I will be bolting a wooden gate post to an inverted T-piece of galvanised steel, itself bolted via M12 stainless studwork to a lump of concrete. The post top is braced in all directions - to a house wall via a rigid cross-beam, to the ground either side by stainless wire rope.
So the lump of concrete need be quite minimal re just providing location. The inverted T-piece lifts the wood off floor level and also permits easy removal/replacement if ever necessary.
A drain passes the post, diagonally about 6in at its closest point and about 12in deep to the top (not the invert). It is a normal brown glazed drain, I have just exposed the top edge to get location & vector info, it is buried direct in clay (no 10mm pea gravel).
Am I right in recalling a rule on "foundations" if there is a drain nearby you go below its invert level due to load spreading at a 45- degree angle?
I was planning on a simple 6in thick slab. The gate hinges away from the drain both when closed and fully open. Moving the gate would be quite messy (scouring the walls of past messy painters for one thing). The gate is wood 1.4m wide, 1.8m high, not 4in box steel.