Liquefaction is a process that temporarily turns firm ground into a liquid. During the Canterbury earthquakes of September 2010 and February 2011, liquefaction caused silt and fine sand to boil up and bury streets and gardens and caused buildings and vehicles to sink. This was a new phenomenon for most New Zealanders, yet it has been a feature during earthquakes throughout this country’s history. There is evidence of liquefaction in most of the largest earthquakes since the 1840s, including the Hawke’s Bay quake of 1931.
Next time you are on a sandy beach, walk across some wet sand a little back from the water’s edge. It’s firm walking, though you might leave footprints. Now stand still, wriggle your toes and feet. If conditions are right, you could sink a little as the sand around your feet becomes sloppy. This is similar to what happens during liquefaction.
Causes of liquefaction
Over thousands of years, rivers deposit layers of silt and sand in many places, especially in low-lying ground and near the coast. The top layer of sediment can become flat, firm and dry – ideal for building on – but this often hides layers of waterlogged sediment beneath.
Severe shaking in an earthquake puts pressure on the silt and water particles in these waterlogged layers, turning once firm sediment into a liquid. Pressure on the liquefied layer is increased by the ground above it pushing down. Water fountains and sand boils can burst upwards through any cracks.
It’s a bit like using a coffee plunger carelessly. If you push the plunger down too quickly, the water and coffee grounds are put under pressure. If they find a gap around the edge of the plunger they spray up all over the bench or table.
Damage from liquefaction
Liquefaction only happens when underlying waterlogged silts are present and when shaking is severe. When the ground becomes liquid, it can no longer support the weight of buildings, so these tend to sink. The overlying ground can also sink, spread and crack. Underground pipes and tanks float and break. Power poles fall and break cables. During the 2010 and 2011 Canterbury earthquakes, over 400 000 tonnes of silt came to the surface, covering streets and gardens in several suburbs.
Liquefaction is not just a New Zealand problem. In 1995, the Japanese city of Kobe suffered extensive damage from an earthquake. The port had been built on artificial islands made of relatively loose sand and silt, and many buildings collapsed during the liquefaction that happened here.
Reducing liquefaction hazards
The Museum of New Zealand Te Papa Tongarewa is on the waterfront in Wellington. The ground was considered to be at risk from liquefaction in the event of an earthquake, so it was compacted before building started. This involved dropping 30-tonne weights on the ground over 50 000 times, which took several months. Together with base isolators, this gives Te Papa considerable protection during an earthquake. Ground compaction also worked to reduce liquefaction damage in some residential areas of Christchurch during the 2010 and 2011 earthquakes.
Another way to reduce damage in areas at risk from liquefaction is to place buildings on concrete pillars that go through the sediment layers to firmer layers beneath. If the liquefaction-prone layers are shallow enough, they can be dug out and replaced with a firmer fill.
Nature of science
Scientific terms are sometimes taken up and used incorrectly by non-scientists. Some reports of the Christchurch earthquakes referred to liquefaction covering streets and needing to be cleared up. The scientific term ‘liquefaction’ refers to a process – it was silt that was covering properties.