Years of experience plus in-depth knowledge of local soil and geological conditions mitigate challenges of predicting shoring performance before excavating
By Barb Feldman
RWH Engineering Inc. was established by engineers Craig Rowe, Jason Weck and Martin Halliwell in 2009, to provide design services for HC Matcon Inc.
Both are now part of the HCM Group, a contractor that has offices in both Calgary and Ontario. The HCM Group offers a full range of geotechnical excavation, shoring and foundation services, including caisson wall, soldier piles and lagging, shotcrete and underpinning, footing tie-down anchors, rock and soil anchors, caissons and belled piles, helical and micropiles, as well as in-house custom fabrication and load testing of helical foundations.
RWH Engineering’s own team of surveyors, engineers, technologists and project managers specialize in shoring and deep foundation systems, with a focus on material usage and sustainability, construction sequencing, site/soil suitability and cost efficiency, providing deep foundation and earth retention design solutions for projects across Canada ranging from solar farms to foundations, as well as geostructural precision monitoring services at all stages of construction to help clients better understand their project’s performance and to help mitigate their risk. HCM and RWH have now been associated with the design-build shoring solutions on the four tallest buildings in Alberta: the Stantec Tower, a mixed-use skyscraper currently under construction in Edmonton; and The Bow, Brookfield Place and the TELUS Sky Tower in Calgary.
The excavation shoring for the TELUS Sky Tower has been one of the most uniquely challenging projects RWH Engineering has worked on, said RWH president Craig Rowe – not least because of the project site’s small footprint and its proximity to existing infrastructure, including the 12-story Telus building directly adjacent to the site, the large utility ducts surrounding it, nearby LRT tracks and a 26-storey building separated from the site only by a laneway. The excavation itself was the deepest in Calgary’s history, and the third-deepest in Canada, extending 110 feet down to accommodate a seven-level underground parking garage. Once completed in early 2019, the 59-story mixed-use tower, incorporating office, commercial and residential spaces and Calgary’s first building designed to LEED Platinum standards, will be the city’s third-tallest building and the thirteenth tallest building in Canada.
The deepest excavation in Calgary’s history
“Downtown-area rock in Calgary is very unpredictable,” said Rowe, “so when you excavate it, it’s really tough to know how much these walls are going to move.” There was no geotechnical investigation completed at the time of tender. Instead, HCM and RWH designed and bid the project based on what they’d learned from their experiences in the same area with the construction of The Bow and Brookfield Place, as well as their in-depth knowledge of local soil and geological conditions.
In Calgary, these are typically non-cohesive overburden soils of varying depth, extending approximately six to nine metres over highly weathered, low-strength bedrock. This geological formation is composed primarily of siltstones, sandstones and mudstones, and can contain weak zones and localized shear band layers that cannot be identified using conventional geotechnical sampling techniques. This makes it challenging to predict shoring performance and impact to the surrounding infrastructure prior to excavating, and creates the risk in deep excavations in Calgary.
“To manage the risk properly we relied on our experience of the last 10 years in Calgary and a lot of monitoring, including the monitoring of the adjacent buildings and LRT tracks, using inclinometers and extensometers to determine horizontal and vertical settlement movements,” said Rowe.
HCM and RWH designed an eight-metre deep perched secant caisson wall that was held back by two to three rows of post-tensioned anchors, the top row installed at 45 degrees into the rock to maintain clearance beneath the existing building’s footings along the north wall. This extended approximately four metres into the bedrock over a 75-millimetre-thick shotcrete system that continued below in the excavated rock to the full excavation depth.
To manage the weak mudstone and control deflections of the caisson wall, the shotcrete beneath it was installed in a panel sequence. Once into the sound rock, a more conventional soil nailing approach could be used and the shotcrete was spaced on equally sized horizontal and vertical panels with the same anchor lengths to tie the rock together as a block. This approach was used on three sides of the site, but because of the existing 12-storey Telus building on the west side, RWH designed a zero-clearance shoring solution for that side that included a shotcrete underpinning system extending 20 metres below the footing.
After the caisson wall was installed, six inclinometers were installed below the base of the excavation and one in the middle of the shotcrete underpinning wall; once the excavation reached rock, four 35-metre-long extensometers were installed at 2.5 metres below that. The site’s geometry helped to reduce the shoring movements, which overall were less than predicted by the two-dimensional Finite Element model, according to a company study.
“Monitoring during excavation and construction is like an insurance policy”
“Typically, the requirements for supportive excavations and shoring are found in urban locations where you have nearby buildings and are planning a deep excavation, so our main markets are Toronto, Ottawa and Calgary, and mostly urban projects, but not exclusively,” said Rob Mara, RWH’s quality assurance manager. “In our own markets we provide construction services for competitors such as vibration monitoring, inclinometers, constructability and quality control,” Mara said. “We occasionally work with other contractors, but more so with developers. We’ll also work with other consultants to look at value engineering on projects such as construction suitability and optimization of foundation solutions.”
Owners are becoming more aware of the risks of excavating next to existing buildings in urban settings and monitoring during excavation and construction is like an insurance policy, said Rowe, “a tool that owners can use to protect against litigation or to minimize their risk – when there are problems with adjacent buildings we can say, ‘Review the monitoring data: the building didn’t really move,’ or within a reasonable amount of what you’d expect.”
“Different areas throughout Alberta and Ontario sometimes have incredibly specific ground conditions,” said Mara. “Understanding how the ground is going to behave requires experience and insight, as well as the technical understanding of the theories that apply.”
RWH in conjunction with HCM continues to work on many unique projects across Canada. Recently, these have included the excavation for a new hotel in Ottawa using a curved lagging wall to accommodate the parking structure and a 40-metre-diameter shaft in Hamilton with perched piles and shotcrete rock protection, which will extend 90 feet deep when completed later this year. 