Liquefaction during the recent New Zealand earthquake has turned the verdant lawns of Elwood Bowling Club in Christchurch into a mud pit. BELOW: Natural hazards expert John Clague, a professor at Simon Fraser University.
By Tom Hawthorn
Special to The Globe and Mail
September 15, 2010
When the ground stopped shaking at 4:30 a.m., the residents of Christchurch poked out of their homes to survey the damage.
Chimneys toppled. Facades fell onto sidewalks. Across the New Zealand countryside, once-straight railroad tracks curved like slithering snakes.
In the city, brick walls collapsed, exposing the interiors like an open dollhouse.
In the eerily silent aftermath, many reported the sound of rushing water, as though water pipes had burst.
The 7.1-magnitude earthquake caused widespread damage. Happily, few residents were physically hurt.
No deaths. Only a handful of serious injuries, including a man injured in a taxitwo serious injuries, both to middle-aged men, one cut badly by falling glass, another trapped by a collapsed chimney. A lemur named Gidro drowned in a moat at a wildlife park.
The world has moved on even as the cleanup continues.
The New Zealand earthquake already has the feel of ancient news, superseded by boorish football players (shocking!) and Lady Gaga wearing a dress made of red meat (horrors!), thus raising the steaks on outrageousness.
In university offices around the world, however, professors whose expertise fall in this area are keenly following developments.
John Clague, a natural hazards specialist at Simon Fraser University, says the antipodean temblor provides lessons for British Columbia.
“It’s the type of earthquake we’ve got to worry about here,” he said. “Shallow, crustal earthquakes.”
Christchurch’s building stock includes many older buildings made of brick. It is a low-rise city with few skyscrapers. It is known as the Garden City,
Sounds a lot like Victoria to professor Clague.
A surprise to many New Zealanders was a process known as liquefaction, or, as Television NZ describes it, “when solid ground turns to sludge.”
Liquefaction sounds like “liquid fiction” for a disaster movie.
The ground shakes and certain soils — silty, sandy, even gravelly material — can be transformed into a liquid the “consistency of heavy jelly,” as one report described it. A muddy slurry of a mess.
Terra turns out to be not so firma.
“It’s a hard concept to get your head around,” Prof. Clague said. “You take seemingly solid earth material and you transform it into a liquid.”
Loose, water-saturated silts and sands lose their strength in the shaking, turning into a liquid. Sometimes, this happens below the surface, causing the land atop to glide laterally. Gravity can then lead this capping layer, as it is known, to slide downhill.
On Courtney Drive in Kaiapoi, about 20 kilometres north of Christchurch, two women were swept away in a river of sludge that materialized in front of their homes. They were rescued.
Puns seem hard to avoid — liquefaction is an issue below the surface, waiting to bubble up.
“It’s starting to sink in about liquefaction,” Lewis Joyce, husband of one of the women, told TVNZ.
Sometimes, the sludge is expelled from cracks in the ground at great pressure, creating geysers. These later form into cone-shaped sand volcanoes, many of which now dot backyards in the Christchurch area.
“An amazing phenomena,” Prof. Clague acknowledged.
For many hours after the quake, residents assumed water mains had broken, as muddy liquid continued to bubble through cracks in concrete. This upward thrusting has damaged many homes and roads.
Now, as well, the drying soil is a health hazard, as the sludge mixed with waste from broken sewer lines.
As television showed residents wrestling with wheelbarrows filled with the heavy muck, it also interviewed professors from the University of Canterbury, the University of Arkansas, and Virginia Polytechnic Institute examining the damage caused by the muddy ejecta.
Christchurch’s mayor said about half the city is susceptible to liquefaction.
In British Columbia, a Geological Survey of Canada map shows extensive areas in the Lower Mainland with loose, saturated lowland sediments.
These include all of Lulu Island (the city of Richmond); most of Delta except for Tsawwassen; along the Nicomekl River in Surrey; along the waterfront in Port Coquitlam and Pitt Meadows; as well as the extensive floodplain along the Fraser River farther up the Valley.
In the Greater Victoria area, the liquefaction hazard areas include a few spots along the low-lying parts of the Inner Harbour and at Cadboro Bay.
In Haiti, where the January earthquake measured 7.1, liquefaction knocked out the port area, delaying the delivery of aid.
Prof. Clague noted that the sludge is not usually a killer, though it creates tremendous damage. Like most earthquake experts, he tempers his knowledge with a desire to lead a normal life. He personally would not purchase property in Richmond, even with the precautions demanded on new buildings, yet is at some risk as he lives in a home on the North Shore. He used his expertise in quake threats and geological hazards in judging the soil surrounding the foundation.
“That would be my worst fear as a professional — having my house taken out by a landslide,” he said. “It’s funny what drives you.”
Suffice to say he teaches atop a mountain.