Cheap and good foundations for a greenhouse

13 Dec 2009
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United Kingdom
I have a largeish 4.5m * 2.5 M greenhouse that I have to make foundations for. It's on a 1 in 4? gradient width wise and I've dug 40 cm deep 25 cm wide trenches for concrete so that the lowest part is about 15 cm deep.

I've been told this is not deep enough really and it should be 40 cm all the way round. This is a hell of a lot of concrete for a greenhouse! Someone said build a wood base out of railway sleepers to fit it on and then use 6 pillar fountations instead. So now I've dug a 50cm square deep hole at the lowest corner to try filling. Six of these are still a fair bit of concrete.

So my questions are: I have a lot of broken, crumbly bricks. Can I use these as ballast and if so in what ratio? I have a lot of engineering quality bricks can I make a tower of these and poor conrete over it to use less concrete and encase it wilst retaining the strength? Or could I get away with 'encasing' a load of broken crumbly bricks in concrete also? :?:
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A 100mm concrete slab would have sufficed. Any concrete pillars should be just concrete, don't try and bulk it up with hardcore. Save this for the floor and blind it with more concrete.
I believe from everything said that a form of bag foundation would suffice, talking in to consideration the Coliseum is built on similar credit to fame?

The material excavated so far could be used to fill the bags, the broken/crumbly bricks would make the perfect trench foundation bottoming. This may be unusual techniques these days but let me just reiterate parts of the Colosseum founds were built this way...a'hem and it's still there,,,pinenot
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