SmartCentres is Canada’s largest developer and operator of unenclosed shopping centres, with over 250 locations across Canada. In 2014, SmartCentres acquired and developed a new property for a new and exciting store, to bring both value and variety to the City of Cornwall. Using innovative techniques and working in collaboration with all levels of local and provincial government, the design team rehabilitated a brownfield site for use by the local community. Included on that site is a new Walmart Superstore, a first for Cornwall.
From the start, the site had significant challenges for the development of a warehouse-style building. Existing as an old industrial site, the soils were not only loose/soft, but were environmentally impacted as well. The Walmart structure, with column loads up to 800 kN and slab loads up to 20 kPa, created significant challenges for this highly variable site. It consisted of loose clayey sand fills, to variable silty clay fills from 1m to over 8m deep. To complicate matters further, a highly compressible medium sensitive native clay was present in several boreholes on site, just below the fill. Zero to two metres of grade raise created the potential for long-term settlement issues, as two metres of new soil can represent an additional 40 kPa of new load.
The schedule also presented other significant challenges. Historically, when Walmart and SmartCentres decide to develop a property, the store’s open date becomes a very significant target. In order to overcome all of the challenges, as identified above, SmartCentre decided to divide the enabling works (site pad preparation) from the building. Through design team consultation, the Geopier Impact System was chosen to reinforce the soils for both the slab and footings. GeoSolv was selected by SmartCentre as the General Contractor, to provide ground improvement to get the pad ready to receive the building. One major advantage that helped us to trim time off of the schedule of this project was that the ground improvement work was able to be completed prior to any foundation permits having to be issued.
One of the major challenges associated with the schedule on this project was that the ground improvement work had to happen in the dead of winter (January and February) in order to allow the foundations of the store to be fully constructed in the spring. Although winter construction is possible with Geopier, the winter of 2014 was a particularly fierce one, with many days having wind chills of -30˚ C to -40˚C. While working in these temperatures slowed down the Geopier installation slightly, as certain safety precautions had to be undertaken, the Geopier installation work went smoothly and efficiently, even in those temperature extremes.
Footing support for the Walmart store consisted of one or more Geopier elements grouped together to increase the composite shear strength of the fill soils below the footing, creating positive settlement control to less than 1 inch, in a variable depth, loose, undocumented fill. Although Geopier elements work by improving the fill and developing lateral stress within the fill, it also provides the friction required to support the load within a few metres from the bottom of the footing. The elements were carried through the fill in all cases, as the risk of leaving undocumented fill in an unimproved state was unacceptable for both GeoSolv, as well as the design team. Although relatively lightly loaded, additional Geopier elements were installed under strip footings to mitigate the risk of variable, undocumented fill.
Slab support on this project consisted of Geopier elements spaced at a maximum of 3.6m on center. Finite element analysis was conducted by GeoSolv to provide the necessary soil stiffness information to the structural engineer, so that the slab design could be verified. Geopier slab design provided for significantly reduced shear and bending stresses in the slab, allowing the slab to be designed as a 4 to 6 inch unreinforced slab.
Because of the variable soils and variable grade raise, the design team had concerns about the exterior aprons and sidewalks outside the building on this project. Rapid Impact Compaction (RIC) was chosen to compact the soils to address risk of differential settlement for the building apron and sidewalks.
Of particular note on this project, the SmartCentres project was supported with the City’s Brownfield Community Improvement Program, which helped to offset the costs of remediating the former industrial site. Paterson Group, Canada’s oldest geotechnical firm, was instrumental in identifying and procuring the funding for this important project.
Carlos Da Silva, Managing Principal and Senior Engineer, had this to say about the project:
“Even though a Record of Site Condition (RSC) was obtained for the entire site by the previous owners of the property, no consideration was given to the future development when the site was backfilled with non-engineered backfill. The anchor building required a ground improvement program to deal with long term settlements. The City of Cornwall has a Brownfield Redevelopment Grant program. Paterson Group was successful in providing a rationale and justification convincing the City that a ground improvement program along with other items was an eligible cost recoverable under the Brownfield program. The $4.5M Brownfield grant was the catalyst required to justify the redevelopment of the subject site. Without this grant, the project would have been cancelled.”
Through innovation and a solid team to provide cost-effective solutions, SmartCentres was able to develop this site for far less than what a brownfield site will typically cost. Open for business in late January 2015, the Walmart store not only employs hundreds of people, it provides affordable goods to the many residents of Cornwall.