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[Eugbug]

So I'm building a concrete gate pier, 18" square by 53" tall. I'm butting it against an existing solid concrete wall. I'm going for more of a spread footing, making the foundation twice the width of the pier, 36" in one direction (so that's a 9" projection each side of the 18" pier).

Some questions:

To extend the foundation towards the adjacent wall which will be butted to the pier, I would have to undercut the wall and remove a section of it's footing. Do I need to do this, or could I rely on the wall (about 2' 6" tall) to prevent any leaning towards it? If I don't, the pier will be sitting on the edge of it's foundation.

I planned to make the foundation 12" deep, should I make it even deeper? E.g. 18"?

What strength mix should I use for the footing? I have seen C25 mentioned on forums?

I don't understand the progression in the strength of mixes

C20 is 1:2:4 cement, sand, stone
C25 is 1:1:2
C30 is 1:2:3

C25 has a higher proportion of cement than C20 and is stronger than it, but a higher proportion of cement than C30 and is weaker? I know the stone/sand ratio is also of significance, but does it follow that less cement and a stronger result can be produced by simply making a C30 mix?
Are these ratios correct?

[stoneyboy]

Hi eugbug
The depth of your foundations for your pier will depend on soil conditions. If you dig down 12" and it is easy to dig you must go deeper until it is difficult to get your spade in. If this means your hole is below the wall foundations you could do a bit of underpinning to stabilise the wall and pier.
On the mix you should be aiming for something between 1:4 and 1:5 cement to ballast.
Regards S