5:30 p.m., Dec. 7)
of failures led to Hwy. 174 sinkhole, report confirms
massive sink hole that closed the eastbound lanes of Hwy.
174 for two weeks was caused by a series of failures dating
back to when the stormwater collector pipe was first installed
under the highway in the late '70s.
to a report prepared by the engineering firm B.M. Ross
& Associates for city council, the root cause of the
sinkhole was a 48-metre section of the pipe that was made
out of non-glavanized liner plate.
is not clear is why the section of pipe wasn't galvanized
before it was installed. According to the City of Ottawa's
infrastructure asset database, the stormwater sewer pipe
is listed as a Corrugated Steel Pipe (CSP). No mention
is made of the 48-metre section of non-galvanized liner
original drawing of the pipe as proposed in 1972, indicated
that approximately 55 metres of the pipe was not CSP,
but rather "10 gauge, hot-dipped galvanized and asphalt
coated liner plate".
as-built drawing submitted to the former City of Gloucester
after the pipe was installed only identified the addition
of 15.2 metres of CSP on the south end of the pipe. There
was nothing on the drawing to indicate that a liner plate
had been installed as part of the project.
liner plate wasn't discovered until a closed circuit television
inspection was done of the pipe in August 2011, which
was also when inspectors discovered major areas of corrosion
in the liner plate itself.
it was this section of the pipe that failed due to corrosion
resulting from it not having been galvanized. Non-galvanized
steel corrodes much more quickly than galvanized steel.
fact that the liner plate wasn't galvanized was not discovered
until the section was removed and sent for testing where
they not only found out that it wasn't galvanized, they
also discovered that it wasn't asphalt coated as was indicated
in the original drawing.
establishing that the collapsed section of pipe was not
treated properly for its intended use, the investigating
consultant focused on the events which led from the initial
inspection of the pipe by closed circuit TV in August
2011 to it inevitable collapse.
particular, the consultant suggests in his report that
the closed circuit inspection showed enough damage that
it should have been followed up by a physical inspection.
He also suggests that a physical inspection may have resulted
in a quicker timeline for repairs.
appropriate immediate response would have been a visual
the goal of better understanding the condition of the
pipe, followed by whatever response the visual inspection
determined was warranted," the consultants state
in their report. "In our opinion, the immediate need
for a more robust assessment of the pipe was not understood,
identified or communicated."
it was, the closed circuit inspection resulted in city
staff identifying the pipe was in need of rehabilitation
and they immediately started the process to reline the
existing liner plate.
it would take a whole year to approve the required work,
send it out to tender and hire a company to do the job,
which is another failure of the system which allowed the
pipe to reach the point of collapse.
final factor which the consultants say "likely"
precipitated the collapse on Sept. 4, was the work being
done in the pipe on both Aug. 31 and the day it happened.
contractor had been inside the pipe on both days installing
lights and removing rocks and debris. Workers were using
a mini-excavator to remove rocks and other debris from
the pipe to prepare it for the new liner.
left the pipe at approximately 4 p.m. after it started
raining. The collapse occurred less than an hour later.
In the consultant's opinion, "vibrations from the
excavator and loader, or changes in the flow path of the
water, as a result of the debris being moved influenced
the timing of the collapse".
the pipe collapsed it created a sinkhole which grew at
a fairly rapid rate and engulfed a car belonging to J.P
Unger who accidentally drove in it.
rest, as they say, is history. The eastbound lanes of
the highway had to be closed for two weeks creating traffic
chaos for commuters, and it ended up costing $5 million
to replace the pipe and fix the roadway compared to the
$1.7 million it would have cost just to reline the pipe.
prevent similar catastrophes from happening in the future
the consultants made five recommendations.
The current definition of a high-risk storm sewer asset
should be expanded, to include consideration of the probability
of failure as well as the consequences of failure.
Storm sewer assets designated as high risk other
than those that were assessed following the event in question
should be examined as soon as possible and reviewed
by persons qualified to assess the condition and judge
the need for further action.
An attempt be made to assess the quality of the information
in the Citys storm sewer asset inventory. Where
there are weaknesses related to the inventorys source
materials or as determined from observations, an effort
to improve the data should be made.
With full consideration of safety issues, and where feasible,
physical inspections be used to supplement CCTV inspections
for high risk assets.
Procedures for scoping capital projects should always
include a discussion of the consequences of not proceeding
response to the report senior staff are already implementing
all five recommendations.
report and the city's infrastructure asset management
procedures an protocols will be discussed by city council
at a special meeting in January.
story was made possible thanks to the generous support
of our local business partners.)
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