Workshop Mk2

[i_am_fubar]

I have a decent brick built workshop in my garden (yay)
It's in a really sunny area and deprives us from more sunny usable space (boo)
I have some space down the side of the house to build a new workshop (yay)
But it's a really tight squeeze and has some technical challenges (boo)

I've been digging the site out for about 5 years now, unsure what exactly to do with it and at the moment it's looking like this:

Measures about 6x2.8m. This is now ~600mm below original grade and is dense clay.

I want to maximise space, so I'm going single skin and due to a roadway beyond the fence, can't do strip foundations or double skin without bringing the wall further into the garden or eating into internal space. I am literally trying to shave off cm to get this to fit to a full block. And while I'm not planning to heat this at all, I would like to insulate my foundations so the temperature swings aren't as bad.

My current cross sectional plan is:

I appreciate this isn't a typical build as it's not a typical site.

The stainless flashing will be custom made to ensure the DPM is protected from termination down to below grade.

My only worry at this point is whether the slab will be strong enough (100mm thick with mesh in centre, 300x350 edge thickening) when situated over the EPS and with the dense block walls around the perimeter. I've found a lot of example foundations that either only have stud walls, or have many more reinforcing bars.

Also, there is an old foundation from an demolished extension that sits 350mm below the top of my formwork in one corner, however the top of the formwork is 100mm above grade on one side, so I'm only 250mm below grade on the lower side (Should say I am WELL below any organic layer and into healthy subgrade all around.

Critique of plans welcome as I've yet to install anything but the formwork frame.

Fubar

 

[cdbe]

Better off with a conventional build and an eccentric foundation on the boundary side (but if you shore up the fence you could dig a bit further under to fit a narrow strip) with your dpm inside. I doubt 50mm of insulation below a 100mm slab will do anything. Unless the ground is much higher on the other side of the fence your retaining wall looks over engineered.

 

[i_am_fubar]

Why do you say better off. Is it a technical issue or a cost/effort issue? I'm keen to pour slab as it involves less soil removal. However if it's unable to take the load, of the build or movement I need to change plans.

I agree it's over engineered for such a short height, but between the roadway and a desire to mount things from it inside, I'm fine with the extra expenditure.

I want to keep the the temperature as stable inside as possible without breaking the bank. It's not much, but it's better than wet soil. I feel it will help with the clay movement, but appreciate its not a typical build

 

[cdbe]

It's just easier. Nothing wrong with what you're doing if you've committed to it - but a conventional build doesn't need rebar, shuttering - just lay a strip - blockwork up to FFL, MOT, dpm and slab.

Unless there's some kind of issue with movement in the area - your house, nearby houses and buildings? Is your house built on a raft foundation? Has an engineer assessed the ground conditions and designed it? (I'm no expert but always believe a raft was for soft ground.)

If not your probably just overthinking it a bit - the ground isn't going to suddenly heave up and tip your shed over like a disaster movie.

Unless it's heated your dense floor and walls will stabilise the temperature.

You are right to focus on the dpm detail - and be aware that while it's pretty good, concrete blockwork isn't waterproof and you'll need an effective barrier over the entire walls above ground - cladding/render etc.

It looks fairly similar to my extension, I have dense clay (I think it's boulder clay given that it has boulders in it!) and had to build a bit below ground.

Just a bog standard strip foundation and slab, built slightly off centre of the slab to get as close to the boundary as possible. I have a cavity wall so was able to bring the dpm up inside the cavity.

If you look where the wheelbarrow is in the first picture, that's now the patio - 600mm of ground behind is happily retaining by vertical paving slabs set in concrete.