Thursday, 23 February 2012

Liquefaction Explained

Liquefaction has been a feature of both Christchurch earthquakes, seen as tonnes of silt on the roads and sand volcanoes or sand boils on people’s lawns. This video shows what was happening under the surface of Christchurch during the earthquake.

Isn’t wonderful what a private citizen can teach with a spade, a wheelbarrow, some cobblestones, and a video camera. How many thousands would it have cost taxpayers like yourself if a government department was told to produce an equivalent demonstration?

The video is great, but ever since the earthquake I’ve wondered what was happening inside the soil during liquefaction. Make yourself comfortable while I explain it in plain English…

If you’re filling a jar with sugar and want to fit as much as possible into the jar you stop part way and shake the jar in order to create some empty space at the top. When you shake the jar the grains of sugar rearrange themselves so they can snuggle up closer and the size of the empty spaces between the grains is reduced.

Much of Christchurch is built on sandy soil and if that soil was perfectly dry an earthquake would shake the sand, the soil would settle like the sugar in your jar, and buildings would crack as they moved downwards to the new soil level*.

However, the sandy soils of Christchurch aren’t perfectly dry and there is water sitting between the grains of sand. The soil is like an old fashioned sponge which has lots of empty spaces that can hold water. When you squeeze a wet sponge you reduce the size of the spaces that hold water. The water is incompressible (it cannot become smaller in response to squeezing) and will not fit into the smaller spaces. When your house is too small you move out, and in a similar way the water moves out of the sponge. When you go the beach and stand on the sand just above the water’s edge you’ll soon find yourself standing in a puddle because your weight has squeezed the water out of the sand.

When a sponge has been squeezed gently the water moves out like water coming out of a garden hose, and when the sponge has been squeezed hard the water moves out like water coming out of a fire hose. Squeezing harder increases the water pressure.

Back to the soils of Christchurch. As the damp, sandy soil is shaken and settles like sugar in a jar the water between the grains is squeezed by the weight of the sand, buildings, roads, etcetera above it. In response to this squeezing the water wants to move out of the sand, but before it has a chance to do that another vibration from the earthquake causes more squeezing. More squeezing causes more water pressure, and two things happen:

1) The increased water pressure greatly reduces the forces that hold the sand grains together. To put it another way, as the water pressure increases each grain of sand finds it harder and harder to hold onto his neighbours. Eventually the water pressure reaches a point where the grains of sand cannot hold hands at all: now the soil has no internal structure and flows like a liquid. Liquefaction has occurred.

A tent with a frame inside it illustrates this. When the tent is set up properly is has shape and is strong. Take away the frame, which is the tent’s internal structure, and the tent collapses. When you take away the tent’s internal structure it goes from being a solid object to an object which flows like liquid. When you watch the video above you can see the sandy soil collapsing much like a tent.
When liquefaction occurs buildings drop down because they are no longer sitting on solid ground, just as the beach sand makes you unstable when you are standing close to the water’s edge.

2) As the shaking of the earthquake increases the water pressure the soil becomes like a little kid who is busting to go to the toilet. The water desperately wants to get out of the soil – just as it desperately wants to get out of a squeezed sponge – and the only way out of the soil is up. When this happened in Christchurch people saw water sitting on top of the ground, much like water sits on top of the beach sand as described above. As the water moves up it carries sand with it and the people of Christchurch saw sand volcanoes (also known as sand boils) and tonnes of sand on the streets.
It’s much like popping a pimple: your squeezing increases the pressure of the pus so that it takes the only exit and hits the mirror :shock: .

 (Thank you to Mandeno Musings for providing this article)

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