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Aerial shot shows roads flooded by liquefaction, June 14 - Source: ONE News -
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The Royal Commission of Inquiry into the Christchurch earthquakes has heard how buildings were pushed to breaking point as the land below became a bubbling mass of sand.
The inquiry into the quakes was told that it is widely known that the city's centre was built on an area considered highly susceptible to liquefaction.
Associate Professor Misko Cubrinovski of the University of Canterbury explained this morning the findings of a geotechnical report into the make-up of the soil underneath the CBD.
He said the top layer is made of gravel, sand, and silt which have a high potential for liquefaction, particularly when combined with the high water table in Canterbury.
Engineers told the inquiry that the silt that blanketed the city should not have come as a surprise.
"Water table is very high, meaning that the soils are fully saturated. When we put all this together (it) leads toward high liquefaction potential," said Cubrinovski.
His findings come following a report earlier this month that found significant areas of the Christchurch CBD were consented for subdivision after 1991 - when the risk of liquefaction was clearly identified.
The fact finding report by consultants Hill Young Cooper and Resource Management Group Limited found up to 1200 properties were granted consent through the Resource Management Act in high risk areas.
Cubrinovski's report, written in collaboration with geotechnical engineer Ian McCahon, also found that buildings on shallow foundations fared the worst in the quakes, sinking into the ground or cracking.
"As CBD was so badly affected, we lost it for a number of months up to this point," Cubrinovski said.
Some areas in eastern Christchurch dropped by several metres, with some homes simply splitting in two.
There has been criticism of a lack of geological investigation.
"Structural engineers need to understand geotech side better," said civil engineer Kevin McManus.
Engineer Dr Jonathon Bray Gray said the city needs to adopt testing methods more specific to each site.
"You can develop robust foundation designs appropriate for the ground conditions and shaking levels that one might see in Christchurch," he said.
The Canterbury Earthquake Royal Commission also heard that the cost of rebuilding the city is likely to dramatically increase.
"There will undoubtedly be an increase in the cost of foundations," said McMahon from Geotech Consulting.
The Royal Commission of Inquiry is due to report on the causes of building failure in Christchurch during the quakes by April next year. It has already made some wide-ranging recommendations from its preliminary hearings.
Full information about the inquiry, including live streaming when in session, here
