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Although I have been away from the build for a few weeks due to to flu, work (yes I went back cos they asked me help out for a few weeks) and parties I have been back on the build for the last 5 days and have reached a harrowing conclusion. The muck away costs for a passive slab are huge! The first lorry grab lorry turned up today and took 15m3 of soil which didn't even dent the pile and we have only done about 30% of the slab area, 600mm deep. When I worked out the budget for this phase I had reasoned that the existing property, a timber cabin, would be hollow down to the soil when in fact, as we demolished it we realised that it was on a slab with some massive foundations - no idea why. These had to be broken up first, and disposed of as rubble before we could start to scrape back the clay to the required depth for the slab build up to commence. So I now think we will need between 10 and 13 lorry loads of muck away which is about 4x the budget I had worked out and will eat a second chunk from the contingency. The expansion (swell) of clay (40%) as you dig it does not help either. I noticed that the grab lorry has load cells, or similar contraption, so it knows just how much weight it has aboard and although we had the volume today we were well down on total weight. So my next job is to run up and down along the pile with the digger to compact it so that it goes back to around 100% before the grab picks it up as I don't think it will swell much in those jaws. Moral of the story is - if you are thinking of a passive slab and you need to dig down, IE you are not building it all above ground, work out the muck away costs carefully.