Town of Oxford could be facing pricey tests to get sinkhole answers
The town received two replies to its request for proposal last month
The Town of Oxford, N.S., could be facing a very expensive test to determine the extent of its sinkhole problem.
In late August, the sinkhole in the town, located about 160 kilometres northwest of Halifax, grew seemingly overnight, swallowing picnic tables and mature trees.
Amy Tizzard, the regional geologist with the Department of Energy and Mines, said the sinkhole has been holding steady at around 33 metres by 39 metres for the last few weeks.
"Overall, it hasn't grown in any leaps and bounds lately, but I wouldn't say that it's stabilized either because it does continue to erode along the edges, but just on a centimetre scale, instead of by metre," she said.
A few weeks ago, Oxford issued a request for proposals for a geophysical survey at the sinkhole site.
The aim is to determine what lies beneath the area and whether there are more empty spaces that could potentially cave in, expanding the town's problem.
Tizzard said there were two formal submissions to the town's request for proposal that ended Sept. 19.
The first involved scanning the site with ground-penetrating radar at an estimated cost of $30,000. The second would involve more in-depth geophysical scans, using seismic methods, at a cost of $100,000.
This does not include any estimate on how much it might take to fix the sinkhole problem, if that's even possible.
A $1,500 test before the tests
Before the town goes ahead with either test, however, Tizzard said the town is doing its due diligence by first hiring a Dartmouth company to perform a GPR test at a cost of $1,500.
"Before they commit to a larger survey, that's required of the area, they'll just do a test run to see how effective that ground-penetrating radar is," she said.
The way it works is the equipment transmits a signal into the ground and then measures how long it takes for that signal to reflect off of different materials.
"Air space will have a very different density than sand or gypsum rock. So that's the sort of thing that the GPR is going to try to investigate is just whether or not it can get deep enough to where the caverns would be in the gypsum," she said.
"We know that there is a bed of sand that overlies the gypsum there and it's probably at least 10 metres deep, just the sand itself. So that's why we want to do this test run first."
Tizzard said success depends on many factors, including the composition of the soil, how far those layers are underground and the conductivity of those layers.
In an ideal scenario, Tizzard said, the radar can achieve up to 15 metres of penetration.
"However, we're uncertain whether the high conductivity of the sinkhole water and the sand around it will interfere with that signal and actually maybe not get to the depths that we need to look at whether there are any more underground caves or caverns in the area," she said.
Ideally, said Tizzard, the radar will work and the contractor can scan the whole region to better understand what lies beneath Oxford.
"But if it doesn't work, then the town will have to look to more sophisticated geophysical methods like seismic profile or resistivity," she said.
Those more advanced methods come with a bigger price tag.
Rachel Jones, the town's chief administrative officer, said in an email the town has been in touch with the province's Department of Municipal Affairs, though she said she has no confirmation what the province might be willing to chip in.
"Neither the Town of Oxford nor the Oxford Lions Club have the financial capacity to pay for the larger testing program, or any of the possible remediation of the site if that option is deemed possible," said Jones.
"There have already been significant costs incurred to ensure public safety."
In a news release, the town said the test will happen Tuesday, weather permitting.
In the meantime, Tizzard and the town continues to watch and wait.
"All I can keep doing — until we have that more information — is just monitor the site and keep the public away because it's still unstable."