Holding Back Time
We are traditionally good at calculating and preventing wall failure. But what happens when that failure takes years to reveal itself, as the lands moves around you?
As towers grow taller, basements get deeper. Often, this results in tight city-blocks using retaining walls to keep the earth from moving under everyone’s feet. However, despite site safety making sizeable progress since the start of the tower boom, some concerns still arise.
“The engineering theory behind retaining walls is well documented, but some engineers who are charged with designing them don’t understand it well – and the fact that they don’t understand that they don’t understand is a problem,” says Dr Chris Haberfield, the principal geotechnical engineer at Golder Associates. “So what ends up happening is that these walls are specified to much more optimistic environmental conditions than the reality on the ground.”
Haberfield recently presented a talk for Engineers Australia, charting the abundant literature on large-scale retaining wall construction. Over the years he says about 30 percent of the legal cases where he’s had to provide expert engineering advice was due to issues stemming from retaining walls over time. While it’s easy to assume that most construction sites are to standard, Haberfield says this has bred a degree of complacency.
“When you have certain companies self-certifying, that’s where you find the problems, because you get a chain of command saying ‘she’ll be right’ and it’s all hunky dory,” says Haberfield. “And because there are different subcontractors to each part of the build, you end up missing the overarching oversight that looks for potential issues that could arise after something’s introduced. Because everyone’s focused on the one job, the full scope of risk isn’t really accounted for,” says Haberfield.
Haberfield wants regulation to get people thinking in and around the box – given that what happens inside a perimeter will also have unforeseen consequences if calculations don’t properly include load, liquefaction and climate factors. He explains that a lot of the issues in retaining wall construction today stem from excavations that don’t properly assess the flow-on effects to neighbouring properties. In contrast to badly bungled instances where retaining walls collapse, Haberfield says the risks that keep cropping up today are in truth, a lot more subtle, some taking years to eventuate.
“You don’t really see many retaining walls collapsing anymore, today the main risk that remains is due to movement of the wall over time. So the examples that you keep seeing are roads cracking,” he says. “The reason why we still have this as an issue is because movement really isn’t factored into the construction. Traditionally, designing retaining wall construction meant that you’d be making your failure calculation to ensure that a wall never fails. That doesn’t account for what we like to call a ‘serviceability’ limit, where you want to eliminate a wall’s movement. This is especially the case when it might be sitting next to critical infrastructure.”
For the average homeowner, Haberfield says this issue has also been brought about due to a lack of literacy: “Unless you’re knowledgeable in the area, you’re not going to be able to see whether the retaining wall next door has been built up to standard. These things take time, and in order to get people thinking about retaining walls, we need to be instituting more checks and balances when we start excavating.”