Most of the pipe is buried to a depth of 600 -- 900 millimetres ( 24 -- 35 in ) .
It is deeper and has thicker walls in areas of high risk such as under road crossings .
Most of the pipe is buried to a depth of 600 -- 900 millimetres ( 24 -- 35 in ) but it is deeper and has thicker walls in areas of high risk such as under road crossings .