C+S March 2021 Vol. 7 Issue 3 (web)

jacent critical infrastructure were some of the many challenges faced by GHD. In order to access the launch and retrieval shafts, a number of tem- porary roads and bridges had to be built. The shaft locations were constructed of crushed stone, filter fabric, and cellular web product to account for soils with low bearing pressure. In addition, these materi- als would help ensure storage volumes for flood events within the watershed were maintained. It was initially estimated the project would take more than four years from start to finish if tunnelling operated withinYork Region’s standard construction hours of 7 am to 7 pm with the use of a single machine. A number of accommodations – from carefully placed shielded lights, sound walls, and silenced generators, to restricted nighttime deliver- ies to and from tunnelling shafts – resulted in favorable community sentiment and support for the project. This eventually led to construc- tion being allowed to take place 24 hours a day, six days a week, with multiple tunnelling machines. As a result, the project was completed in just over one year. Details as the Baseline for Quality Careful consideration was given to the design of the tunnel’s align- ment. The appropriate tunnel easement width and horizontal curves were selected to allow the use of precast tunnel segment without the need for hydraulic joints. Vertical curves were selected in certain areas to promote clearance to existing natural water bodies and minimize the depth of shaft construction. Curve radii, while employed within the design, were kept to a minimum deflection angle to maximize forward thrust from the jacking frame hydraulic cylinders. Hydraulic pressure within the jacking frame was monitored continuously, as was the sam- pling of the soil to ensure consistency with the geotechnical condition stated within the geotechnical baseline report. Utilizing the latest technology, the microtunnelling machine was operated in a manner that allowed for the monitoring of continuous face pressure. This meant the earth could be excavated in a controlled manner, minimizing face loss and settlement to the utilities beside and

ers, town staff, Indigenous Peoples, and the general public was imple- mented to ensure all residents were aware of times when their travel would be impacted. The stakeholder program was also used to com- municate the measures being taken to mitigate the impact on the area’s various water crossings and local conservation lands such as Fairy Lake and Bailey Ecological Park, a 10-hectare bird and animal sanctuary. Project Legacy Having a new forcemain in place allows York Region, for the first time in over 40 years, to inspect and maintain the original forcemain when repairs are needed. This fast-paced project included the successful construction of two of the longest curved microtunnel drives in North America, measur- ing 820 m and 1,132 m in length respectively. In less than a year, the project team successfully completed all eight micro-tunnelling drives, including installation of carrier pipe and construction of chambers. Complexity and Challenges The proximity to conservation land, numerous water bodies, and ad-


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