Foundation for timber framed double garage

[pigman8]

I will be building a gluelam framed 30sqm double garage. 150mm square posts off simpson strong tie post bases with 220mmx150mm top beams connected to posts via concealed beam hangers. Timber stud walls with OSB, membrane and ext. cladding. Standard cut roof with RB and common rafters, sarking, battened out and ideally clay tiled to match existing house.

National Soil Resources Institute show that i will be building on loamy/clayey soil with impeded drainage with bearing capacity in the range 215-270kN/sq

Here is the question:

Could i use a 200mm post hole auger with 1m depth capacity, bore a load of holes (mini mini piles in effect), fill with compacted Type1 hardcore out of my local quarry(proper non recycled granite) then top up with Gen1 mixed on site, sit a beam and block floor off the pads with a 75mm void and then throw the garage up off that? The pads would be done yanky style with sonotubes(cardboard form tubes) suspended off the bottom of the hole so that the Gen1 would spread out at the bottom to form a footing. Is the thing just gonna pencil away down down before my very eyes-next stop Australia- or has this total self sufficiency idea got any legs?

The 150mm concrete beams would be on 488mm centers as would have to be the augered holes.

Anyone attempted anything like this or any feedback most welcome. Thank you.

 

[catlad]

If you are building of clay then is it not better to have a concrete slab with re-bar in it, you can have it done in a couple of hours if you get ready mix in.

 

[pigman8]

thanks for the reply catlad. my concern with casting a slab is that the area where i will be siting the garage was quite densely planted with small to medium sized trees and although i chainsawed them down last year there is one silver birch that i left in (trying to keep a bit of landscaping- maybe unwisely) A cracked slab due to heave is the concern.

 

[r896neo]

Augering a hole and then filling it partly with hardcore, no matter how well compacted is a bad idea. Lean mix concrete would be much better if its the money your concerned about

Sorry i can't offer any other more helpful ideas.

 

[pigman8]

thanks for the response r896neo. 2 questions now then - why would it be a bad idea and what exactly is lean mix concrete?

 

[noseall]

You don't lay hardcore in foundation trenches or in any foundation holes for that matter. you are just creating more compaction problems. Rather you would just fill with concrete.

What's wrong with building traditional strip foundations along with heave protection rather than all this faffing about with bore holes?

 

[pigman8]

noseall - The thinking was that i can hire an auger for £50 for the week. I can borrow a mixer and pour each hole as and when after work. Hiring the mini digger - £200 for the week, £150 for the dumper and approx £400 for about 4cubes of Gen1.

How does this sound as an alternative:

Dig out the footprint of the garage 500mm deep and use a 300mm bucket on the digger. Dig an identical trench mid width running lengthways,Get a kerbside delivery and dumper in Gen1 straight into the trench up to grade. Protect it and let it cure properly, put a beam and block floor down off the concrete foundation and build the gararge off that?

The BCO tells me 1m down and 600mm wide but ive done 4 exploratory holes 2 weeks ago down to 700mm - topsoil stops at 300mm, then loamy clay(hard to get your finger in much beyond 25mm) and the apparent frost line is at most 75mm. Weather up here at 700ft has been such that snow has been lying for a fortnight and nighttime temps have been consistently low.

 

[IJWS15]

One of the issues will be holding it down, not holding it up, concrete in the ground forma a good anchor when the wind blows.

Even more relevant if you ar at 700 ft!