Bricks it is, then.

Besser blocks model 15.01, to be precise. 40cm long, 15cm wide, 20cm high. Six of them to a side on three sides, the fourth side being the edge of the concrete apron around the house. 18 per tank site, 36 blocks in all, delivered for $156.20. About the same price as the equivalent quantity of pine sleepers soaked in copper, chrome and arsenic.

They were delivered late yesterday and today I spent a few hours sculpting the trench in which the first six of them now sit. I say "sculpting" because below the turf in my back yard is a whole lot of very dense clay. Using my 150mm spade and a mini-mattock I carved out a surprisingly neat rectangular slot. The blocks went in with a little clay soil packed into the slight gaps along the long sides. The goal is to build a nice rigid wall that will hold a base of crusher dust firmly in place beneath a five tonne water tank. The clay is so solid I'll probably not use any additional concrete to bed the blocks as I had planned.



If it takes a few hours to get one side done, I have an awful lot of hard work remaining. It would have been so much easier just to pour two slabs of concrete - and probably cheaper given that I have a builder, an underground cable layer and a concreter as neighbours. Why the heck am I spending my weekends scratching about in the clay?

The primary environmental concern with concrete is the energy used in manufacturing cement. I read somewhere yesterday that 8% of the world's anthropogenic (man-made) greenhouse gas emissions result directly from the production and use of cement. Scary, if it it's true. But concrete is such a practical building material I can understand its appeal: even mixing by hand I could have poured two slabs in an afternoon, and they would easily outlast the tanks they'd be supporting.

In the end I compromised. I needed something heavy, strong and durable for the border; needed to prevent termite damage to my home; wasn't keen on the copper-chrome-arsenic approach. Pre-formed concrete bricks are going to do the job nicely, are non-toxic, and would probably be re-usable in the future if I ever had reason to take them out of the ground. I think it's an acceptable use of resources.

As added bonuses, I've had my kids out "working" with me, I've performed some much-needed physical exercise, and so long as it turns out OK I'll have the satisfaction of knowing I did it with my own hands.