A former electrical sub-station in the Junction District of West Toronto underwent site remediation for PCB and hydrocarbon contamination in order to permit re-development. The Remediation Plan called for installation of a braced interlocking (secant) caisson wall excavation support system around the site perimeter, followed by excavation of contaminated soils to a depth of 6 m below grade. In the course of identifying and removing abandoned subsurface utilities prior to caisson drilling, the remediation contractor, Priestly Demolition Inc. (PDI) discovered a live electrical duct bank within the site footprint. The duct bank serviced an adjacent heritage building and could not be taken out of service or relocated. The caisson wall alignment had to be modified to avoid damaging the duct bank thus leaving a zone of contaminated soil below the duct bank. In order to satisfy Ontario Ministry of Environment requirements, an alternative means of remediating this contaminated zone was required.
Geo-Foundations was retained by PDI to design and install an array of overlapping jet grout columns beneath the duct bank in order to bridge the gap in the caisson wall. The drill rods used to advance the jet grout nozzle were small enough to penetrate the ground between and immediately adjacent to the electrical ducts, and the resulting soil-cement block created a contiguous zone of impermeable ground stretching - from the underside of the ducts to the caisson tip elevation - from one side of the caisson wall to the other.
Geo-Foundations mobilized to the site and installed 3 pre-production test columns, each constructed using different lift rates and injection pressures. The test columns were exhumed the following day and the results were used to select the installation parameters for construction of the production columns. In order to provide complete coverage of the contaminated zone between the ends of the stopped-short caisson walls, three overlapping 1.4 m columns were constructed. Following installation, core drilling at the perimeter of the contaminated zone was carried out to verify the presence of the jet grout columns. Upon completion of jet grouting, removal of the contaminated soil and backfilling to grade with clean fill was completed by PDI.
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