YORKVILLE

SINKHOLE REPAIR

OVERVIEW

When a void in a well traveled, outdoor public walkway in a high end retail district of Toronto, Ontario presented a danger to public safety and impacted local business to operate efficiently FORSMITH was retained to determine the cause, develop a solution and ensure execution of the remediation was completed as specified so the pedestrians, residents and business owners could get back to safely traversing the dense, historic retail village.

PROBLEM

FORSMITH was contacted to investigate the breadth and causation of a below grade subsidence (sinkhole) situation below a public walkway in a high-end retail district of Toronto, Ontario. Once the as built conditions were confirmed FORSMITH was then responsible for developing an efficient, nonintrusive, and cost-effective solution that would return the walkway to a safe and sustainable condition.

CHALLENGE

Determining the cause for the sinkhole development was the initial challenge associated with this project. The outdoor walkway had been in service for over a decade, remaining open to the public as old buildings within the dense retail area were torn down and new ones erected in their place. FORSMITH was retained at the first discovery of the issue to determine the scope of danger the current conditions below the walkway presented and identify the source for the sudden change in the ground’s stability. Because changes in below grade conditions could have come from a range of deficiency in building envelope systems or design, the source of the subsidence issue had to be methodically evaluated. The systematic, forensic approach the team at FORSMITH employs on every project they undertake allowed for a rapid response to this time sensitive problem.

SOLUTION

FORMSITH undertook a targeted investigation to both efficiently determine the cause of the subsidence issue but not to be too intrusive to the neighboring businesses and residences that overlooked the site. FORSMITH systematically reviewed the design drawings and archive photos of the construction process for the building adjacent to the initial subsidence issues site, which was only constructed 5 years ago. The design review provided FORSMITH with details regarding potential drainage systems present at the foundation and the history of the walkway through the construction process revealing the walkway was kept in operation through the entire duration of the construction project through an extensively designed shoring system deployed on that particular stretch of building foundation. In parallel FORSMITH coordinated the surveying of the entire length of walkway by Ground Penetrating Radar (GPR) to detect if and where additional subsidence issues were present but had not manifested into a safety issue at the surface yet and the complete camera scoping of the ground water drainage and weeping tile system for evidence of sediment or granular material being able to evacuate from below the walkway into these systems. When additional voids along the same section of wall were detected and no evidence of material evacuation through the available ground water systems the focus of the investigation shifted to the as designed vs. as build conditions of the recently constructed Building.

SOLUTION

The shoring design for the building constructed along the duration of the subsidence issue had called for a unique shoring system consisting of piles featuring potions of lagging on top and a transverse steel beam running horizontally to support the top 7’ of the pedestrian walkway while a void below this beam would be created during excavation given an existing secant wall from construction of the previous building ran parallel to the new foundation. The designers of record on the project had been aware to the potential for this void to occur and had specified appropriate considerations be made to ensure that these voids were backfilled prior to the foundation wall being formed but our investigation to date indicated that these instruction may not have been headed. Using a remote controlled camera the existing sinkhole location was mapped with the void measuring approximately 17’ deep, 6’ in length and 18” in width, aligning with the design features of the shoring pile locations and size. Most notably the void was observed to be clean. No evidence of any granular material on the face of the forming ply or sides of the piles were observed at all. The observation was confirmed when a suction truck was brought to sight and a full 12 section of the top walkway removed. After the first 7’ of material were suctioned away a void was present along the entire duration of the exposed area. We were then able to determine the magnitude of the repairs.

We designed a remedial solution to solve the issue in quick fashion allowing the owners to open the walkway buck up to the public while ensuring no further subsidence issues could pose a risk to the safety.