New workshop build on pier foundations

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You have contradicted yourself by saying a raft would have no benefit if the ground was subject to movement, on suspect ground a reinforced raft foundation would be the first choice, only the top soil needs to be removed, a thick layer of MOT type 3 is not needed for a shed, just a sand blinding for the DPM, even on a piled foundation the pile caps would be tied together with ground beams, compared to a Raft foundation, piers are more likely to be impacted by tree roots not less.

I shudder at the pic's of joist on very small rubber supports, the metal bracket idea introduce's two point loads with the bolt heads, but what do I know as a retired Chartered Structural Engineer you would think I would have learned something in the fifty years I was in practice. 🥴
As I said, you need to consider my post in relation to the OP.

he stated he lives in a terrace house, making removal of top soil on a slab of 20 square metres unrealistic, If the top soil is 300mm deep that’s 9 cubic metres of soil out and 3 cubic metres back in. (I’ve assumed 50% bulking factor and 150mm concrete)

where you state “only the top soil needs removing” well how deep is the top soil? I’ve seen sites where top soil is 200mm deep and I’ve seen top soil 800mm deep.

You don’t know the soil type, top soil depth, it’s load bearing capacity, or it’s soil volume change potential, so I’m not sure how you can decide a raft needs no hardcore.

I agree that individual piers wont have the stability of piles with a ring beam, however we are talking about a timber structure which has considerable more flexibility compared to a masonry structure, constructed with cementitious mortar.

In my opinion, 16 piers would be better than using a thin slab on top soil with risk of cracking.
Topsoil in the case of a shed construction would be defined as the depth of soil with sufficient nutrients to support ground growing plants, no need to go any deeper.

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