City mulls ‘single-bore’ tunnel for the Green Line
A massive, ‘single-bore’ tunnel could be built for the underground portions of the Green Line, a move that would reduce street-level disruptions during construction of the $4.6-billion project.
The design would involve one large-diameter tunnel, big enough to accommodate two tracks stacked on top of each other, stretching four kilometres from 20th Avenue N. to Macleod Trail, city council’s transit committee heard Thursday.
The single-bore tunnel — a lesscommon configuration than the double-bore design employed in most metro tunnels — could have big implications for the Green Line’s station designs and eventual operating budget.
Project manager Paul Giannelia told committee members Thursday that constructing the underground portions of the line will mean reckoning with Calgary’s “poor soil” and risks associated with the deep foundations required for stations.
Giannelia said the project team has concluded that a single-bore tunnel would be the most costeffective and user-friendly design for the route.
One of the main benefits of the approach, he said, would be that it requires less tearing up of roads and disruption to utilities.
Double-bore design is typically closer to the surface and involves more excavation, Giannelia said.
“During construction, there will be less impact to the travelling public, whether it’s pedestrian or vehicular.
“That’s a big plus, a big gain,” Giannelia said.
Giannelia said a final decision on tunnelling has not been made.
The project team is expected to return to council in December with further details, including the effect on access points for Green Line stations.
But as work on the project proceeds into the next phase of construction preparation, some members of council are concerned that the flow of information from the Green Line team has “gone silent.”
Coun. Druh Farrell complained Thursday that it has been difficult to get a meeting with the Green Line team, and that after years of consultations with the public and councillors on this project, communication had stopped.
“I feel that there’s a firewall that’s been put up,” Farrell said in council chambers Thursday. “And that gives me concern because I wonder what’s changing.
“The communities were hyper engaged for years and then it’s just gone silent.”
Farrell said she worries that previous discussions about the vision for development around Green Line stations is being lost while technical and budgetary concerns dominate — a sentiment that was echoed by Coun. Jyoti Gondek.
Giannelia replied that there was no intention to “firewall” anybody, but that it is difficult to engage with communities while key details of the project are still being settled.
Committee chair Shane Keating downplayed the concerns raised by his colleagues, suggesting they were a natural response to the project entering a new phase.
“Every time you go into a different phase or stage there’s uncertainty,” he said.
“We had three years of engagement, planning, talking to the public (and) then it went into the zone of silence.
“We changed from, ‘let’s talk to people about what they want,’ to, ‘let’s actually now get to the construction.’”