Toronto's news media outlets were abuzz one afternoon in the summer of 2006 after a garbage truck was partially swallowed by a large sinkhole that developed in a back alley laneway in midtown Toronto. The laneway dates to the 19th century and in the weeks following the drama, investigations determined that the sinkhole was caused by a combination of poor initial construction, layers of organic soils underlying the surface paving and, most predominantly, a leaky sewer. Prior to reconstruction of the sewer and repaving of the laneway in the winter and spring of 2007, Geo-Foundations spent 8 weeks on site performing compaction grouting to remediate the soft conditions prevalent due to the organic soils and decades of disturbance.
The work Grout holes were spaced at 1.75 m centres on three gridlines, parallel and offset 1.5 m relative to one another (holes on the middle grid were staggered such that there were grout holes every 0.875 m along the laneway) and positioned to miss the sewer and provide ample clearance to neighbouring basements. Compaction grouting casings were drilled to depth using double-head drilling with drag bit and compressed air flush, as well as double-head auger duplex. Grout was delivered to site via ready mix trucks and injected using a Puttzmeister P240 concrete pump. Drilling of compaction grouting casings totalled 2385 lineal metres, with a maximum depth of treatment of 11.5 metres. A total of 204 cubic metres of compaction grout was injected into 221 holes, covering 130 lineal metres of laneway. 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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