Required depth of 5" drains in back garden?

Hi all, Is there any legal requirement regarding the depth of 5" plastic drains in a house's back garden? I've just acquired a property where a pipe run has been left only partially buried in shallow trenches. I guess the owner planned to raise the surface of the surrounding ground. I have now raised the level of the surrounding ground using broken concrete blocks, (which needed disposing of anyway). If I then make sure the drains (and broken blocks are covered by 4" of crushed limestone aggregate, is that sufficient? Is that a suitable material to cover the work with? Part of the pipe run is for sewerage and part is for rainwater drainage which runs unto the sewerage section.
The drain is close to the back of the house, so no vehicles are likely to drive over them, except at one section, which is the rainwater drainage section. Should I encase the drain in concrete at that section?
Thank you, Alan
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mmm legal requirements or not I'd start off investigating as quietly as poss. if anyone (in authority) knows about this 1/2 finished work - if they do and they are expecting and requiring it to be done properly then they are the ones you need to please and they are the ones who will tell you what to do...
If no-one knows or you fancy a gamble - cover it over as you see fit - 10mm pea gravel is usual for bedding and covering drains - depth I don't know offhand but someone will be along with the "offical" line soon. Where the vehicles "may" run then concrete encasement would probably be a better bet.
JimK
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JimK wrote:

I got to know a lot about bloody drains, building this house. And got to like them, too.
The most overriding requirement is to get the fall right: This is subject to building control as is the second most important thing, to keep them straight and arrange a manhole or rodding point when they are not.
They should ideally be bedded in something that wont cut into them - pea shingle is a good cheap solution. Limestone and rubble is not. Its OK higher up, juts not round the pipes, if any vehicles are ever to go over them.
I do not believe there is a minimum depth: they are not as far as I remember subject to protection against freezing, as half the time hot water goes down them. This is in direct contrast to incoming mains that do have a minimum depth for anti-freeze reasons!
The final requirements are to be moderately gas tight to ensure no leaks of foul water into the local ground water, and to be vented againts build up of methane and the like.
That is an entirely different reason for venting compared with the other requirement that may be fulfilled by a vent or air admittance valve, and that is to not have the suction of a flushed loo or emptying bath suck traps dry, and allow noxious odours into the bathroom..
Oh and there is a minimum diameter constraint for non pumped systems as well.
That's about it.
no minmum depth. Heck lots of soil pipes run down the sides of buldings from upstairs loos, and those can, and do freeze!
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wrote:

<<<some snipped for brevity>>>

Many thanks for the helpful response! Yes, you confirm my suspicion: that the work was actually approved by building control, but has just not been covered over. I must go and buy the said pea grave...
Al

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