Because the concrete wasn't level all round we decided to start
with a layer of trenchblocks to make sure both leaves of the cavity
wall started out level. Thermalite blocks are remarkably easy
to cut with a wood saw although it blunts the blade. Several blocks
were sliced up to fill the dips in the concrete.
There was still a fair amount of clay left within the 
walls and it had to be dug out before the
oversite concrete could be layed, It took a day with a jackhammer
to shift the frozen clay.
The top of the concrete slab is six courses of brick below the
damp proof course. The first two courses are now taken up with
the trenchblocks, leaving two rows of brick below and two above
ground level. (the DPC has to be at least 150mm or 6 inches clear
of the ground). There are some parts of the site where we will
have to reduce the level of the surrounding soil to keep it below
the DPC.
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