One of my favorite childhood games was called The Ground is Quicksand. I would leap around my backyard from rock to stump to patch of grass avoiding any sandy patch of ground. If I lingered on a patch of sand for more than just a few milliseconds, the ground would swallow me whole! As I grew older, I was more than just a little bummed that quicksand turned out not to be such a big deal in my life. So much for backyard adventures and harrowing escapes.
Then, I found out about liquefaction. It’s not quite quicksand, but it’s still super cool and actually relevant to us adults. Liquefaction is a phenomenon where solid ground behaves like a liquid. This phenomenon needs just a couple simple ingredients: 1) wet sediment and 2) repetitive stress. Here is what it looks like:
Why did my feet sink into the ground? It has to do with the sediment-water structure. Sand grains are normally round(ish). This means that there are pockets of open space between the sand grains. When the sand is wet, that open space is filled with liquid. When the sand is compressed by my foot pressing down onto it (Stomp!), the grains are pushed together, the water pressure increases, and water is squeezed out of those open spaces. The sand grains remain touching and the friction between them keeps the ground behaving as a solid.
When the pressure is applied rhythmically and quickly (Stomp! Stomp! Stomp!), the water doesn’t have time to escape before the next compression cycle starts. This makes the water pressure build up so much that the sand grains are pushed apart and the framework of sand grains is destabilized. The sediment structure loses its strength and the sand grains flow like a liquid.
Liquefaction often occurs during the rhythmic shaking of earthquakes and can cause the ground to swallow up entire cars and buildings! In the 2011 Christchurch Earthquake, dirt roads turned to dirt soup:
On the scale of earthquakes, liquefaction is impressive and a bit terrifying. On the scale of feet stomping, liquefaction is amusing and a lot of fun. So, the next time you’re out in the backyard on a soggy day, try playing The Ground is Quicksand with a new set of rules. Avoid rocks and patches of grass. Instead, see just how deep you can sink yourself with the power of liquefaction!
Feature Image credit: Jon Sullivan