New garage foundations

[Voidcomp]

Hi everyone, new member here. With apologies, as I couldn't decide whether this was "building regs" question or just plain "building"....

This is my first major build (previously only drives, paths, walls, but not buildings). I'm planning to build a 36m2 square garage at the back of my garden, backing onto an existing rear shared driveway. Construction: cavity rendered medium density concrete block wall, pitched roof (I know it needs planning and BC). I want this as a dry, lit and powered workspace, so will be insulating walls, DPM/DPC, etc.

My question is about foundations, primarily to get it right structurally, but secondarily to pass muster with BC.

I live near the top of a hill, with a gentle slope from the back where the garage would be, downwards towards the house (I estimate about 3%). Subsoil is Essex clay.

Stuck choosing between a full steel reinforced trench (currently planning 600mm wide, 600mm deep (nearest the house), and footings/foundation blocks. Should I prefer a trench, given the above, and should I be going deeper than 600mm?

Any criticism or suggestions around my plan very welcome.

 

[Norcon]

About the max depth we go on retaining walls up to 4.5m tall is about 250mm. Thats double reinforced of course.
Seen us laying foundations three or four inches deep for large industrial building. Works ok.

 

[noseall]

600mm wide sounds good.

The depth however will be determined by bearing strata conditions and whether the found's can be affected by nearby trees etc.

A typical dig would be in the realms of 1m deep to trench bottom. However, once down to virgin clay, this depth can be amended provided the BCO is familiar with the locale and he is satisfied the sub-soil is good and is unlikely to change at depth.

Trench foundations are the simplest and least expensive so why bother looking for an alternative if you don't need to. Adding reinforcement is unlikely to affect the problems associated with with clay volume changes but it may deal with localised moisture changes in the sub soil.

Thickness of concrete can be anything from 225mm upwards.

 

[Voidcomp]

Thanks for the advice. I know the answer to the depth question is always "it depends" but getting the benefit of someone's experience is always a good place to start.

Just one big dumb question @noseall: when you say concrete thickness can be upwards of 225mm, are you talking about the minimum for a footing? Reason I ask is that I was considering the whole trench being filled with concrete, making it effectively 600mm (or according to your advice, 1000mm) deep and 600mm wide, or is that overkill?

 

[noseall]

225 is a typical minimum thickness of concrete for domestic extensions and the like.

However, much more common is to mass fill the trench with concrete as this saves awkward time consuming and expensive blocklaying in a narrow trench.

If the ground is fairly level there is no reason why you can't fill to within say 450mm of dpc.

Don't over fill the trenches as this may make gully installation difficult.

 

[Voidcomp]

Thanks noseall, that's exactly what I was thinking. Didn't really fancy laying blocks into that trench, not even wide ones.

Cheers.