Creating a level backyard and installing a retaining wall in a Sydney home, hit complications when engineers hit rock. Here, how they found a solution.
The hard reality of striking rock while constructing a retaining wall will be familiar to many an engineer and builder. Just when you think the ground will yield and the post holes will be drilled without issue, you hit an impenetrable layer. After all, we rarely know what lies beneath until the clang of metal on rock tells us what we are up against.
This is what engineers from Tailored Retaining were faced with recently while working at a residential site in Shailer Park, Queensland. Contracted to level a sloping backyard and install a terraced retaining wall to make the area more entertaining-friendly, the installers discovered they couldn’t drill far enough to secure the posts before hitting rock.
While using much heavier-duty tools can get the job done, Andrew Taylor, director of Tailored Retaining, says in his experience, few residential sites can accommodate the 12-tonne machinery required to keep drilling under those circumstances. Such was the case at Shailer Park, where steeply sloping, unstable ground and lack of access points meant a bigger drill was out of the question both logistically and financially.
A cantilever system offers an effective alternative – and often a less expensive one. (Note: while Taylor refers to this system as “counter lever” and believes that phrase more accurately describes the engineering principles involved, the term “cantilever” is more widely used.) Indeed, Taylor estimates that some 90 per cent of masonry-block retaining walls over a metre high would be constructed with cantilever footings. The design of the cantilever footing itself is very simple but very powerful. The footing takes the form of a T-shape, with one side more abbreviated than the other. Working on the principle of leverage, the weight of the backfill stops the footing from sliding and overturning. In other words, the weight and pressure of the soil on the longer section of the base counters the lateral pressure of the wall itself.
many residential retaining walls, the purpose of the Shailer Park
retaining wall was to restrain the soil of the sloping ground so the
area could be levelled to make a more liveable backyard. While a
straightforward gravity retaining wall is often the logical first
option, discovering what lies beneath means engineers and builders
are always prepared to adapt.
On this occasion, Austral Masonry constructed customised footings from Pioneer galvanised steel posts with welding modifications. “The footings still have to be concreted in,” explains Andrew. And for this, ironbark concrete sleepers were used.
Whether you’re an experienced builder and know exactly the measurements and specifications your cantilevered footings require, or just trying your luck on some DIY backyard blitzing, these kinds of made-to-measure products mean you can get on with the job effectively and efficiently.