S. Kamao, M. Takezawa, K. Yamada, S. Jinno, T. Shinoda & E. Fukazawa
Soil liquefaction describes a phenomenon whereby a saturated soil substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or another sudden change in stress condition, causing it to behave like a liquid. The liquefaction phenomenon due to the Great East Japan Earthquake 2011 occurred to the reclaimed land around the Tokyo Bay area. Urayasu city was caused extensive damage by liquefaction due to the earthquake, and recovery from the disaster is still going on. This paper describes some remarkable damage against the structures due to the liquefaction by the earthquake, and indicates the possibility of liquefaction by current determination methods using published soil profile and N value of SPT (Standard Penetration Test). The authors also carried out a series of laboratory liquefaction tests (the cyclic undrained triaxial test) in order to make clear the characteristics of liquefaction, using Urayasu sand taken from the liquefied site in Urayasu city. Keywords: earthquake, liquefaction, Urayasu city, damage types, liquefaction strength.
earthquake, liquefaction, Urayasu city, damage types, liquefaction strength
Tectonic setting & Cause
There is some DISPUTE over the earthquake. New Zealand’s GNS science have called the February 2011 event an “aftershock” of the earlier September earthquake. Others have said it is a spate event on a separate fault system.
Regardless, the earthquake was created along a conservative plate margin where the Pacific Plate slid past the Australian Plate in the opposite direction. The earthquake occurred on a fault running off this major plate boundary. It was a strike slip event along the fault, mostly horizontal movement with some vertical movement upwards.
The earthquake generated its own significant aftershocks,
· 2 Hours after the main earthquake - Largest aftershock of magnitude 5.9
· The first week post event - 361+ aftershocks (magnitude 3+ foreshocks) in the first week,
· 16th April - a 5.3-magnitude aftershock caused further damage, including power cuts and several large rock falls.
· 13th June - a series of aftershocks occurred. A tremor of 5.7 was followed by a 6.3 tremor just over an hour later, with a depth of 6 km, located 10 km east of the city. Power was cut to around 54,000 homes, with further damage, casualties and liquefaction in already weakened areas.
EFFECTS OF THE FEBRUARY 2011 EARTHQUAKE
Primary effects (caused directly by the earthquake)
1. 185 people were killed in the earthquake. Over half of the deaths occurred in the six-storey Canterbury Television (CTV) Building, which collapsed and caught fire in the quake.
2. 3,129 people injured (source)
3. April 2013, the total estimated cost had ballooned to $40 billion
4. Significant liquefaction affected the eastern suburbs, producing around 400,000 tonnes of silt
5. Building damage to the central city and eastern suburbs of Christchurch. The damage was made worse by buildings and infrastructure already being weakened by the 4 September 2010 earthquake and its aftershock. Up to 100,000 buildings were damaged and about 10,000 buildings needed to be demolished.
6. 3.5 m tsunami waves in Tasman Lake, following quake-triggered glacier calving from Tasman Glacier
7. Water pipes, roads, bridges, power lines, cell phone towers and ordinary phone lines were broken or damaged.
8. 50% + of Central City buildings severely damaged including the city’s cathedral which lost its spire
"Ruins of the Canterbury Television (CTV) building, 24 February 2011" by Gabriel - Flickr: PGC building. Licensed under CC BY 2.0 via Wikimedia Commons.
Secondary effects (things that happen after the primary effects but often as a result of them)
Some economists have estimated it will take the New Zealand economy 50 to 100 years to completely recover.
1. 80% of the water and sewerage system was severely damaged
2. An additional 1,293 people were injured in the aftermath
3. Psychological impacts – in a survey post event (2013) 80% of respondents stated their lives had changed significantly since the earthquakes. Almost a third said the earthquakes had caused them financial problems, while 64% said they felt guilty that other Cantabrians were more affected by the earthquakes.
4. The divorce rate per 100,000 in Christchurch increased from 211 during 2010 to 249 in 2011, and 261 the following year.
5. The population, which had been growing for decades, declined in the years after the quake
6. Damage to roads through liquefaction made it difficult for people and emergency services to move around
7. Christchurch could no longer host Rugby World Cup matches so lost the benefits, e.g. tourism and income
8. Schools had to amalgamate
Weaker with a worse impact?
The February 2011 earthquake was smaller in magnitude than the 2010 quake, but the earthquake was more damaging and deadly. The reasons for this are;
1. Epicentre was closer to Christchurch
2. The Focus was shallower, only 5 kilometres underground, whereas the September quake was 10 kilometres deep.
3. The February earthquake occurred during lunchtime on a weekday when the CBD was busy, and many buildings were already weakened from the previous quakes. People died in collapsing city centre buildings. The 2010 earthquake happened at night and caused less building damage.
4. The Peak Ground Acceleration (how much the ground shakes) was extremely high and made it "almost impossible" for buildings to survive intact.
5. Liquefaction was significantly greater than that of the 2010 quake, causing the upwelling of more than 200,000 tonnes of silt. This undermined many foundations and destroyed infrastructure, damage which "may be the greatest ever recorded anywhere in a modern city".
Rescue crews from all over the world came to help. There were crews from Japan, the United States, the United Kingdom, Taiwan, Australia and other countries.
Aid money poured in – Australia gave $5 million (Aus $) in aid.
The government declared a state of national emergency, which stayed in force until 30 April 2011
Domestic help was available - The Farmy Army was made up of 800 farmers who brought their farm machines and muscle to help clean up the city.
The most vulnerable people were cared for and people were kept safe away from dangerous buildings
Chemical toilets were provided for 30,000 residents
Christchurch was divided into four zones after the earthquake;
· Green Zone land was undamaged and could be built on again.
· Orange Zone more checks were needed before the land could be built on.
· White Zone was the area that had not been checked at all yet.
· Red Zone areas were very unstable: building on it safely would be difficult.
The government provided temporary housing and ensured all damaged housing was kept water tight
Water and sewerage was restored for all residents by August
Roads and houses were cleared of silt from liquefaction by August and 80% of roads & 50% of footpaths were repaired
Many Non-government organisations including Save the Children helped with long term recovery efforts
Canterbury Earthquake Recovery Authority was created to organise the REBUILD of the region. It had special powers to change planning laws and regulations.