The
northwest corner of St. Clair Avenue West and Weston Road in Toronto is
home to a 600,000 square foot big box retail centre called The Stockyards.
The site is a part of a large redevelopment project that consists of the
construction of multiple commercial retail units in two levels and a three-storey
parking garage. The site had been historically used to raise livestock,
process meat and related products after being used as landfill from 1920
to 1950.
The geotechnical investigations required ground improvement due to poor
soil conditions and the loading configuration. As an alternative to costly
large scale excavation and replacement, Menard (formerly Geopac), which
is a subsidiary of the Soletanche Freyssinet Group (similar to RECo), proposed
a value-engineering solution utilizing Dynamic Compaction (DC), Rapid Impact
Compaction (RIC), and Controlled Modulus Columns (CMC), to improve the
upper 6-15 meters of fill materials on this challenging project.
The
site grading plan required retaining walls to retain the backfill and support
the footings of commercial buildings and the parking structure along Weston
Road, Gunns Road as well as the north entrance ramp. The project consultant
proposed mechanically stabilized earth (MSE) walls to a maximum height
of about 9 meters. Reinforced Earth Company Ltd. (RECo) was awarded the
project that included the design and supply of a 2300m2 vertical facing
Reinforced Earth (RE) retaining wall with precast TerraClass panels and
high adherence steel strip soil reinforcements. The ground improvement
work was designed and carried out by Menard. This affiliation provides
clients with innovative and cost-effective solutions that are offered within
one group.
In the north and northeast of the site, with a commercial retail unit only
one and a half meters behind the wall, Menard and RECo planned to utilize
CMC’s to support the retaining wall and use a preload of 30 kPa after
wall construction to induce the initial settlements before the building
is constructed. The wall in this section is 7 m high and the CMC’s
are to a maximum depth of 9 meters. Sharing the same parent company played
a big role in securing this project and this project is the first of many
synergy projects between RECo and Menard across Canada.
The
subsurface investigation indicates a heterogeneous fill at the site and
further investigation identified a thick layer of soft to firm silty clay
fill right underneath the northeast retaining wall. Spacing, diameter and
depth of the inclusions were defined to respect a maximum settlement of
25 mm.
The approach in the design of load bearing MSE walls is developed from
an extensive research in modeling, testing, field instrumentation, and
the verification of material behavior versus design theory, uniform analytical
design procedures based on finite element analysis. This method is confirmed
and supported by measurements from earlier structures which allows the
design of load-supporting RE in the project and enables analysis of external
loads from superstructure footings.
Prime considerations in the design of this type of load bearing walls are
the bearing capacity, the load distribution, and tolerance to settlement
and horizontal deflections. The flexibility of the facing enables the RE
wall to better withstand movements in the vertical, horizontal and longitudinal
directions. Accordingly this type of wall is suitable to withstand the
internal settlement and foundation movements due the preloading suggested
in this section of the wall.
The
arching effects from the CMC may cause differential settlement and add
stresses to the lower strips at the base of the RE wall. In order to mitigate
the additional stress, a load transfer platform (LTP) was considered. The
LTP distributes the CMC axial loads to soil reinforcements and eliminates
the local stress on the first layer of steel strips. The LTP consists of
a compacted granular fill with 100% compaction and about 1 meter thickness
that is placed between the top of CMCs and the first layer of soil reinforcement.
A monitoring system was implemented to regularly observe the movements
of MSE wall after wall construction and during different stages of loading.
In order to do that, reflective targets were placed on the facing panels
in defined intervals. Targets were monitored for a total duration of 6
months prior to the building construction. According to the monitoring
results, the settlement of the wall was in the range of 15 to 20 mm under
the surcharge. Horizontal displacement readings also showed a fairly uniform
movement of the wall face, approximately 22 mm without any tilting. Monitoring
readings are consistently lower than the range calculated in finite element
analysis. Based on the results of post-construction monitoring, it was
concluded that no significant wall movement has occurred.