a drainage plan.

 

I have read through part H and it indicates that I should have all plastic pipe at a depth of at least 600mm. Am I understanding this correctly? Because on the initial submission for building control my architect has some inspection chambers with inverts 100mm to150mm or so shallower than this 600mm from table 10. All of the areas in question are lawn with no vehicle access requirements. 

 

I would prefer to go with architect on this because the site has a high water table in winter and this could easily flood the treatment plant if it has to be placed lower in the ground (as happened to my neighbour this winter)

Edited April 20, 2020 by dnb

[andyscotland]

Marley say it can be shallower in non-trafficed areas so long as covered by at least 75mm gravel then paving slabs as per their installation document:

 

I emailed to confirm what the "where necessary" meant in case there were exceptions to this, they said it meant where necessary to put it shallower than 600mm.

 

They also said the paving slabs can be replaced with 50mm poured concrete if easier (I had spare concrete materials on site so did this, scored at regular intervals to encourage any cracks to span the width of the trench rather than randomly fracturing over the pipe.

 

Yours seem very shallow though. A 100mm invert (assuming 110mm pipe?) means the pipe is only partly buried.... 

 

NB although the drawing shows the paving slab buried when I spoke to them they confirmed it doesn't have to be e.g. if very tight for depth you could have them under a paved path so long as 75mm gravel between the pipe and the path bedding.

[dpmiller]

Your BCO might have his own thoughts depending on local conditions. Ours was happy with 300mm cover

[PeterW]

As good practice, any pipe under 450mm depth should really be concrete encased, even under pedestrian areas. It is not a big cost and will give piece of mind. 

[ToughButterCup]

19 minutes ago, PeterW said:

[...]  It is not a big cost and will give piece of mind. 

 

Peace brother, peace! ?

[PeterW]

I hate autocorrect ... ??‍♂️

[dnb]

33 minutes ago, dpmiller said:

BCO might have his own thoughts

He certainly does! He is most concerned about ensuring the site doesn't flood and turn back into a swamp. See earlier comments about the high water table. 

[andyscotland]

27 minutes ago, PeterW said:

As good practice, any pipe under 450mm depth should really be concrete encased, even under pedestrian areas. It is not a big cost and will give piece of mind. 

 

Interesting. When I spoke to Marley technical they were of the opinion that uPVC pipe should only be concrete encased where unavoidable. They said it could cause more problems than it solved as the concrete was less tolerant to ground movement etc than the pipe so could crack and put pressure points on the plastic. If it was possible to use one of their standard details e.g. gravel and slab, they reckoned that was a much better option.

 

I'm not an expert in any way, just reporting what they said...

[Temp]

+1

I'd go with good quality pipe, gravel and paving slabs. Much easier to dig up if there are any problems but I reckon it will be fine.