Oh good, it's forecast to rain for the next week, just in time for the heavy clay foundation trench to fill up like a swimming pool. How we laughed.
When we finished laughing we strung up an enormous tarpaulin to try to keep as much of the rain off the trench as possible. You see, once the rock-hard clay/stone mix of ground got even the slightest bit wet, it turned from ground which you'd have serious trouble getting a spade in to, to ground which you'd have serious trouble getting your shoes out of.
That was a horrific couple of days of soul-destroying shoveling to get the trench to final depth (4ft-plus below finished floor level) and the entire area of the slab down to a flat and stable 3ft below finished floor.
Clay, you see, shifts and moves and grows and shimmies as it absorbs water, so the regs say you have to go deep, and provide a solid, heavy, reinforced base which won't cause your extension to crack. To hell with the fact that your house's foundations are (literally) above the finished floor level you're aiming for, and that it's just plonked down on the clay.., that's old building with stone and lime versus modern thinking and materials.
And the rain, as if to illustrate the fact that the clay was absorbent, kept on coming. Our tarp served to redirect most (not all) of it in to a bathtub we rigged up, where we pumped some away, and watched as more filled our trench repeatedly. It was frustrating, shall we say.
But in the end we got there, and you can see the bottom of the trench, where we threw in some spare Glapor to give us a walking surface, versus the finished floor level (see the gravel line on the house wall).
It took 3 days of manual digging and fettling to get the levels right.