manhole cover/pipe positions

[jb1468]

I am preparing the ground for a deck area in my back garden and have exposed a manhole cover that I never knew was there (someone at some point had covered in about 8 inches of soil when levelling an area of the garden) My problem is I need to place a post in that area to support the decking and am concerned about accidentally causing damage to whatever pipes are under or around it. Without going into fine detail I really need to put a post in that area and cannot just avoid it so a solution may be required.

The manhole cover itself is surrounded by or set into concrete about 45 inches by 24 inches. I assume its sewage/waste water but not really sure if it can be for something else. I live on a slope in the middle of 5 terraced houses (2 higher above me and 2 lower below) and I assume that this drainage runs at least for these 5 houses. The property was built in the 60’s. Questions –

1. How deep can I assume the pipes are below ground or can I assume anything.
2. Does anyone know the circumference size of the pipes underground as I may be able to move enough sideways for the post to avoid it if it is not deep enough to avoid. 3. If I was to move sideways is it safe enough to assume that these pipes run straight down the hill/garden from the manhole cover to serve the houses above/below and not sideways or any other angle for example to serve houses backing on to these 5.
4. What are the underground pipes made of and if careful digging down how easy are they to damage/split if I calculate wrong or they are places at a height/angle that is unexpected.

Thanks in advance – I know little about outside drainage/waste

[collectors]

1. How deep can I assume the pipes are below ground or can I assume anything.

From 12" to many feet. Its all down to what fall was needed when it was installed.

2. Does anyone know the circumference size of the pipes underground as I may be able to move enough sideways for the post to avoid it if it is not deep enough to avoid.

Usually from 4-6" is used

3. If I was to move sideways is it safe enough to assume that these pipes run straight down the hill/garden from the manhole cover to serve the houses above/below and not sideways or any other angle for example to serve houses backing on to these

Usually the pipe will serve a small block of in line house (5ish). But best bet is to lift the manhole cover & have a look.


4. What are the underground pipes made of and if careful digging down how easy are they to damage/split if I calculate wrong or they are places at a height/angle that is unexpected.

I would avoid placing anything directly above a pipe as it may damage in time.
Pipes from the 60s could be salt glazed earthen ware or plastic. Would opt for the 1st one.