Christchurch
Earthquake Response
March 2011
Liquefaction
Frequently asked questions
How does liquefaction work?
Liquefaction happens in loose silt and sand that is below the
water table. It does not happen in clay soil, which tends to stick
together, and is uncommon in gravelly soils. It only happens in
soils that are below the water table, where the spaces between
the grains of sand and silt are filled with water. Dry soil does
not liquefy.
tend to be denser and less susceptible to liquefaction than sands
near streams, rivers and estuaries. The severity of liquefaction
that occurs also depends on the strength of ground shaking and
the length of time the ground shook, so it can be hard to predict
exactly where it will occur in any one earthquake.
What has happened in this earthquake, compared to
September 2010?
When an earthquake occurs the shaking is so rapid and violent
that the sand and silt grains try to compact, and they compress
the spaces filled with water. The water pushes back and pressure
builds up in the water until the silt and sand grains “float” in
the water. When that happens the soil behaves like a liquid
and loses its ability to support the weight of whatever is lying
above it. This could be surface layers of dry soil, concrete floors
of buildings, or roads and footpaths. The pressurised water
is forced up to the ground surface through the easiest way it
can find – often through cracks or crevasses in the ground or
concrete. It takes silt and sand with it to create the “sand boils”
or “sand volcanoes” seen on the ground. Sometimes sand boils
are created under roads or lawn without the asphalt or lawn
breaking, so it looks like a hump in the ground.
Liquefaction occurred in many parts of Christchurch, Kaiapoi
and rural Selwyn district during the 4 September 2010
earthquake. In the 22 February 2011 earthquake liquefaction
occurred over a much larger area of Christchurch, but there was
little liquefaction in Kaiapoi and rural Selwyn. This is because,
even though the magnitude of this earthquake was smaller than
the 4 September earthquake, the epicentre was much closer to
Christchurch so the ground shaking in Christchurch was much
greater than in September. This meant that soils that had not
been affected in September liquefied this time. Soils that had
liquefied in September also liquefied again. However, by the time
the earthquake waves got to Kaiapoi and rural Selwyn district in
this earthquake, there wasn’t enough energy left in the waves for
major liquefaction to happen again.
As liquefaction occurs water, silt and sand rises to the ground
surface while the ground surface lowers or subsides. It doesn’t do
this evenly, which leaves the ground surface lumpy and bumpy.
Buildings can sink or tilt to one side and things buried in the
ground such as tanks, pipes and manholes can float upwards.
Liquefaction also creates a mess in people’s backyards and
on streets.
To give an idea of the difference, Christchurch City Council
removed 54,000 tonnes of silt from areas affected by liquefaction
after the September 2010 earthquake. As a result of the 22
February 2011 earthquake, Christchurch City Council has
removed 322,000 tonnes of silt as of Thursday 10 March.
Liquefaction silt from both events has been taken to Burwood
Resource Recovery Park.
You can create your own liquefaction by shaking the very wet
sand at the beach with your foot – it’s the same process, just on a
smaller scale.
Does liquefaction happen in peat?
What is lateral spreading?
Lateral spreading is when liquefied soil moves sideways, towards
streams or estuaries, during earthquake shaking. This creates
cracks in the ground parallel to the stream or edge of the estuary.
These cracks are not earthquake faults, as they only go down a
few metres at most into the ground surface.
Where does liquefaction happen?
Much of eastern and some of southern Christchurch is built
on loose silty and sandy soil, so these are generally the areas
that could liquefy. However, even within these areas soil types
vary a lot from one location to another, which explains why
liquefaction can occur beneath one property with no evidence
of liquefaction on the neighbouring property. Beach sand dunes
No, liquefaction does not happen in peat, because it is made up
of plant material, rather than sand and silt. However, there may
be sand or silt layers above or below the peat that can liquefy.
How much has the ground sunk?
The ground surface sinks or settles to different levels to fill in the
spaces left by the water, silt and sand that has come out of the
ground. After the September earthquake ground levels in most
places affected by liquefaction appeared to have sunk by up to
10 centimetres. Because so much more silt and sand has come
to the ground surface in the 22 February earthquake the ground
in some places is likely to have sunk by more than that. We
are currently doing very detailed aerial surveying of the city to
determine how much areas have sunk by. This information will
be available over the coming weeks.
Christchurch
Earthquake Response
How deep down does liquefaction occur?
Liquefaction mostly occurs within the top 10 metres below
the ground surface, although it can occur up to about
20 metres deep.
Does liquefaction leave big holes underneath the ground?
The ground surface sinks to fill in the spaces left by the silt and
sand that has come to the ground surface. However, there may
be hollows in the ground surface after liquefaction that you can
see. There may be holes that you can’t see where high pressure
water pipes have burst and scoured out sand and silt under the
ground, or where lateral spreading has happened under asphalt
or lawn. Lateral spreading occurs in sloping ground near streams
or estuaries.
In areas affected by severe liquefaction you should be careful
for the first 2-3 weeks after the earthquake until the soil is fully
stable again.
Should I fill in holes left in my backyard after liquefaction?
Yes, you can fill in any hollows in the ground surface. We also
recommend that you inspect your property for any hollows
or dips in the ground to make sure you don’t step into them
by accident and trip. If you can, take photos of your property
before you fill in any holes so you can show the Earthquake
Commission engineer when they come to visit.
How stable is the soil now?
The water pressure in the soil decreases quite quickly in the days
after the earthquake. After 2-3 weeks the soil is almost back to
the same strength as before the earthquake. There may be areas
where the ground will continue to subside over the next couple
of months, but this will only be by a small amount.
Does liquefaction improve the soil? Can it happen again in the
same place?
No, liquefaction generally does not improve the soil. In most
cases the soil goes back to pretty much the same condition it
was in before the earthquake. This means that liquefaction
can happen in the same place more than once if the soil is not
treated. For example, areas of Christchurch that liquefied in
September, liquefied again in February.
March 2011
The Earthquake Commission is rolling out a new assessment
approach to give people with houses damaged by the 22 February
earthquake in Christchurch more certainty faster.
The new approach will start with a rapid assessment of all
residential properties in Christchurch, starting on 11 March and
covering metropolitan Christchurch, Lyttelton, and affected parts
of Selwyn and Waimakariri over the next eight weeks.
The rapid assessment of Christchurch households over the next
eight weeks will place dwellings into damage categories with the
timeframe for a further full assessment:
• Houses with severe structural damage will be revisited for a
full assessment within four months. Those needing repairs
of more than $100,000 + GST will then be passed on to the
homeowner’s insurance company for follow-up.
• Houses with minor structural damage will be revisited for a
full assessment within four to six months.
• Houses with no structural damage will be revisited for a full
assessment within six to nine months.
Anyone in need of emergency repairs should contact EQC on
0800 DAMAGE (0800 326 243).
If you are able, please take photos of any major land damage
before you tidy it up to show the EQC engineer. Also remember
to tell the engineer if you have had silt or sand on your property
after the earthquake that has since been removed.
What can be done to treat the soil to reduce the chance of
liquefaction happening again?
There are many ways of reducing the chances of liquefaction
happening. This includes compacting the soil and placing
columns of gravel in the soil, which make it easier for the
water to escape from the ground during an earthquake. The
foundations of buildings can also be better designed so that if
liquefaction does occur the building can better withstand the
sinking of the ground beneath it.
The method used to reduce the likelihood of damage from
liquefaction depends on many factors, including whether the
area already has houses on it or not and the specific type of soil
in the area. The method chosen will also depend on economics.
When will the Earthquake Commission come to inspect
my property?
Tonkin & Taylor Ltd, who do geotechnical work for the
Earthquake Commission (EQC), are currently busy doing rapid
mapping assessments of liquefaction land damage in the areas
affected by the 22 February earthquake to give a “big picture”
view of the situation.
This factsheet incorporates information from the Liquefaction factsheet compiled by the Institute of Professional Engineers of New Zealand.