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 build-ing
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 geotech-nical
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, includ-ing
the monitoring of the adjacent buildings and LRT tracks,
using inclinometers and extensometers to determine horizon-tal
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 shot-crete
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 conven-tional
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
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