LIQUEFACTION: A new word in our vocabluary - when solid ground is shaken to mush...
A sand volcano after the Christchurch earthquake. Large tracts of silty, low-lying land compounded the effects of Saturday's earthquake in Canterbury, as whole streets were transformed from firm land to sludge.
In what's known as liquefaction, Christchurch's sandy soil was shaken violently, causing water to rise through its pores. Scientists compared it to jumping on wet sand at the beach - it soon turns to a murky soup.
Professor Michael Pender from the University of Auckland geology department said the Canterbury quake was one of the most significant cases of liquefaction in New Zealand history.
He said the process could affect any town or city near a river, estuary or coastline. Auckland's waterfront, built on reclaimed land from the Ferry Building up to Shortland St, would be very vulnerable to large tremors.
Coastal developments which encroached on sand dunes, such as Mt Maunganui and some North Shore beaches, were also susceptible during a major earthquake.
Large sections of Christchurch were built on soft sediments which remained saturated after a wet winter.
Roads, bridges and pipe infrastructure have been unsettled by the water squirting up through the soil during the 7.1 magnitude tremor.
As many as nine out of 10 homes on the city's flat have been damaged by the quicksand-like effect. Much of this damage was superficial rather than structural. But in Bexley, a 5-year-old subdivision near New Brighton, at least 100 new homes were left uninhabitable after silt, sewage and grey sludge cracked the road and squeezed through floorboards.
The worst-affected areas were coastal spots such as New Brighton, and suburbs that skirted the lower reaches of the Avon and Heathcote Rivers, in particular Dallington.
Homes in Kaiapoi, near the Waimakariri River, were also reported to have sunk into the soil.
Geologists said that when rising water was concentrated into small cracks it could create sand volcanoes - mounds of sand that pushed up to the surface, as seen on the streets of Avonside and Linwood.
Professor Pender was surprised that some new developments were susceptible to major damage from underground water.
"You'd expect the engineers doing the site investigation to realise that there is this loose sand present and do some remedial work before they built foundations."
New Zealand Society for Earthquake Engineering spokesman Win Clark said most of Christchurch's structures performed well under the stress of the quake. "The buildings responded as expected - most damage was to unreinforced masonry and buildings built before World War II."
I understand this phenomenon has been experienced before in California, for instance. This has encouraged scientists from the US to comever and investigate.
http:/www./redcross.org.nz/ Christchurch Earthquake Appeal
