NZ researchers say that waste tires could be reused as seismic isolation for medium-rise buildings.
As in most nations formed near fault lines, earthquakes are seen as a part of life in New Zealand, and they are weaved into the indigenous mythology of the islands. Rūaumoko, a Māori god, is said to cause the rumbling of quakes and the hiss of volcanic activity as he moves below the earth. Māori have recorded multiple large-scale earthquakes throughout their oral history, including one at Rotorua said to have claimed the lives of 1,000 people. European colonists in Wellington were ill-prepared for their first significant earthquake in 1848 – the quake and its aftershocks destroyed the many stone and brick buildings they’d constructed. Since then, there have been at least ten major quakes in NZ *, and all have left indelible marks on the landscape and the people.
Something else that has changed in that time is the way buildings are constructed. Pre-1931, NZ’s European population favored the use of heavy masonry for large structures in urban centers. Their heavy mass, high strength and rigidity were expected to confer a sort of ‘quake protection’ on the buildings. And it worked, to a degree. But then, a huge quake hit Hawke’s Bay. In the cities of Napier and Hastings, facades and cornices fell from shopfronts, and buildings swayed violently before collapsing into the street. Internal floors in tall building detached from walls, their bricks crumbling inwards. In the wreckage, 256 people lost their lives, and thousands more were injured. The tragedy marked a turning point in construction history – engineers on the newly-founded Buildings Regulations Committee picked through the evidence to identify the most dangerous building practices, and produced guidelines that were the forerunner of today’s building codes.
Designing a structure that can withstand an earthquake is, unsurprisingly, very complicated. For smaller structures, it’s possible to ‘tie’ the roof, walls, floor and foundation together, so that they form a rigid box that can withstand the lateral (sideways) forces produced in a quake. For taller structures, though, you need to take a different approach to structural protection – you need to let the building move a little, while maintaining its structural integrity. For that, engineers often turn to seismic base isolation. These systems work to decouple the building from the ground it sits on, by separating them via flexible, energy-absorbing support structures. That way, when a quake hits and the ground begins to move, some of that energy is directed into the base isolation structures, rather than into the building. These isolation systems can take lots of different forms, including heavy-duty springs that contract and bend, and padded cylinders that roll when a seismic event occurs. But it was a kiwi engineer, Dr. Bill Robinson, that invented the most ubiquitous base isolation system in the world; the lead-rubber bearing (LRB).
Back in the 1970s, Robinson was a scientist in a government lab here in NZ, working on metallurgy and solid-state physics. A conversation with his colleague Dr. Ivan Skinner changed his course. Skinner had been successfully developing base isolators from steel and rubber – the steel would melt and deform, absorbing the majority of the seismic energy, and the rubber provided flexibility. Robinson was fascinated by this idea, but became convinced that steel wasn’t the best option. So, he locked himself in his office and worked his way through the Periodic Table, hunting for an alternative. In Robinson’s mind, there was one clear winner – lead – and he set about designing a device that would make the most of its properties.
In 1975, Robinson’s lead-rubber bearing design was released, and it shared some similarities with a car’s suspension system. Each pillar was made from laminated layers of rubber and steel, which acted like a spring, pulling the bearing back into shape after each shock. This multilayered sandwich of steel and rubber surrounded a central core of lead, which was the shock absorber. It stretched sideways when the earth shook, stopping most of the quake’s energy from passing into the building. The combination of flexibility and damping quickly made LRBs the isolation system of choice, and their design has hardly changed since.
Lead-rubber bearings are currently used in more than 8,000 buildings and hundreds of road and rail bridges across the globe. While they can’t make a structure earthquake-proof, they can minimize damage which can be enough to protect people from serious injury. Their effectiveness has been repeatedly tested – the LRB-protected Telecommunications Computer Centre in Kobe, Japan, survived the 1995 earthquake that destroyed much of the city around it. And the University of Southern California Teaching Hospital remained operational during and after the 1994 Los Angeles quake. Robinson died from cancer in early 2011, between the two earthquakes that hit his home city of Christchurch. His invention saved countless lives in those tragic events, and played a vital role in the rebuild that followed.
LRBs can be installed as part of a building’s construction – this was the case for the Museum of New Zealand, Te Papa Tongarewa, which opened in 1998, supported by 152 of Robinson’s bearings. Alternatively, they can be retrofit them to existing, at-risk structures, like Parliament House in Wellington. Either way, though, these systems are expensive. As a result, they’re usually installed only in large and/or important, buildings. But New Zealand’s urban landscapes tend to be dominated by low- and medium-rise buildings. In fact, in the complex Kaikōura quake of late 2016, it was Wellington’s medium-rise buildings (those between eight and 15 stories) that suffered the worst damage, despite many of them being built to modern design codes.
Is there a way to make those structures more earthquake-resilient, without the hefty price tag of LRBs? That’s a question that Dr. Gabriele Chiaro from the University of Canterbury found himself asking several years ago. And in 2018, he and a group of colleagues were awarded a $1 million grant by the NZ government to find an answer. Their proposal, titled ‘Eco-rubber seismic-isolation foundation systems’ aims to make use of a major source of waste – tires.
“Five million tires reach their end-of-life every year in New Zealand,” Chiaro told me over the phone. “And 70% of them are destined for landfills and stockpiles, or are illegally-disposed of or otherwise unaccounted for. Waste tires are a big problem.”
But, they’re also a useful source of high-quality rubber – a material that has been widely used in base-isolation systems. As we know from LRBs, rubber’s flexibility and shock-absorbing performance can confer some seismic protection to a building. The Canterbury team’s design is rather different to anything currently available, though. For example, the ‘eco-rubber foundations’ won’t look like an array of flexible pillars. Instead, it takes the form of a solid, steel fiber-reinforced rubberized concrete raft, supported by a thick layer of a waste rubber-gravel mixture.
Let’s start with the raft. This would sit immediately beneath the building, acting somewhat like a traditional foundation. Rubberized concrete is used because it can be more easily deformed than standard concrete – this makes it less susceptible to large cracks, but the rubber also reduces its mechanical strength. That’s where the tiny steel fibers come in. They act as micro-reinforcing – a scaled down version of rebar – which hold the mix together, and stop even the smallest cracks from forming.
The rubber-gravel mixture that sits below the raft is the key seismic isolation element in the design, so its performance is a major focus of this research. Combinations of waste rubber, soil and sand have previously been used in civil engineering projects, but according to Chiaro, this has largely been in countries that aren’t at risk of earthquakes. “We did a preliminary study of the structural properties of such mixes back in 2015 – that’s what kick-started our current project,” he says. That work has now accelerated, as they look for the optimal mixture of materials that will disperse the vibrations of a quake. But how these mixtures behave structurally is only part of the challenge. Arguably, a bigger question is around their potential impact on the environment.
Tires are hardly ever just made from rubber. I’ll write a separate piece on their construction at a later date, but for now, I’ll just say that they also contain quite a lot of steel: 15% (by weight) for car tires and up to 25% for truck tires. And as groundwater scientist Dr. Laura Banasiak told me, embedding them in soil without careful handling can lead to contamination. “From steel fibers you can get high concentrations of aluminium and manganese. In most overseas studies, we’ve found the predominant leached compound is zinc.” But, she emphasises, “most of the concentrations are well below guideline levels”, despite these studies being carried out in highly acidic conditions – a chemical worst-case scenario.
Banasiak works at NZ’s Institute of Environmental Science and Research (ESR), and she is Chiaro’s collaborator on the eco-rubber foundations project. While the Canterbury team are exploring the optimal mixture for seismic isolation, Banasiak and her colleagues are examining the exact compounds that are being leached from those mixtures. She says, “We don’t want to just remove tires from where you can see them and hide them under the ground. Because, while we want to introduce this product, we absolutely don’t want to introduce a whole new environmental problem.”
Chiaro, Banasiak and their colleagues are less than a year into this project, but they say they’re “moving fast.” Key to the foundation system’s success will be getting buy-in from regulators, the construction industry and government departments, but they seem confident that they’ll be able to get that. “Bearing isolation systems are too expensive for lots of buildings – especially housing developments and small-scale commercial properties,” says Chiaro. “There’s a demand there for earthquake strengthening, and there’s nothing on the market.” And for Banasiak, the goal is clear, “Ultimately, we want to produce a system that’s not just good on paper, but is something that will make a real difference in NZ.”
* And many thousands of minor quakes.