Complexity and Challenges The proximity to conservation land, numerous water bodies, and ad - 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 within York 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 above. The face of the tunnelling equipment was armed with rear load - ing cutting disks and tools that would accommodate for the excava - tion of the highly varied soils with the potential for cutting through large cobbles and boulders. The tunnelling equipment was specified with face access as a mitigation measure to accommodate changing out of disk cutter should it become necessary. Hyperbaric intervention was also used to give workers safe access to the face of the tunnelling equipment under pressure. Technical Excellence and Innovation It was critically important for the project to have as little impact as pos - sible on the surrounding area’s urban setting, and to ensure protection of the East Holland River floodplain and natural environment setting, which cuts through the heart of Newmarket. As a result, the 5.6 km pipeline was constructed with a limited number of shafts and with the rare use of back-to-back microtunnel drive lengths in excess of 500
meters. Extensive geotechnical analysis, along with value engineering consultation and risk mitigation actions, provided the Region with added confidence for the project to proceed as designed. The placement of shafts was key to productive mining, so shaft sites were selected using a carefully established decision matrix. These sites allowed for the delivery of material along a controlled haul route to minimize community traffic and noise impact. All equip - ment and material entered the shaft and was installed within the shafts and the newly constructed tunnelled sections. Even though microtunnelling took place 24 hours a day, traffic to the shaft loca - tions was restricted to regular construction hours. This required all excavated material and new precast tunnel segments to be supplied strictly within a 12-hour window. Constructing the forcemain within the tunnel also required a unique approach. This meant using pre-stressed cylinder pipe segments within the precast tunnel sections to address the internal pressures from the forcemain as well as the external soil loads. Pipe segments had to be carefully placed within the tunnel to account for both horizontal and vertical curves and all connection points. Pipe segments would need to be interconnected, blocked in place, tested and then the annular space filled with grout to ensure system reliability. It Takes a Village The planning efforts of York Region leading up to the project were critical in laying an important foundation for success. Up-front com - munity consultation, engineering studies, and quality assurance efforts all contributed to the successful completion of the project—ahead of schedule, within budget, and mindful of the needs of the community. York Region listened to community concerns and responded with prac - tical solutions to address traffic concerns, noise, vibration and light issues, tree and wetland protection, as well as minimizing disturbance to existing utilities. The project minimized the number of tunnel shafts to ensure community services would not be adversely affected. The natural environments of the Town and the East Holland River Flood plain were protected by reducing shaft compound size, selecting locations outside the flood plain and maximize the spacing between shafts. With microtunnelling being used as the construction method, long distances of forcemain could be installed between shafts. This meant trees and vegetation were not disturbed and parkland remained in use throughout construction.
BRADLEY MARIN is GHD North America Tunnel Service Lead. GEORGE GODIN is GHD Water Market Principal. TOM CASHER is GHD Water Market Principal.
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DECEMBER 2021 csengineermag.com
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