I back-filled the base with some of the leftover bessa blocks, some broken bricks and large rocks I found, plus the earth I’d dug out when preparing the site for the foundation, plus whatever solid, non-decomposing space-filling material I could find etc etc...
At times, I almost felt like climbing in myself and having my wife cover me over, entombing me in my own vainglorious edifice. But with a steady supply of cold drinks and paracetamol, I got there in the end (thanks Love!).
About 200mm from the top, I back-filled with a 100mm layer of brick sand, compacting it with a board that I would place on top and jump up and down on, and then by tamping it down all over with a brick, like so:
The sand apparently works well as an insulator, but on top of the sand I decided to cap things off with a 100mm layer of insulated concrete. This can be made by substituting the aggregate (i.e. small stones) normally used in concrete, with some hard aerated material like perlite. Perlite is an industrial mineral used extensively in hydroponic gardening – it’s cheap and light and I’d recommend it for this job. Vermiculite is another product you can use. I couldn’t get hold of either at the time so instead I used scoria – that dark red volcanic rock used in landscaping. Here’s some:
The scoria worked fine. The ratio of cement-sand-scoria I used was 1-3-3 (plus water, of course).
I also opted to lay down some internal reinforcement for the top layer, and here I really was making things up as I went along!
First I cut some notches in the outer walls with my grinder, to support a couple lengths of rio rod running across the top in both directions.
My thinking here was to ‘tie’ the insulated top layer to the outer ‘skin’ of bessa blocks, thus preventing it from sinking. In retrospect I think I was wasting my time – that top layer wasn’t going anywhere – but I had the materials handy so thought I’d use them.
I placed a layer of shade cloth over the sand, then added the rio rod, followed by a layer of wire mesh. Here’s how it looked:
Finally, with all that extra crap in place, I filled her up with my insulated concrete mix, levelled the top and, hey presto, the base was complete!