TO HIGHLIGHT the overlap of construction activities in Ottawa’s downtown tunnel, the contractor’s Humberto Ferrer cites a long list of the current work. "Now we’re at the east portal excavating soft ground at the face; we are doing rock bench excavation with a roadheader at the station at the same time; we are doing drainage and water proofing; and we’re doing the starter walls and the final lining inside the tunnel as well."

Underground construction is only one part of the Confederation Line LRT project, which comprises 12km with 2.5km of the alignment in said tunnel. The underground works include 2,530m of running tunnel and three mined stations: Lyon, Parliament and Rideau. The CAD 2.1bn (USD 1.58bn) system also includes 13 stations, and a maintenance and storage facility.

Rideau Transit Group (RTG) is a P3 delivering the project with the City of Ottawa and Infrastructure Ontario. As part of RTG, the design-build joint venture includes Dragados, SNC-Lavalin Constructors Pacific and EllisDon.

Sequential Excavation Method (SEM) is being used for the underground construction, which on average is about 15m below the urban surface.

In addition to changing ground conditions along the alignment, the project has many complexities from constructing the large station caverns in a narrow right-of-way to overlapping activities.

The contractor describes the geology as sound limestone interbedded with shale from the Lindsay and Verulam formations. Compress strength varies from 50 to 90 MPa and its RQD from 30 to 90. Overlaying this sedimentary ground, glacial deposits of clay and fines including a paleovalley at Rideau.

The contractors chose three Sandvik MT-720 roadheaders for tunnel and station excavation, which started in late 2013. Initially the reference content during the procurement process was based on TBM excavation, says Gary Craig, with the City of Ottawa. "This team reviewed the geotechnical requirements and the reports we provided them during the procurement phase and they made the decision to use roadheaders.

"I think the fact that they can move these machines around very easily — and if they run into an issue in one location they can just move the roadheader to a different location and continue to achieve production–from our perspective that’s certainly been one of the successes on the project," he says. "Any challenges they encounter they have the capability of being flexible."

Cavern ConstruCtion

At the time T&T spoke to members of the project, excavation for Lyon station had just recently reached completion. Ferrer, deputy director of RTG says of the three caverns for the three underground stations each represents a different challenge. "Lyon because of the proximity to buildings with lower basements," he explains.

Many of those buildings are at a similar depth as the cavern’s cover, around 10m, and each cavern has a span of about 18m. Upon completion the stations will be approximately, 16m by 13m in range and 120m in length.

Craig says both Lyon and Parliament stations are on Queen Street, "which is a relatively narrow right-of-way for a downtown street. That is why the excavation is very close to the foundations and the parking garages under the buildings. Whereas Rideau Street the right-of-way is much wider." One of the project’s successes is the use of tension ties in Lyon and Parliament station caverns.

"If you are building right next to existing buildings, when you open your arch support, you will transfer loads to the buildings," explains Ferrer. He describes the installation and excavation process to avoid transferring the loads: "We first did the two, side lower drifts, and on those we had done what we call the L-walls or the final walls on the final lining. Then we put the tension ties that will avoid the arch transferring the loads to the adjacent structures."

He explains the next step is to fill (using material from the tunnel) and build a slab for the roadheader, which would work on the upper drift. "We did the upper section the side drift first, and then as we moved to the central pillar we were putting in the tension ties. That way as we excavate the central pillar, where the arch forms, the union between the arch and the walls puts the load on the walls, and then the tension ties avoid the walls having a reaction on the adjacent structures."

Soft ground

Dr. G. Sauer & Partners Corporation is providing design services for the temporary works, engineering support and site supervision during the mined construction works. In a report on the project last fall it says, "with the different layers of station construction ongoing the impressive magnitude of the station caverns is becoming more visible.

"The Rideau cavern in the East has already passed through the solid rock portion and now faces the challenging Rideau valley section. This is a 120m wide and 35m deep valley formed during the glacial period and filled with closegrained deposits necessitating special underground design and construction measures.

"Upon completion of the Rideau cavern structure in the valley section, the running tunnel shall continue west until it leaves the Western flank of the valley and enters the sound bedrock." At this point it will meet with the counter tunnel advance from Parliament Station.

The section of soft ground is one of reasons for using SEM and by the end of 2015, the JV was in the process of excavating in this portion of the alignment. The soft ground, Ferrer notes, isn’t too different from that of other projects but it’s made additionally challenging due to all of the adjacent buildings.

"On the soft ground we’re completing the north and south side drifts first," he explains.

"Once those drifts are completed, we’ll do the central section. That’s difference between Rideau and Parliament [stations] — where we first completed the upper level." At the time, he says, "in the north drift the excavation is complete. We did the head wall already, in the south drift we have one more round to go, by end of next week we will have completed the south drift including the headwall. It’s going well. Then we will start by the following week, the second level or the bench of the north drift."

RTG is using three sets of formwork to line the tunnels, one for the running tunnel, one for the arches of Lyon and Parliament caverns and a third for the Rideau cavern. Final lining is poured between the west portal and Lyon Station. "They started to pour lining from the east portal towards Rideau cavern," Craig says. "There is completed, lined tunnel from west portal to Lyon station and started from the east portal now toward Rideau."

In late spring the tunnels will connect in the running tunnel between Parliament and Rideau. In December 2015 the contractor had two roadheaders at Parliament Station. "One is completing the upper level of Parliament. Then we will start the tunnel between Parliament and Rideau," Ferrer says. "The second roadheader is going to be kept in the station to complete the excavation of the bench of the station."

By late February Parliament Station had reached 92.7 per cent completion and Rideau Station 77.3 per cent. There were 26.5m of the top heading excavated as of February 26. Construction is scheduled to finish in autumn 2017 with the system in service the following spring.