DFI Magazine January/February 2020 - 92

* Thin caisson walls (thickness less than
about 24 in or 610 mm) have higher
susceptibility to frost and can exhibit
different behavior than thicker (at least
32 in or 813 mm) secant pile walls.
* Lack of frost protection at the face of
shoring walls can cause overstressing of
the excavation support bracing
requiring remedial action to protect the
integrity of the excavation shoring
system.

Acknowledgements
We would like to express our gratitude to
Clark Construction Management and to the
rest of the project team for its collaboration
in undertaking the construction of
Toronto's first supertall residence.

View of northwest wall and super corner during excavation of elevator pit

Isherwood has observed damaged lagging,
cracked caisson walls, broken tieback
connections (including projectiles) and pile
movements greater than 1.5 in (38 mm) in
one week and more than 3.5 in (89 mm)
total. These forces due to frost present safety
concerns, can cause site delays and require
increased field presence, monitoring costs
and require remedial action. Our recommendation regarding frost move-ments is to
protect the shoring and soil faces from
freezing between November 1 and March 31
using insulated blankets or tarps, heating
tents or spray on polyurethane foam.
Prior to November 1 at The One, frost
protection was installed on the micropile
shoring wall only, which showed little-tono impact from frost throughout the winter.
The "skinny" caisson walls on the northwest
and south walls posed an additional
challenge for frost susceptibility. These 23 in
(584 mm) thick walls were the first to show
impact from frost, exhibiting movement
over the entire depth of the excavation, and
required remedial action to protect the
integrity of the shoring. Insulated blankets
and active heating were required on the

92 * DEEP FOUNDATIONS * JAN/FEB 2020

northwest and east walls and the south wall
was backfilled with a 23 ft (7 m) high soil
berm to the third anchor level to protect the
anchors from yielding. The total additional
movement due to frost ranged from about
0.5 in (13 mm) on the northwest wall to
0.75 in (19 mm) on the south wall. After the
application of active heating, the frostinduced movement was recovered on the
north and east walls and the movement on
the south wall was recovered after
temperatures rose above freezing. Frost
movement had little-to-no impact on the
north wall (i.e., south facing), which
received the most sun exposure.

Lessons Learned
Several lessons were learned throughout
the excavation and shoring of The One:
* Design-build alternatives at tender can
lead to innovative solutions.
* Vibratory installation of the casings for
the large diameter structural caissons at
subgrade had small impact on performance of the support of excavation
system in the stiff-to-hard silt/clay and
d e n s e - t o - h a rd s a n d ( m a x i m u m
movements of about (1/2 in or 13 mm).

Shawna Munn, P.Eng., is a project manager and
design engineer with Isherwood Geostructural
Engineers in Mississauga, Ontario. She has more
than 10 years of experience in the shoring and
specialty foundations field with a focus on complex
excavation retention systems, subway infrastructure, anchors, bracing. and micropile shoring
and foundations.
Nadir Ansari, P.Eng., D.GE, is the CEO of
Isherwood Geostructural Engineers and has more
than 30 years of experience in the geostructural
industry. He has been involved in the analysis,
design, review, field inspection and monitoring of
more than 1,500 structures across Canada and in
the U.S., Mexico and the Caribbean.
Toben Jerry is the senior vice president of the shoring
and foundations division of GFL Infrastructure
Group in Vaughan, Ontario. Since joining the
industry in 2003, he has obtained experience with
specific management roles in operations, estimating
and project management.

DFI Magazine January/February 2020

Table of Contents for the Digital Edition of DFI Magazine January/February 2020