New sewage forcemain runs below dense urban area including 11 river crossings, three railway crossings, parks, and major city streets
By Bradley Marin, Tom Casher, and George Godin
The York Durham Sewage System Forcemain Twinning Project (YDSS) is a sustainable sewage servicing solution undertaken in the summer of 2019 by the Regional Municipality of York, located north of Toronto, to provide relief for the Town of Newmarket’s original forcemain, as well as resiliency for the area’s overall conveyance sewage system. As the lead design firm on the project, GHD was tasked with all of the pre-design investigation as well as the design, bid, and build process, and coordination of all activities related to approvals and construction.
The work involved mining through highly variable ground conditions so concrete forcemain could be laid for a 5.6 km-long sewage pipe running parallel to an existing 36-year-old forcemain through the heart of Newmarket, a city of 85,000. The objective was to provide a dual forcemain system to ensure resiliency in operations and maintenance for the system.
A key requirement was to use a construction method that would have the least social and environmental impact due to the area’s highly congested urban setting and natural environment. The solution was microtunnelling construction of a two-pass system consisting of a new forcemain within a micro-tunnel carrier pipe.
Part of the initial investigations included a phased geotechnical program to evaluate ground conditions and later identify where additional geotechnical information was needed to mitigate risk during construction. Part of the process also included establishing value engineering sessions with construction leaders for guidance on the capabilities of the tunneling equipment and procedures to be specified within the design.
The project’s engineering scope included all preliminary geotechnical, hydrological, and archaeological site investigation and environmental studies, route evaluation and selection, constructability reviews, geotechnical data, and geotechnical baseline reports. GHD was also tasked with hydraulics modelling, geotechnical settlement analysis, noise impact assessment and attenuation, detailed design, environmental management, and a comprehensive public engagement program.
From a design perspective, GHD was responsible for all approvals, contract document specifications, development of cost estimates, tendering and construction contract award, review of shop drawings, contract administration, and construction site inspections. GHD was also responsible for administering the commissioning and warranty of approximately 5.6 kms of 1,050-millimetre diameter concrete forcemain to be installed via microtunnelling in a 2,250-mm diameter tunnel.
Mitigating the impact of the YDSS Project on the community was a high priority in the planning process. Since the project included crossing several regional roads and three live regional transit railway lines, accommodating daily high traffic and rail times was paramount to keep residents getting to work and school on time.
A robust stakeholder engagement program involving agencies, landowners, town staff, Indigenous Peoples, and the general public was implemented to ensure all residents were aware of times when their travel would be impacted. The stakeholder program was also used to communicate 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.
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, measuring 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 adjacent critical infrastructure were some of the many challenges faced by GHD.
In order to access the launch and retrieval shafts, a number of temporary 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 materials 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 deliveries to and from tunnelling shafts – resulted in favorable community sentiment and support for the project. This eventually led to construction 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 alignment. 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 sampling 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 loading cutting disks and tools that would accommodate for the excavation 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 possible 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 equipment 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 locations 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 community 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 practical 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.
This article was originally published in March 2021.