The City of Ottawa has received Rideau Transit Group’s (RTG) technical memo summarizing the results of their root cause investigation and their proposed next steps to resume tunnelling at the East Portal.
As indicated in the technical memo, extensive monitoring points in the area have confirmed that no building settlement of concern occurred and damages were limited to City utilities that crossed its path.
The root cause report concludes that the sinkhole event of February 21, 2014 was primarily caused by the presence of a previous excavated construction pit deeper than the known utilities. The shape of this pit closely matched the shape of the sinkhole and the material from the pit was indicative of poor quality, un-compacted backfill.
Neither the presence of the poor quality backfill, nor the presence of water in the soil are insurmountable tunnelling problems.
Tunneling will continue in two phases, the first phase consists of a smaller tunnel shape to keep it in known and solid geotechnical conditions. It will involve investigative drilling, and grouting to confirm stable tunnel face conditions. The second phase of tunneling works will start only after all new borehole data is fully analyzed.
City staff and their technical experts have reviewed RTG’s proposal and are satisfied with the proposed approach and have given RTG authorization to resume work at the East Portal as described above.
Source: City of Ottawa