How deep should sewage pipes be?


I'm just in the process of buying a house and I noticed that the sewage "pit" in the back yard is only about six inches deep, so the top of the pipes are only about two inches under the surface. I know that pipes are usually much deeper than this but I wondered if there is any problem with the pipes being at such a shallow depth? Has anyone else seen pipes at this depth? I'm going to phone the BCO on monday but thought I might get some info before I do, so that I know a little more first.
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On Nov 10, 1:15 pm, clangers snipped-for-privacy@yahoo.co.uk wrote:

Sewage drainage systems rely totally on gravity so the pipe fall usually 1:40 is the main criteria to ensure is maintained. Certainly do not lower the pipe without knowing the full details of where this connects into the Main Service otherwise you might start getting your own back (so to speak) and possilbe everyone elses..
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Ask the vendor why they are so high. If you are worried about them you could either use stronger pipe material, or protect with a strong galv steel sheet under the paving/concrete. Min gradients for peak flow: up to 1 litre/sec =1:40 over 1 litre/ sec =1:80.
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IIRC building regs says encase them in concrete if less than a certain depth below the surface. I can't remember the depth off-hand. Simon.
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