A plastic toy, like a person, isn't dense enough to sink entirely into a mass of quicksand.© Daniel Bonn

Although horror films frequently depict victims disappearing in quicksand, the truth is much tamer. People cannot fully sink into this type of soil, and laboratory simulations now bear out this little-known fact.

Quicksand is simply ordinary sand that is so saturated with water that the friction between sand particles is reduced, making them unable to support any weight.

The mixture most frequently appears near the deltas of mighty rivers. It can also form after an earthquake releases water from underground reservoirs. When quicksand causes the collapse of bridges and buildings, it truly can be dangerous, experts say.

The probability that a person will be completely sucked into the sand, on the other hand, is nil. "The Hollywood version is just incorrect," says Thomas Zimmie, an expert in soil mechanics at the Rensselaer Polytechnic Institute in Troy, New York.

Dutch courage

But scientists have not tired of disproving the myth. Daniel Bonn's quest to recreate

quicksand in the lab began on a trip to Iran.

The researcher from the University of Amsterdam saw signs alerting visitors to the danger of this grainy soil near the Namak Lake, located in the north of the country. Bonn also heard cautionary tales from local shepherds.

The warnings piqued his curiosity and inspired him to take a sample back to the Netherlands, where he and his colleagues analysed its composition.

Once they had determined the proportions of fine sand, salt water and clay in the quicksand, they mixed up larger batches of the same.

Walking on water

Bonn and his colleagues then placed aluminium beads, which had the same density as an average human, on top of their homemade quicksand. The team shook the system, and watched as this action partially submerged the beads.

An object that falls into quicksand can cause the sand particles supported by water to loose their stability and flow downwards in a liquid fashion. Bonn likens the disturbance to the toppling of a stack of neatly arranged oranges.

"An extremely small variation in stress can cause the complete collapse of this material," he says. According to measurements taken from the bead experiments, increasing the physical stress on the particles by just 1% can cause their flow speed to increase by a factor of a million, creating a downward pull. Bonn adds that getting out of the quicksand at this point is tough; the force required to pull out a foot equals that needed to lift a medium-sized car.

"The real danger of quicksand is that you can get stuck in it when the high tides come up," says Bonn. But patience can be a life-saving virtue. If you wait long enough, the sand particles settle and the buoyancy of the mixture will cause you to rise up to the top.

As proof, the beads in the experiment did in fact float, and never became more than half submerged in sand. Although each bead measured only four millimetres in diameter, Bonn says that the findings still apply to people, as they have the same density. The results from the study appear this week in Nature¹. 

University of Amsterdam, The Netherlands

• References

  1. Khaldoun A., Eiser E., Wegdam G.H., Bonn D. Nature, 437. 635 (2005). | Article | PubMed |