MOREE, NSW – FEBRUARY 16: Luke Vanderwerf, site manager at Grain Corp’s Moree receiving depot, stands on a huge pile of Australian wheat February 16, 2006 in Moree, Australia. The wheat from this year’s harvest is being stored in massive grain sheds, awaiting transportation to ports for export. Australia currently exports around 65% of it’s total wheat crop to countries like China and Indonesia. Recently, Australian wheat exports to Iraq have been thrown into turmoil over the AWB Food for Oil scandal revealed by the Cole inquiry. The Iraqi Grains Board has suspended business with the Australian Wheat Board (AWB) until after the Cole inquiry which could mean Australia losing the chance to sell Iraq almost a million tonnes of wheat. (Photo by Ian Waldie/Getty Images)

Source: BBC

By Claudia Hammond

29 March 2016

We’ve all seen the films.  A man is caught in quicksand, begging onlookers for help, but the more he struggles, the further down into the sand he is sucked until eventually he disappears. All that’s left is sinister sand, and maybe his hat. There are so many films featuring death by quicksand that Slate journalist Daniel Engbar has even tracked the peak quicksand years in film. In the 1960s, one in 35 films featured quicksands. They were in everything from Lawrence of Arabia to The Monkees.

Yet the evidence that the more you struggle, the further you sink until you drown, is rather lacking. Quicksand usually consists of sand or clay and salt that’s become waterlogged, often in river deltas. The ground looks solid, but when you step on it the sand begins to liquefy. But then the water and sand separate, leaving a layer of densely packed wet sand which can trap it. The friction between the sand particles is much-reduced, meaning it can’t support your weight anymore and at first you do sink. It is true that struggling can make you sink in further, but would you actually sink far enough to drown?

John Dimech in the 1962 film Lawrence of Arabia struggles in quicksand (Credit: AF Archive/Alamy)

Daniel Bonn from the University of Amsterdam was in Iran when he saw signs by a lake warning visitors of the dangers of quicksand. He took a small sample back to his lab, analysed the proportions of clay, salt water and sand, and then recreated quicksand for his experiment. Instead of people, he used aluminium beads which have the same density as a human. He put them on top of the sand and then, to simulate the flailing of a panicking human, he shook the whole model and waited to see what happened. Would the aluminium beads “drown”?

The answer was no. At first they sunk a little, but as the sand gradually began to mix with water again, the buoyancy of the mixture increases and they floated back up to the top. Bonn and his team tried placing all sorts of objects on his lab-made quicksand. If they were of density equivalent to a human they did sink, but never completely, only half way.

Although quicksand doesn’t continue to pull you right under, if you can’t get free in time, a high tide can sweep across you

Why then, if physics predicts that you don’t endlessly sink further and further down, are there occasional tragic accidents where people do die, such as a mother of two who drowned in 2012 while on holiday in Antigua?

The reason is that although quicksand doesn’t continue to pull you right under, if you can’t get free in time, a high tide can sweep across you. This is really when quicksand can be dangerous.

So struggling alone won’t drown you, but we do still need to be wary. If you want to free yourself without waiting for rescue or for the sand to liquefy again, then Bonn’s research showed that just to release one foot, you would need to provide a force of 100,000 newtons – the equivalent of the strength to lift a medium-sized car.

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