Shifting sand and rubber crumbs help homes handle quakes

Earthquakes are among the most destructive forces on Earth, but research suggests that foundations seeded with sand and rubber could help them withstand shocks

Home life Shifting sand and rubber crumbs help homes handle quakes
Shifting sand and rubber crumbs help homes handle quakes
Shifting sand and rubber crumbs help homes handle quakes

Before bicycles (and cars) had pneumatic tyres, they were known as boneshakers. And not just because the roads were either cobbled or poorly maintained. With no air-filled rubber tyres to soak up the bumps and jolts, taking a spin on a two-wheeler or tooling about in a Tin Lizzie was a painful way to pass the time. 

It is much more pleasant today thanks to the combination of air and rubber that helps to soak up the shocks and smooth out bumps. 

Now, some scientists are using rubber's ability to absorb impacts to handle the biggest bumps and shocks of all – earthquakes.

Mix magic
A research project run by Anbazhagan Panjamani, a civil engineer at the Indian Institute of Science, has found that old tyres, when ground up into crumbs of 1mm-8mm in size, and mixed with sand, can soak up the sharp shocks that can level buildings.

Through a series of tests using rubber crumbs mixed with sand in varying proportions, Anbazhagan's team has found that they can improve the ability of this mixture to absorb energy by up to 70 per cent. A mixture that uses 25 per cent rubber crumbs appears to work best. This, he says, seems to be because, at that concentration, the rubber crumbs fill in the gaps between the sand grains. This turns the loose material into something almost sprung – that can flex as a quake shakes it. 

So far the tests of these materials have been relatively small scale using centrifuges and shake tables. A shock to represent an earthquake “signal”, or tremor, is induced to the flat surface of the shake table to see how the sand and rubber crumb samples respond. "The next stage we are planning to go for is a large scale model test for an earthquake signal,” says Anbazhagan, "then it may be tried in a real building." 

Dig deep
The idea is to use the material around foundations and pilings to stop the destructive shaking reaching a building. Instead, the tremor will be absorbed by the looser material added all around the structure. The technique should work for both low and medium-rise buildings, he believes. 

Using these cheap, widely-available materials might be a more effective defence against earthquakes than established techniques that involve making a building stronger as it is constructed. Strengthening a dwelling in this way adds up to 25 per cent to building costs, said Anbazhagan, putting it beyond the buying power of most people.

And few developing nations, like India, that are experiencing rapid urbanisation, want to spend more money on building. Few of the projects being put up to house all the people migrating to the big cities are well protected against a significant quake.

In contrast, says Anbazhagan, seeding foundations with the absorbent sand and rubber crumb mixture could be a great – and cheap – step towards providing some much needed protection.

Shake down
Designing buildings to survive quakes is a particularly tricky business, says Dr Andrew Brennan, a senior lecturer in civil engineering at the University of Dundee, because of the way earthquakes rock buildings.

"We design buildings for vertical loads and gravity and the weight inside them," he says. "But if you shake them the load is horizontal and that's not what they are built to cope with."

The heavily compacted ground around many buildings does nothing but pass on earth movements, he says, so changing the character of that material by adding sand and rubber crumbs could help a lot.

"Greatly reducing the stiffness of the soil causes a drop in natural frequency and the whole thing becomes a low-pass filter, cutting out higher frequency vibrations. That means the big shocks do not reach a building, so even if it lacks that strengthening it has a better chance of surviving.”

Nature’s time scale
However, cautions Dr Brennan, there is only so much that good engineering and building practices can do to help protect against the ravages of a big quake. The unpredictability of these events makes it hard to be completely prepared. 

One example of this was seen in New Zealand where anti-quake efforts had concentrated on Auckland as a place subject to regular tremors. Less had been done in Christchurch, which really suffered when a quake passed through in 2011. On a geological time scale, the region around Christchurch might well be as active as that underlying Auckland, but human records did not extend that far, so the city was caught out. 

As Dr Brennan warns, there are limits to what even the best anti-earthquake preparation can do. "There is no such thing as being 'earthquake-proof',” he says, “as nature can always find something worse than we can economically design for."

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