Construction tiebacks, a component commonly used to enhance the structural stability of retaining walls, have been around since the ‘60s. More than two million have been installed in the GTA alone.
The underground system quickly gained in popularity over bulky steel braces as they free up construction sites, making it easier, faster and more cost-effective to build while protecting neighbouring buildings.
However, while there is a long history of tieback use in Ontario and around the world, some municipalities in the province have recently begun moving to restrict or ban the use of tiebacks.
Toronto is one of those municipalities. The city has commissioned a probe of alternatives. In 2019, the Region of Peel looked into banning tiebacks altogether but decided, after considering the impact to construction and housing developments, to instead refresh its policies to allow them, with extra documentation required to ensure utility tunnelling can avoid old tiebacks.
Given the current state of the housing market, it would seem illogical for any government to consider new regulatory measures that make it more complicated and costly to deliver new housing.
Restricting the use of tiebacks makes no sense. The systems enable projects to be built faster and cheaper by reducing site congestion and simplifying excavation and restoration procedures. Modelling shows construction and forming timelines are speeded up which reduces costs.
Recently, a report and case study on the issue was prepared at the request of RESCON and the Ontario Association of Foundation Specialists by Isherwood Geostructual Engineers, a Toronto area consulting engineering firm that designs geostructures for new and renovation construction.
The firm examined the benefits of tiebacks compared to the primary alternative of steel bracing.
Bottom line?
The findings demonstrated tiebacks enable faster and more economical project delivery by reducing site congestion, allowing concurrent construction activities, and simplifying excavation and restoration procedures.
A tieback, also called an earth anchor, consists of a small hole drilled into the ground with steel cables inserted. The hole is then filled with cement grout, the steel is attached to the retaining wall and the cables are pre-stretched to snugly support the wall.
Tiebacks were introduced into the Toronto market in the 1970s and quickly gained popularity.
Steel braces were the main method of supporting deep excavations before the adoption of tiebacks. However, they become more challenging when sites become too deep or too wide, needing complicated frames and supports to keep them from becoming inadequate to do their job.
Steel braces also clutter up excavations, make it more difficult to transport material, obstruct site lines, lengthen construction schedules and create more pinch points.
Across Ontario, tiebacks are regularly used by the industry in building subways, utilities, tunnels and underground parking, keeping an excavation open and safe while a structure is being built.
Tiebacks promote industry competitiveness by enabling construction methods aligned with modern scheduling, logistics and safety expectations. Meanwhile, the engineering profession has established robust design, construction and testing standards for tiebacks.
The author concluded permitting practices in Ontario should continue to reflect best engineering practices and construction realities, and continued acceptance of tiebacks, particularly under the municipal right-of-way, would support housing supply objectives, minimize building construction impacts on communities and reduce unnecessary cost burdens on developers and contractors, but more importantly, reduce costs to end users and not further erode housing affordability.
Tiebacks are permitted in the public rights-of-way in the majority of Ontario municipalities, including Hamilton, Burlington, London, Windsor, Oakville, Milton, Mississauga, Vaughan, Markham, Toronto, Waterloo, Kitchener, Cambridge, and Ottawa, with varying limitations, conditions and fees applied.
However, some municipalities are rethinking the practice due to concerns of potential damage to utilities during tieback drilling or concerns over an incident in 2022 in which a micro-tunneling-boring machine for a sewer became stuck on old tiebacks along Old Mill Drive in Toronto.
Had the available records been provided to the tunnelling consultant or contractor, the problem could have been avoided, just as other utilities along that street were avoided.
Tiebacks are often too deep underground to be dug up after construction of a building is completed. There are some that are removable but almost all options leave a portion of steel cable or bar in the ground, as the mechanisms they rely upon to release the steel can be unreliable.
There is no solid reason to discontinue the use of tiebacks as they are a safe and proven state-of-the-art technology to support excavation retaining walls. The Ontario construction industry is geared up for the use of tiebacks and they have clear advantages over alternatives.
The industry has more pressing challenges, namely reducing the cost of building new homes. We cannot afford the cost of retooling this vital way of supporting retaining walls and structures.
Richard Lyall is president of the Residential Construction Council of Ontario (RESCON). He has represented the building industry in Ontario since 1991. Contact him at [email protected].
