We needed some new, low retaining walls in our top garden. “I’ve been thinking“, I said to my husband. This phrase always strikes fear into the heart of my family, though I can’t imagine why. And my idea? Reusing old, broken up concrete to make the new walls. When I first ran the idea past a couple of designer friends they were distinctly luke-warm. “Instead of classy-looking real sandstone? Hmmmm………not sure that will be a look you’ll really want” said one. But I thought I could ‘see’ something beautiful in my mind’s eye.
While excavating behind where the old, nearly-collapsing 2.6m (9ft) high brick retaining wall had been removed, we discovered where some previous owners of our house had chosen to put most of an original driveway, which had been replaced over 30 years before when the now-removed brick retaining wall had been built. Yes, there it was, as fill in the slope, with some pieces about a metre (3ft) square, creating huge voids beneath them. There was just no alternative – all of them would have to come out, or planting and maintaining the slope would be impossible.
The landscape plan (helped along by my garden designer friend Arthur Lathouris – although I’m a designer by trade I find it impossible to design my own garden) called for two new walls, the lower straight and the upper with some gentle curves, but both with an end point that had to blend into the new, rendered concrete kerbing that edged the driveway. Plus we had to figure out a way to incorporate some matching steps up to the top level.
After wall Plan A ‘Sandstone Boulders‘ was rejected on the basis of cost, Plan B ‘Concrete Blocks‘ on ugliness and Plan C ‘Timber Sleepers‘ on longevity, that’s when I had my in-the-middle-of-the-night epiphany. You know, the sort when it’s such a revelation you have to wake your partner and tell them about it right away, even though there is distinctly less enthusiasm for your idea than you think warranted. My new Grand Plan D re-used the old broken concrete, cut up into smaller slabs, to create 500mm (20″) high gravity walls, keeping it on site in a very environmentally-virtuous way. (OK – we’d also already sent tonnes of bricks to landfill and it was costing a bomb). [Note – if you want to build higher walls, or they will have heavy loads nearby (like near a driveway) you should seek engineering advice.]
With lots of grunting, heave-ho, sweat and a bent crowbar, we managed to remove all the large concrete slab pieces from inside the slope. There was still considerable suspicion that my Grand Plan D was actually D for delusional, and it soon became apparent that we wouldn’t have nearly enough concrete to do the job. Not sending away concrete to landfill now became begging for other people’s concrete. Driving up the street a few days later, I noticed a neighbour had stacked pieces of sawn concrete from what had once been his driveway, and there were obvious plans to pull up more. A note in the letter box, a very surprised sounding ‘you want what?’ phone call and 5 trailer trips later, we had a base course for our first wall. But still we needed more.
The concrete-for-reuse specifications were: no reinforcing mesh, a standard concrete ‘grey’ with no coloured oxides, and between 70-120mm (2¾”- 5″) thick, which dictated something laid more than 25 years ago. First we found a friend with an old concrete pad, revealed after demolition of his old shed, plus an old path. A hired jackhammer, lots of trailer trips and several aching backs later, we had enough to build the first wall. We developed an insatiable appetite for old concrete paths, particularly those old skinny path/driveway tracks so popular in the 1950s-70s. And each time we asked, the owner would agree (incredulously) and ask “but how are you going to cover up the concrete?”. When we finally bullied my friend Adrian into surrendering his old driveway, we were set.
As I’d never thought of covering it up, this was a bit disconcerting. Was I building something that would be so bug-ugly I’d have to plant densely to disguise it or, worse still, render over the offending broken concrete slabs?
Construction was the same as a bookleaf dry stone wall, with the roughly broken 300 x 600mm (1ft x 2ft) concrete slabs laid stretcher-bond style, and the smooth sawn pieces from our neighbour forming the base course, where their textural difference would be less noticeable. Our soil is a very stable clayey sand, only barely removed from concrete itself (it hadn’t moved at all since the wall came down), so we didn’t need to make a strong base course. But in other soils you’d need to excavate a bit further and spread and compact some aggregate first. The base course was angled about 5 degrees backwards into the slope and had the occasional ‘deadman’ – a longer piece of concrete that extended back into the slope – for extra stability. Although 300 x 600mm slabs was the ideal, in reality we had to use up a lot of triangular and oddly-shaped pieces in the lower courses where only their leading face would be seen.
As our acidic, sandy soil sets so hard, to create the level area for the upper wall we hired a small Dingo excavator. While the top wall was being built, I also started improving and planting in the lowest level so we could get some growth happening on our two small trees. As I wanted the wall to ‘weep’, there’s no waterproofing behind it. I also didn’t use any geotextile/fabric behind it, reasoning that the slight backwards angle and the wall’s thickness would be enough to prevent soil washing through, and that’s proven to be correct.
The tricky part was making up for variations in thickness; concrete from old paths is often only about 50mm (2″) thick but parts of the driveway were up to 130mm (5″). Added to that was the unevenness of many of the pieces, as cheaply poured slabs from several decades ago didn’t tend to have a nice smooth substrate. And some of the concrete had a lot more aggregate making it much darker than other pieces, so there was artistic blending of colours required as well. I was a bit worried that such uneven pieces could make it all look too haphazard, but trial and error and my husband’s skill created a great result. Although he has an engineering background and pretends not to be artistic you can see that’s not really true.
An electric rotary hammer drill worked well to chip off the lumps and break up slabs that were too big into more manageable pieces. A fairly dry mix of mortar, concrete chips as packing pieces, trial and error, months of hard slog, a good eye and occasional “why am I doing this again?” finally got both walls built.
And the result? Well even my long-suffering husband had to admit that my idea combined with his strength and skill had produced something really attractive, practical and unique. Visitors to the house expressed their admiration (and astonishment) at how good it looked. I hope it’s also more environmentally friendly, as it excluded the manufacture of new materials and prevented old concrete ending up in landfill.
The wide top on the wall also makes convenient access paths through the garden. I do often look at these beautiful walls and wonder whether we could have incorporated some sort of water cascade into the design too. But maybe enough was enough.