In the early hours of Friday, September 29 last year, in the eastern suburbs of Melbourne, locals woke to find the busy intersection of Kooyong and Glen Eira roads a swarm of emergency vehicles and road workers. At around 2am, the ground underneath this busy road gave way and collapsed into the earth. Official reports say a leak in the underground plumbing caused the earth to collapse on itself, forming a sinkhole.
“It was a massive shock, you never expect the road to collapse like that in this sort of area,” says barrister and former candidate for the Glen Eira council, Joel Silver. “Thank God nobody was on top of that road, the hole looked big enough to swallow an entire car.” Silver is just one of many locals who were given a shock introduction to this naturally occurring phenomenon.
Sinkholes form when highly porous rocks in the subsurface – such as limestone or gypsum – are dissolved by flowing water. This erosion leads to underground cavities, which continue to grow until the overburden no longer has subsurface support and gives way. Sinkholes also vary in size. While some are barely a metre in diameter, others can be as large as 600 metres.
While they are a natural phenomenon, recent years have seen faulty human infrastructure and highly intrusive activity lead to the rapid formation of sinkholes in urban areas. Last August, an enormous sinkhole opened up in the middle of a busy street in central Zhengzhou, in the Chinese province of Henan. Provincial firefighters believe the 20-metre wide and nine-metre deep sinkhole was the result of fragmented underground water pipes, similar to last year’s sinkhole in Glen Eira.
“There’s a variety of ways in which humans influence and create sinkholes,” says Mark Quigley from University of Melbourne’s School of Earth Sciences. “For instance, the most common thing we get in Melbourne is a road that’s underlain with water pipes.” It’s no secret that Melbourne’s inner-eastern suburbs are burdened with dated infrastructure. Professor Quigley, who previously worked in Christchurch as one of the chief scientists studying the city’s string of earthquakes, says there is a simple explanation for how plumbing can trigger sinkholes. “The pipes might leak and dissolve or wash away some of the surrounding material underneath the road. Eventually the road will collapse.”
Abandoned mines have also proven highly susceptible to sinkhole formation. Dr Oswald Maroni, a researcher from the CSIRO’s Department of Land and Water, says history shows a lack of damage control following the closure of mines. “If mines are abandoned, then normally they would have to be filled per some standards,” he says. “But sometimes they didn’t do a good job of that or they didn’t fill it at all and that can definitely lead to some problems.” There are numerous examples of unfilled mineshafts leading to the formation of sinkholes. Last August, a couple from Ipswich, Queensland, woke to find an eight-metre-wide, six-foot-deep sinkhole had formed in their backyard. This raised questions about the geological stability underneath the couple’s home. “It is known that an historic coal exploration hole was dug in the early 1900s near where the sinkhole appeared,” Queensland Mines Minister Anthony Lynham told ABC News.
Such instances prove the extreme speed and unpredictable growth of these occurrences. Dr Maroni says the rate at which sinkholes grow largely depends on the depth of the cavity. “Sometime they can grow gradually and sometimes they appear suddenly,” he says. “If the cavity is shallow than that might indicate a high possibility that a sinkhole will form, and this is much more dangerous.” However, Professor Quigley says there are several tell-tale signs that a sinkhole is forming in a certain area. “Subtle circular depressions at the surface are one,” he says. “There’s also a whole bunch of geophysical techniques we can use.”
These include the measurement of electrical currents in the ground, the density of the ground, structural materials or voids in the ground. Dr Maroni says sinkhole prevention can also be as simple as creating a “susceptibility map”.
“This shows where sinkholes may most likely form,” he explains. “A geologist can then look at a geological map and search for the presence of those highly soluble rocks.”
However, these techniques have proven difficult in highly-concreted, urban areas. Where most urban sinkholes are triggered by faulty sewerage or waterpipes one would think the answer would be as simple as monitoring or even upgrading this infrastructure. But for Joel Silver, the task of replacing entire sewerage systems in a suburban area would prove far too difficult. “It’s a challenge when you’ve got an area where people are living and working and you’d be having to rip-up entire streets,” he says. “It’s just not viable.” He remains optimistic that last year’s sinkhole was nothing but a “freak occurrence” and thinks it’s unlikely a similar event will happen anytime soon. “If it happens again then obviously that’d be a concern because you never want the road collapsing.”
But Professor Quigley says other methods are available to combat deep or well-hidden cavities in urban areas. “Using ground-penetrating radar or gravity surveys or electric surveys, we can actually map out these voids before they turn into sinkholes and then take preventative measures,” he explains. These preventative measures include “destabilising” the void by injecting the subsurface with grouting material or completely replacing the weak subsurface materials with stronger gravel. “We can then make it stronger and more homogeneous,” he says. “And that’s really the end game.”