Rainwater drainage pipes - depth/cover

[Rad]

Only joined this forum today and it's proving a great help.
Two more questions, hope someone can offer advice...
1. I am in the process of lowering the height of a section of patio, this means the cover over some 110mm brown plastic drainage pipe from a 450mm inspection chamber is down to 300mm. What is the recommended covering over this kind of pipe at this depth? It is under just a general use patio, nothing heavy duty.
2. Part of lowering the patio is to create two different levels, therefore I need to build a low retaining wall which will have 2 courses above ground level and 2-3 below ground level to make a step 3.1m long. This wall has to pass over the 110mm brown plastic drainage pipe in 2 places at an angle. Should I use a concrete lintel at these points, then encase the surrounding area (and pipes) in concrete? Or should I just use the lintels on concrete footings (at each end of the lintel) and then backfill on top of the remaining exposed pipework as advised in answer to question 1?
Any advice would be great.
Cheers
Phill

[lutonlagerlout]

300 cover is fine phil,you will need to lintel over the pipe though
pipes are best surrounded by 10mm gravel 100mm all round then leanmix on top of that
cheers LLL
Ps make sure the lintels have at least 100mm bearing each end
LLL

[Rad]

Again thanks for the advice. Here's my spec'

Will use 100x140x2100mm lintel, this will get me over the 2 pipes in one go and I figure that a 140mm wide lintel will be more stable and give me more room to lay the bricks on top. This will have min 150mm bearing at each end and will be supported in the middle too. Does this sound ok?
Cheers
Phill

[lutonlagerlout]

you want to lay it with the 140 mm side upright nd the bricks sit on the 100mm flange
the BMs round here call them R15 lintels made of reinforced concrete
cost you about £20
LLL