Liquefaction is a process where by firm ground takes on a liquid form temporarily. This generally happens during seismic events such as earthquakes. A notable example of this was during the Canterbury earthquakes of September 2010 and February 2011. In these events liquefaction caused silt and fine sand to ‘boil’ up and cause widespread damage. Entire streets were buried with this silt and sand, swallowing vehicles and buildings alike.

Causes of liquefaction

Over the course of thousands of years, rivers and streams deposit layers of silt and sand. Low-lying areas such as swampland and peaty marsh have higher concentrations of this silt and sand. Coastal areas also generally have higher concentrations as well. The top layer of sediment can become flat, firm and dry – which is ideal for building on. However, the waterlogged layers underneath are often hidden.

When seismic activity such as an earthquake occurs, severe shaking often accompanies this. This results in immense pressure being placed on the silt and water particles in these waterlogged layers, turning once firm sediment into a liquid. The ground above this land is pushing downwards, which further amplifies this pressure. If there is enough pressure water fountains and sand boils can burst upwards through any fissures. An analogy often used to describe how this works is one involving a coffee plunger. If you were to plunge a coffer plunger down too quickly, the water and coffee grounds are placed under pressure. If they are able to find a gap around the edge of the plunger they will spray up.

Damage from liquefaction

Liquefaction only occurs if the underlying waterlogged silts are subject to severe shaking. If the ground becomes liquid, it will be unable to support the weight of buildings that are on top of it. This means they can sink into the ground. Liquefaction can also cause the overlying ground to sink, spread, and crack. Underground services such as pipes and tanks ‘float’ in the liquefied ground and can also break. 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 limited to New Zealand.

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 there.

Liquefaction can occur anywhere that has seismic activity and where the right soil types are present.

Reducing liquefaction hazards

Compacting ground that is prone to liquefaction has proved effective in reducing liquefaction hazards. This involves putting large amount of weight on the ground before construction begins. 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.