Vancouver’s earthquake-resilient tunnel project addressing tricky conditions

One of the largest tunnels ever built by Metro Vancouver is underway, making the utility more resilient to earthquakes while better serving the region’s growing population. 

And though tunnel projects are not new, the Second Narrows Water Supply Tunnel’s size, depth, site geology and marine crossing are creating unique challenges. 

“We know that there is a strong likelihood that earthquakes will happen in this region,” said Marilyn Towill, general manager of water services for Metro Vancouver. “As utility managers, we need to be prepared for that.”

Metro Vancouver provides wholesale regional utility services to 2.8 million people in British Columbia, Canada, through more than 20 member jurisdictions. In the 1990s, spurred by the 1989 San Francisco Bay earthquake, the utility began identifying high-risk infrastructure that needed upgrades to improve the water system’s resiliency. 

“We’ve seen the impacts recently of earthquakes in New Zealand and Turkey,” Towill said. “It’s hard to understand the devastation that can come with an earthquake. We need to be ready.”

Crucial marine crossing

The Second Narrows Water Supply Tunnel is the second of five new regional water supply tunnels designed to meet current seismic standards. It extends 1.1 kilometers (0.7 mile) from the District of North Vancouver to the City of Burnaby, 30 meters (100 feet) below the bottom of the Burrard Inlet, at one of the most important marine crossings in the area, said Murray Gant, director of major projects -tunnels in Metro Vancouver’s Project Delivery Department.

Once complete, the 6.5-meter-diameter (21 feet) tunnel will house three steel water mains that carry water from two supply reservoirs on the North Vancouver side to customers on the Burnaby side. It will replace three shallow water main crossings built in trenches in the 1940s through 1970s, which are nearing the end of their service lives and do not meet current seismic standards. 

The project also will significantly increase capacity to serve the region’s growing population, and its depth will protect the tunnel from third-party impacts in the busy channel, such as pile driving for bridges. 

Much of the region is built on the Fraser River Delta. When glaciers retreated 10,000 years ago, they left behind sand and gravel that could liquefy during a major earthquake, Gant said. The ground could lose strength when it shakes, flowing into rivers and the Burrard Inlet, “and any infrastructure would flow with it,” he said.

To prevent that, the design required the tunnel to be buried deep in dense glacial soils and bedrock that won’t liquefy in an earthquake. Crews first excavated sizable vertical shafts on either side of the inlet 50 meters deep on the north side and more than 100 meters on the south — before tunneling horizontally between the shafts. 

The utility then brought in a special slurry-type tunnel boring machine — the first time such equipment was used in Canada — that also required one of the largest cranes in the country for setup, he said. 

For 13 months, the machine crept deep below the surface, creating a clay-slurry mixture that could be pumped out. It took about an hour for the equipment to bore 1.5 meters, or 5 feet, before stopping so a team of workers could install precast concrete rings — 719 total — to support the ground as the machine bore through. 

“It’s a unique working environment, and it’s not for everyone,” Gant said. 

Complex challenges

Crews completed boring the tunnel in 2021, but it was a bit anticlimactic, Gant said. The south shaft in Burnaby was partially filled with water to counteract the high groundwater pressure, which meant crews watched the two sides meet via a remote camera. 

“All you could see when the tunnel broke through was bubbles, but it was still so exciting for everyone,” he said.

The truly nerve-racking part came earlier, when crews tunneled under the inlet’s 150-meter-wide shipping channel. If the machine were to get stuck there, officials would have had to stop marine traffic as they made repairs, “and we couldn’t let that happen,” Gant said.

Before they bored beneath the shipping channel, crews froze the ground in front of the cutterhead so inspectors — clad in diving gear just in case — could examine the equipment. 

“Then we’d let the ground thaw, and we’d keep mining,” Gant said. 

Now that the tunneling portion is over, crews are installing water pipes and other infrastructure to complete the six-year project by 2025. The total budget for the project is $445 million ($325 million U.S.) and is funded through water sales from Metro Vancouver’s 20 member agencies. 

General Manager Towill said decades of careful planning and analysis of earthquake-prone infrastructure prepared Metro Vancouver for the massive project.

“You can’t do it all at once, but you have to get started,” she said. “In our case, this project is of strategic importance because two-thirds of our drinking water comes from the sources that can be fed through this tunnel into the infrastructure on the other side. We had to secure it.”
Gant agreed, adding that the project will benefit the region for generations. 

“We’re building this for a one-in-10,000-year earthquake,” Gant said. “It has a 100-year design life, but we hope it will last at least 200 years. We’re protecting our drinking water system for a very long time.”