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Disan Court slope restoration a complex stabilization

Dan O'Reilly
Disan Court slope restoration a complex stabilization
TRCA—Severe erosion behind homes on Disan Court resulted in non-engineered timber retaining walls to fail and seriously compromise the working platform for the storm and sanitary lines which extend between the houses.

Reducing threats to life and property from flooding and erosion is a core responsibility for the Toronto Region Conservation Authority (TRCA) and one which extends well beyond massive, lengthy, and expensive projects.

Small scale restorations such the recently completed $425,000-Disan Court Stabilization Project often come with their own scales of complexity, says TRCA project manager Courtney Rennie.

In the wake of a major storm in July 2013 which caused damage to hundreds of public and private properties throughout the Greater Toronto Area, the steep slope behind two houses on Disan Court in northwest Toronto was severely eroded. The erosion caused the decades-old non-engineered timber retaining walls to fail and seriously compromised the working platform for the storm and sanitary sewers lines which extends between the houses, she says.

Undertaken by a small TRCA crew between June and November of this year, the stabilization included demolishing and removing the old retaining walls, excavating unsuitable fill from the slope, the reconstruction of the sewer infrastructure platform, and erecting a two- to four-metre-high (6.5- to 13.1-foot) segmental block retaining wall.

Project consultants included Sarafinchin Associates Ltd. who completed the slope stability and erosion risk assessment and EXP which developed the detailed designs for the retaining wall.

In late October the crew began the site restoration which included planting some trees in the homeowners’ back yards and erecting privacy fence on top of the retaining wall. Trees were also planted in a nearby park to compensate for tress loss in the ravine stemming from the construction, she says.

Conducted in the rear yards of the two adjacent homes which overlook the Humber River valley, the project came with a list of challenges, not the least of which was squeezing small machinery in the tight space between those homes, she says.

First step in the schedule was laying down a trail of mulch to lessen the impact of a Tri-axle, a skid steer, and a Cat 305 excavator which were used in the various phases of the project. An estimated 650 cubic metres or 65 tri-axle loads of the fill was excavated and then stockpiled on a small staging area on Disan Court until it could be removed, says project supervisor Norman Pires.

And in the middle of last summer’s heat there were a number of extremely hard tasks for the crew including demolishing the old three-layer concrete working platform and building the retaining wall, says site supervisor Anthony Fortunato.

“Each block weighed about 250 pounds (113 kilograms) and they had to be saw cut on site for the custom corners.”

Although the retaining wall was required on this particular site, the authority’s preferred option is to regrade slopes wherever possible to lessen the environmental impact and avoid future maintenance issues at some point in the future, says Rennie.

The need for some form of remediated action was brought to the attention of the authority shortly after the 2013 storm by the local city councillor. But the project didn’t commence until this year because the TRCA has a priority list of projects and only so much funding, says Rennie.

Under its Erosion Risk Management Program, the majority of that funding is directed towards the maintenance of existing erosion control structures along rivers and valleys which protect public greenspace, park amenities, and municipal infrastructure. But it does apply to protecting private property where homes and other essential structures confirmed to be at risk from erosion or instability.

 

TRCA—A panoramic photo of the completed $425,000 Disan Court Stabilization Project, which included slope stabilization, demolition and removal of old retaining walls, excavating unsuitable fill and reconstruction of a sewer infrastructure platform.

In the case of those private sites a three-step assessment process is used to determine what can and should be done. That process starts with an initial site visit and then can be accelerated to a detailed risk assessment to a final decision on whether work is required, she says.

“Properties where no risk has been identified are recommended for monitoring only.”

On properties where work is deemed necessary there is period of pre-construction negotiation with the homeowners who are expected to bear a portion of the project cost or transfer ownership of the affected land parcel to the TRCA.

Unless a parcel is landlocked with no adjacent public lands, the authority prefers the property transfer route. Not only does it foster the expansion of public greenspace, it ensures the parcel is owned by one owner (the TRCA) instead of multiple owners, says Rennie.

“One single owner ensures consistent monitoring, maintenance, and repairs to erosion control structures throughout their lifespan.”

With hundreds of reports of property damage following the 2013 storm, the authority has been working to address the most severely damaged sites and that ranges from four to six sites annually depending on funding.
“We’re getting to the end of the 2013 list.”

However, subsequent storms have caused similar slope problems and they will also have to be addressed, she says.

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