503
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
1
Performance of a deep excavation in downtown Toronto
Performance d'une excavation profonde au centre-ville de Toronto
Cao L.F., Peaker S.M., Ahmad S.
SPL Consultants Limited, Ontario, Canada
ABSTRACT: This paper presents field measurements of soldier pile walls installed in the clayey soils and shaly rock in downtown
Toronto. The method of deducing wall bending moments from the inclinometer measurements was evaluated and discussed. Back-
analysis using a finite element program has been carried out to evaluate the shoring wall performance as well as the creep behaviour
of the shaly rock. Recommendation for the design of soldier pile walls in the similar soils and bedrock conditions were provided.
RÉSUMÉ : Cet article présente des mesures de terrain pour des murs de pieux soldats installés dans les sols argileux et le roc shaleux
du centre-ville de Toronto. La méthode donnant les moments de flexion du mur à partir des mesures inclinométriques a été évaluée et
discutée. Une analyse a été effectuée avec un programme d'éléments finis pour évaluer la performance du mur étayé ainsi que le
comportement en fluage du shale. Des recommandations, pour la conception des murs de pieux soldats dans des conditions
semblables de sols et de roc, ont été fournies.
KEYWORDS: deep excavation, field measurement, inclinometer, bending moment, finite element, time-dependent deformation
1
INTRODUCTION
Underground structures such as basements and subway have to
go deeper today than in the past due to limited space in densely
populated urban environments. As deep excavations induce
large stress and strain, underground structures and the adjacent
structures/utilities will confront risks of being damaged. As the
soil/rock stress-strain behaviour is non-linear and affected by
many factors, it is difficult to predict the ground movement
induced by excavation. In practices, field measurements are
widely used to monitor soil/rock behaviour and to control
ground movement.
This paper presents a case study of a deep excavation in
downtown Toronto. Soldier piles with tiebacks were used to
support the excavation. Two inclinometers and one hundred and
twenty seven reflective targets were installed to monitor the
movements of the shoring walls during and after excavation.
The inclinometer measurements have been used to deduce the
wall bending moments. A finite element program has been
carried out to evaluate the performance of the shoring walls. It
is found that the total stress analysis leads a good prediction of
wall deflections during the excavation, whereas the effective
stress analysis is required to model the behaviour of shoring
walls after excavation. The back-analysis also shows the
evidence of the creep movement of the shaly rock.
2 GROUND CONDITION AND TEMPORARY SUPPORT
SYSTEM
The site is located at 352 Front Street West in Toronto, Ontario.
Field investigation with drilled boreholes revealed that the site
stratigraphy was made up of about 1 m thick, compact sand to
gravel fill with asphalt surface overlying 3 to 4 m thick, firm to
hard clayey silt fill over 2 to 5 m thick, stiff to very stiff clayey
silt till. Both clayey fill and till are low plasticity soils. Georgian
Bay formation of shale and limestone/siltstone was encountered
at 9 to 9.5 m below existing ground surface. The groundwater
table was about 5 m below grade.
The Georgian Bay formation is generally massive shaly rock
with widely spaced jointing and sub-horizontal bedding planes.
The influence of sedimentary shaly bedrock formations on the
engineering performance of underground structures in Southern
Ontario was summarized by Lo (1989). The shaly bedrock
formations are subjected to high in-situ horizontal stresses with
typical coefficient of lateral earth pressure K
o
of 4 or greater.
Upon relief of the high residual horizontal stresses, time-
dependent, creep-like deformations take place. These time-
dependent deformations that are highly stress dependent, persist
well beyond the initial elastic deformations and generally
exceed the magnitude of the elastic movements.
Soldier piles of steel H-beam W610x82 at 3.05 m spacing
with wood lagging were employed to support an approximately
14 m deep excavation in which 9.3 m excavation was inside
overburden soils and 4.7 m excavation inside the bedrock. The
soldier piles were installed typically 16 m below the existing
ground surface in 910 mm diameter drilling holes. The drilling
holes were backfilled by 0.4 MPa concrete with the exception at
the pile toe, where 20 MPa concrete was used to support the pile
toe. Two layers of tiebacks were installed at approximately 3.3
and 8.3 m below the existing ground surface, respectively to
support the soldier pile walls during excavation. The tiebacks
were installed within 150 mm dia. cased boreholes and bonded
in bedrock. Each tieback was made up of 6 to 7 numbers of 15
mm strand tendons. The upper and lower tiebacks were installed
at 45
o
and 25
o
to the horizontal direction, respectively. The bond
length of the upper tiebacks was typically 5 m and the free
length 9.4 m. The bond length of the lower tiebacks was
typically 3 m and the free length 3.9 m. The tiebacks were
generally post-grouted the day after they were installed. The
typical design loads for the upper and lower tiebacks were 1000
and 800 kN respectively. Figure 1 shows outlook of soldier
piles with wood lagging supported by tiebacks.
Two performance tests for the tiebacks were conducted up
to 138% and 200% of the design load, respectively. The test
loads were maintained for 0.5 to 1 hour and the tests met the
PTI criteria (PTI, 1996). Proof tests were carried out for all
