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May 16, 2011
TORONTO TRANSIT COMMISSION
A drilling rig takes core samples from boreholes on the subway extension project.
Toronto Transit Commission testing soil for subway extension
DON PROCTER
correspondent
Tunnel construction of the 8.6-kilometre Toronto-York Spadina Subway Extension (TYSSE) represents a formidable job for the construction team, but even before building starts the geo-engineering crew has been busy with its own subsurface work along the line.
The Toronto Transit Commission (TTC) requires testing and monitoring of the soil for potentially combustible or hazardous gases and vapours prior to starting the underground project. Monitoring wells are installed in the hundreds of boreholes drilled to investigate soil and groundwater conditions 20-30 metres deep at tunnel level.
“Testing and monitoring is done to identify and minimize risk during construction of the subway,” says Hossein Bidhendi, geotechnical co-ordinator for the TYSSE project. The TTC started geo-engineering investigations in late 2008 and retained Coffey Geotechnics shortly after to undertake geotechnical investigations including vapour/gas sampling and monitoring.
So far, tests have revealed “no unforeseen challenges or surprises,” Bidhendi says, pointing out that methane has been recorded at various concentration levels in the Toronto area. Methane migrates from bedrock and possibly as a result of biodegradation of ancient matter (fossils) in lower deposits.
Coffey is also testing for concentrations of volatile organic compounds (VOCs). Small amounts of acetone, methyl ethyl ketone and tetrachloroethylene detected during vapour testing are not a health and safety concern. Hydrogen sulphide and light hydrocarbon gases and derivatives of gasoline such as benzene are also being tested for.
While many of the samples retrieved from the wells and boreholes have shown only trace amounts of methane gas to date, in one area the concentration was recorded at 39 percent by volume, Bidhendi says, adding such high recordings have been rare along the line.
“However, for the necessary due diligence exercise to identify and minimize any potential risk during construction of the subway, results of the sampling program were used for case analysis,” says Bidhendi, adding concentrations of vapours along the extension are not expected to present serious issues.
To deal with any high readings, high-capacity ventilation systems will be installed during tunnel construction by tunnel boring machines, he points out.
“The amount of methane expected to be released into excavation areas will be negligible,” Bidhendi adds.
Still, to be on the safe side, ongoing monitoring will be conducted.
Hand-held photoionization and combustible gas detectors are screening tools to test for gases and VOCs, says Fisher.
Another instrument measures for such gases as methane by changes in resistance with a catalytic platinum element. Gas samples can be collected by attaching tubing from specially designed cap at the top of the monitoring well to a Summa gas canister — a stainless steel container about the size of a volleyball. Under vacuum pressure, the canister allows for easy collection of air samples, explains Fisher, adding the samples are sent to a lab for analysis.
Boreholes on the site are typically drilled with augers four to eight inches in diameter to 20- to 30-metre depths. Two-inch diameter pipes used for wells incorporate screens or slots at various depths, usually below the water table surface to extract vapors, Fisher says, adding water-level readings are also collected.
Ontario’s Ministry of Labour requires contractors to test and monitor for harmful gases and oxygen deficiencies on underground projects.
Most site owners take the geo-engineering monitoring process seriously because underground conditions are unpredictable, says Bidhendi. Even in some open excavations for buildings, testing and monitoring is conducted, although health and safety risks are generally low because any hazardous vapours aren’t in a confined space.
Testing and monitoring similar to the TYSSE was conducted on the Sheppard subway project but there have been technological advancements in measuring equipment and lab analysis since then, says Fisher. For example, the contents of Summa gas canisters can be thoroughly analyzed through a process called gas chromatography which separates and analyses compounds that can be vaporized.
The subway line is scheduled for completion in 2015.
