Hi, - more problems!

The main drain pipe from our house seems to have broken in 4 places in the
field adjacent our house. Some large vehicle must have driven over it at
some stage.

If I was to replace this section of pipe, is there any regulation as to how
deep it should be buried? I guess to maintain the drop this will involve
building up the ground around it.

The pipe is 9 inch internal diameter, made of clay(?) and goes into a mains
sewer.

TIA Colin