Tunnelled under live rail lines to service community
By Adolfo Emer, Marshall Macklin Monaghan and Adam Polski, Con Cast Pipe
How can a 3,000 mm storm sewer and a 600 mm sanitary sewer be run across two major railway lines north of Canada's largest city without building two tunnels?
The answer was to suspend the sanitary sewer from the top of the pipe, while monitoring carefully to ensure deviation of no more than 6 mm at the top of the track above the tunnel during installation. Yonge-Bayview Holdings Inc. (YBHI) hired project managers Metrus Development Inc.(MDI) and engineers Marshall Macklin Monaghan (MMM) to work with Con-Drain and their manufacturer, Con Cast Pipe, to design and construct the largest jacked concrete pipe tunnel in Ontario history.
The project is located in Richmond Hill some 10 miles north of Toronto.
The challenges that YBHI, MDI, MMM, geotechnical consultants Geo-Canada Ltd., Con-Drain and their subcontractor, Peran Tunnelling faced were considerable.
With an average of 22 freight trains a day, six GO trains, and four VIA trains, there were limited choices. Together with CN staff, all were reviewed in light of schedules, feasibility and costs. They were left with two options: jacking of a primary steel liner with a secondary liner to be installed later, or jacking of a concrete pipe. They also had to consider difficult, unstable subsurface conditions.
In the end, the trains' frequency, ground conditions, and CN's maintenance schedules, plus the need to install the tunnel quickly in advance of CN track maintenance schedules, made jacking concrete pipe the answer. The team satisfied itself that jacking could work and meet the tight working window of opportunity. Geo-Canada Ltd. determined that the soil's properties would permit jacked pipe versus poured-in-place. When the sewer-in-sewer design was approved by Maureen McCauley, Richmond Hill's Commissioner of Engineering and Public Works, they were ready to begin.
The first step was dewatering, undertaken by Atlas Dewatering. A two-tiered system was required to deal with ground water at two elevations one above the tunnel, the other below. Because standard well points are only effective to 5 m, Geo-Canada suggested the use of eductors to "vacuum" out the water. The use of compressed air was ruled out because of the relatively shallow nature of this tunnel less than two pipe diameters of cover. On-site tiered piezometers were installed early in the design process to measure water, and shallow and deep settlement points were used to monitor ground movement during tunneling.
While this was going on, Con Cast Pipe was preparing the specially designed 3,000 mm stormwater pipe. To accommodate the sanitary sewer that would be hung from the ceiling, special stainless steel bolts and brackets were installed and extra care taken to ensure the strength and integrity of the pipe and its connections. Specialized pipe design software, PIPECAR, was used to verify the detailed steel design requirements for the pipe. The pipe was produced to CAN/CSA-A257.2-M92 standard. Prior to shipping, a Tylox jacking spacer was glued around the shoulder of each pipe and super seal gasket was used for each spigot.
Installation began in June, 1998, with the delivery of the first of 21 sections of pipe, each weighing up to 17 tonnes. Installation was in the downstream direction, from east to west a non-traditional approach but required in this case due to the existing services on the west side. Peran custom-built a 12-tonne, 3.6 m diameter by 4 m long shield that allowed for pneumatic hand mining of the tunnel with a mucking machine removing material as the tunnel advanced. Pipe sections were jacked in behind the liner using jacking pressure of 19 MPa and bentonite slurry to fill voids and lubricate the pipe. The installed jacking spacer worked well to minimize rebound and concrete crushing.
During the entire operation, Con-Drain's survey crew was on site living in tents to record any change in track elevation each time a train passed. With an allowed tolerance of 12 mm total settlement, by the time Peran finished the job on July 3, maximum settlement at track level was only 6 mm. After grouting the lift holes, the sanitary line was installed and sprayed with Shotcrete for additional protection.
"Each of the technical components of this project has been proven in the field, but not to our knowledge brought together in a project of this size and sensitivity. We therefore appreciate all the more the confidence shown in us by CN and the municipality," said Fred De Gasperis of YBHI, on completion of the project.