Footing depth below pipe

[jamesdean1986]

I have been helping my friend dig his footings for a single story extension at the rear, as wide as the house and 3m deep.

He is replacing waste water pipe that runs through that area but was suggesting that all 3 sides of the footings need to be 600mm below the bottom of the pipe. which due to the land at the side of his house already being lower, it will actually make them all about 6ft deep.

I was suggesting that only the footings around the pipe need to be 600mm lower.

Can anyone clarify?

 

[noseall]

Is that a foundation trench going directly in line with drain pipe?

 

[jamesdean1986]

the trench running on top of the pipe has been dug out so he can replace the pipe, but there is a trench to the left which runs parallel with it

 

[noseall]

It is only those foundations within an influencing proximity to the drain, which need to be deeper than the drain.

You would assume the force or push on the footings typically has a 45 degree diagonal influence on the ground.

You don't necessarily need to be 600mm deeper than the drain either, but you do need adequate clearance above and around the drain should you lintel over it.