When the perimeter spread footing of a 1970's era mid-rise located in midtown Toronto were undermined by a neighbour's deep excavation, the options for remediation came down to micropiles or compaction grouting. Due to the cohesionless character of the foundation soils (compaction grouting works best in self-draining soils) and the relatively shallow zone of disturbance, compaction grouting was chosen for its expeditiousness and cost effectiveness, and Geo-Foundations was selected to carry out the compaction grouting on a unit rate contract basis.
The work took place from inside the building's parking garage, in just 2.5 metres of headroom. A series of cased holes, all drilled at a slight angle to vertical to gain access to the suspected worst affected soil zones, were drilled using drop-off drill bits and polymer flush to ensure protection of the existing footing at all times. Strict pressure refusal criteria and monitoring measures were put in place to ensure that grouting did not induce superstructure movement or surface heave of the preserved slab-on-grade, and any holes exhibiting significant grout takes were followed up days later with secondary holes to ensure that all disturbed soils were duly pressurized by compaction grouting.
A total of 19 lineal metres of perimeter strip footing were successfully rehabilitated by compaction grouting at this site, to a depth of 8 metres below underside of footing, by injection of 5 cubic metres of compaction grout through 15 drilled holes. Including restoration of the slab-on-grade, all work was completed in just 4 weeks.
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