Physicists have calculated that the perfect sandcastle is made from eight parts sand to one part water.
The study, released before publication in the journal Nature Physics, deals with the stability of sandcastles.
But the research could also help to determine the stability of retaining walls and the material they hold back, says author and physicist Associate Professor Arshad Kudrolli, from Clark University in Worcester, Massachusetts.
“Our study is the first step, in some sense, in trying to understand what’s the most stable angle that one can build, say, a retaining wall,” he says.
“And if it fails, where would the material end up? How much part of the land will give way?”

Wet sand, dry sand
Scientists who specialise in granular physics know that wet sand is more stable than dry sand.
This is because adding a small amount of water forms ‘bridges’ between the grains, leading to an attractive force not seen in dry sand.
But the details have long-been debated.
Inspired by childhood memories of the seaside, the researchers worked on a simple model of what makes for the most stable construction involving liquid and particles.
They ran a series of experiments in transparent, rotating drums to see the stability of sand under different conditions. Among the things the researchers looked at were different sizes of sand grains, different proportion of liquid to sand and how far the drum had rotated before the sand slid.
They then used a mathematical model to explain what they saw that was based on what was happening at the surface of the sandpile and to the pile as a whole.
From there, the scientists calculated what would work best for the most stable sandcastle.

The perfect ratio
The answer came out as use roughly one-eighth the water to the amount of sand, though Kudrolli says there is a range of possibilities that would work.
“You need a mix of the two to get it right,” Kudrolli says. “It could be more than that, or less than that would be just fine.”
Despite the playful nature of the sandcastle example, Kudrolli notes that their model for what keeps a sandpile from disintegrating could lay the groundwork for questions about what kind of retaining walls would be most stable.
Even though engineers have designed such structures for hundreds of years, the science behind them is not very well developed, Kudrolli says.
The study uses ‘idealised sand’, glass beads the size of sand grains, and water, seeing how the piles would stack up and when they would topple.
Now, the scientists are interested in how natural sand behaves, which is made from non-spherical grains and grains of different sizes.

Source: http://www.abc.net.au/science/articles/2005/09/29/1470831.htm