Hunting sinkholes — before the earth opens
But high-risk areas worth a check, experts say
People wondering how Rideau Street caved in without warning might spare a thought for a Florida man named Jeffrey Bush, who was out of work and down on his luck in 2013.
A friend helped him out. Buddy Wicker (given name: Leland) offered Bush a room in his own home near Tampa. But Bush’s luck didn’t turn around.
In the middle of the night, a sinkhole swallowed a corner of the house — only one room, but it was the bedroom where the luckless Bush was sleeping. They never found his body.
The story gets stranger. Months earlier, Wicker had seen a State Farm ad for sinkhole insurance, and signed up. The insurance company sent a crew to check out the property and make sure there was no sign of a sinkhole. Wicker’s place passed the test a mere five months before it was sucked underground.
But what was happening silently underground had been developing for years, even centuries, in the Florida bedrock, which is prone to sinkholes because it’s limestone permeated with water, and the rock tends to dissolve.
Water-filled cracks grow into pockets, and pockets become major cavities underground. The roof of the cavity can thin as more water dissolves more rock. And one day the thin layer of unsupported rock just collapses — sometimes a few metres down and sometimes dozens.
Tony Randazzo studied and taught about sinkholes at the University of Florida for 35 years, and he now does private-sector engineering work, looking for them.
Rideau Street, he says, didn’t technically experience a sinkhole. He suspects washed-out soil was our problem, whereas a sinkhole means the slow erosion of rock over time.
Either way, the road collapsed. And while there is technology for searching for weak spots, he argues against surveying every road in the city.
“It’s really kind of impractical to go around the streets and say, “I’d like to check the condition of the roadway because there’s always a chance” of a collapse, he said.
“The most economical way if you wanted to do it — if you have deep pockets and a lot of resources — is to use ground-penetrating radar.”
A crew drags a sled along the surface, and this carries a radar that “looks” straight down by shooting radio waves into the earth. The waves that bounce back to it from different underground structures show a skilled reader what is rock, what is soil and what is a cavern waiting to collapse.
Typically, it makes images down to a depth of about five metres, Randazzo said.
“You walk it along the roadway. You have a person pulling it. It runs at one or two miles per hour. You can survey mile after mile after mile relatively inexpensively — but the operative word here is ‘relatively.’ ”
He estimates the work would cost US$2,000 to $3,000 a day. That makes it economical and useful to survey a small area where someone has suspicions, but not to do a whole city.
Suspicious areas, he suggests, tend to be those with very old infrastructure — sewers, underground conduits for utilities and water mains that may have corroded or broken. (The hole that opened at the edge of Highway 174 in 2012, swallowing a Hyundai, was caused by the collapse of a section of large metal stormwater pipe. The Hyundai owner is now suing the city.)
And Randazzo says “it would make a lot of sense” to also survey areas near tunnelling, though he is not blaming the LRT tunnel for the Rideau Street hole.
Water mains often have slow leaks that gradually erode the soil around them, he said. If so, radar could show a washed-out area before it caves in. However, if the water is running fast, the ground could look solid one day and be washed out a short time later.
Another limitation is that radar doesn’t work as well in clay because the radar signal is partly absorbed instead of bouncing back, which leaves an image with poorer resolution than with rock or sand. Ottawa has a lot of clay.
“It obviously has limitations, but it would help, particularly (with) the imminent types of collapses — like, within five years of a collapse.”
His view is echoed by another engineering researcher, Alister Smith of Loughborough University in England. He did his PhD in Canada at Queen’s University.
“Geophysical technologies that could be useful in this application include, but are not limited to: ground penetrating radar, seismic tomography and electrical resistivity tomography,” he said in an email.
“Such geophysical surveys allow 2D/3D images of the subsurface to be produced, and if surveys are performed through time, the formation of voids could be detected.
“The problem is that these surveys are expensive and it is uneconomical to perform them unless there is a concern in a specific area,” Smith said.