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So I need to make my drive way wider and intend to demolish the existing pier and rebuild it. The pier will about 18 inches wide and 4 feet high and solid concrete. My plan is dig a foundation, fill with hard core to a depth of about 4 inches and then cast a concrete slab 3 times the width and length of the pier and about 4 to 6 inches thick, adding some rebar for strength. In the center of this slap, there will be a vertical section of 6 inch girder, maybe half the height of the pier. Then I'll shutter and cast the pier in place on the slab.
Some questions:

1) Does the foundation slab need to be this wide and long? As far as I know wall foundations are supposed to be a width equal to 3 times the thickness of the wall.

2) Is 4 inches thick enough for the slab? For the section which projects into the gate entrance side of the pier, it probably does to cater for e.g. delivery trucks

3) The existing wall is solid concrete, about 6 inches thick and 3 feet high. Originally the wall had no support pillars and spanned 60 to 80 feet between passenger gate entrances. Presumably the foundation was cast with the wall and was sufficiently heavy with a low centre of gravity that there was no danger of it moving or toppling in wind. When I cut the wall back to make the gate wider, would it be a good idea to allow a piece of it to project into the shuttering so that when I cast the new pillar, it helps to support the wall?
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