Slab edge formwork is usually supported off the sides of the excavation.
But sometimes I'm able to do it a bit more easily, cutting less timber.
I like to set out and bang in wooden pegs before I blind with concrete. Get the blinding concrete all round all the pegs. Fix a scaffold board to the pegs so the top of the board is top of concrete. The gaps can have some mud behind that stops the flow of concrete underneath.
Use 2x2 not 4x2 because you will need to snap the pegs to get the boards out.
These examples would be more usual.
This is so neatly done they used expanding foam to prevent concrete leaking out.
It is important that the waterproof concrete does not lose much grout through narrow gaps because to be waterproof it needs enough cement to fill all the spaces between all the grains of sand.
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