I'm digging the footings for my conservatory, and the original drawing suggested digging down 600mm for a 600mm wide, 150mm strip footing, which i made the decision to make a minimum of 250mm. I got to 600mm and the ground was still soft. Incidentally, it is pure clay where i am..completely orange/red! I spoke to a friendly local surveyor who said i should go down to 1 metre which i have done. I then read that if you can drive a 50x50 post more than 6 inches into the ground that it is not firm enough...i tried this, and using a lump hammer i could get it at least 8 inches in...i didn't try any deeper for fear of not getting it back out!
So what do i do now?? Dig closer to australia, dig wider than the 600mm, add reinforcing to the strip, or trench fill? It seems an awfully deep trench for a 600mm high wall and i'm already down as deep as the footings for the main house.
It is not clear what your conservatory is constructed of or its dimensions but if it is subject to planning control approval the short answer is to get your local building control officer in for a site visit and say to him this is the problem this is how far I have got what the resolution. Most of these people are very helpful. I had the same problem with a project with a foundation and went down 2.8 metres before hittting ground that the BCO was happy with. Just be careful if your structure should have had planning permission and it has not beeen applied for then its worth doing it properly (ie get permission) as when you come to sell you will need a completion certificate. Another approach is check your local neighbours (assuming you have ones) and see if they have a similar structure then go and bash on doors with a bottle of wine and ask to see the plans!!!
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