On 09/09/2020 22:22, Tricky Dicky wrote:
Since you already have laid the base and the purpose of the extra width is there to prevent all the weight from being right on the edge thus prevent it crumbling. I would dig out a trench all round the base to the depth of the keels allowing an extra 150mm all round to be added. This could be reinforced with rebar and as a belt and braces to tie the extra concrete to the existing base I would drive some long concrete screws into the side of the existing base with about half the screw protruding.

I was hoping not to have to do that. Would it really crumble? A quick
calculation on moment just now (!) (assuming downward force exerted by
wall/roof is 1750kg/side at 0.05m in from edge, force of keel alone
centred 0.3m in 1440kg/side) in will easily balance, unless I have ocket
that up too, so rotational force won't be an issue, Mix is C20, approx.
However, if I am to do anything, perhaps I could go out further on one
side/front, and leave the other sides as they are. 2/3 cubic metre of
concrete, about. I blame the booze/.