Wit A new hockey arena and a new museum both expected to complete in the next several years, downtown Edmonton, Alberta, should look like a whole new city thanks to a rejuvenation program. Adding to this face lift are amenities including light rail extensions, and plenty of office and condo space housed in high rise buildings. Space for work sites is in high demand. And in one particular instance the same could be said of underground space.

Around the same time in 2007 that the city was working on the concept design for its next light rail extension near downtown, plans were being fast tracked for a proposed 28-storey building. The Epcor Tower was to be located on the approved alignment for the 3.33km North LRT (NLRT) extension.

"We basically had to go underneath a building that was being built," says Lubko Stebelsky, a senior civil engineer with the City of Edmonton. "The timing was kind of funny. If they would have been later we would have just tunnelled up and came up. But they were ahead of us, so we struck a deal with them."

The tower’s contractor would build a sub-basement while building the tower that would later house the NLRT lines.

"They built the Station Lands structure underneath their parkade so we tunnelled through there and came up as quickly as we could," Stebelsky explains. Three new stations are included on the line, all of which are above ground meaning the tunnels needed to be on the surface as soon as possible.

NLRT will run from the existing Churchill Station in downtown Edmonton to the Northern Alberta Institute of Technology (NAIT). Twin tunnels comprise the alignment between 105 Avenue and Churchill Station (Figure 1), some 700m long excavated by SEM to approximately 6m in diameter.

The City of Edmonton awarded a contract worth CAD 300M to the North Link Partnership (NLP), a joint venture of SNC-Lavalin and Graham Infrastructure to provide construction management.

Flatiron Construction, as a subcontractor to NLP, is responsible for tunnelling.

Aecom is the lead consultant, securing a USD 20M contract for program management and design services. The CAD 755M (USD 719M) project is jointly funded by the governments of Canada, of Alberta, and the City of Edmonton (see box).

ALWAYS UNDERGROUND

As this expansion ties into an underground portion of the current system, some amount of tunneling was inevitable for the line. The original alignment had been planned in the 1970s for a line all the way to the northwestern end of the city from downtown. The length and depth (22m at its deepest) of the underground alignment was then dictated by the agreement with the Station Lands development below Epcor Tower.

The ground in this area is primarily till. The top 6-8m is a glacial lacustrine clay, with silt. Below that is till, a mixture of clay and some sand zones.

"That was one of the areas we had some concern because we had these sand pockets and during our geotechnical and some of them were wet," Stebelsky recalls. "We had some concerns they may run when excavated."

For the portion of the alignment between Station Lands and Churchill station approximately two-thirds of the bottom of the tunnel was in a fairly compacted sand formation, the Empress sands, he explais, which had crews digging in solid sand for about 100m.

Fortunately, when the contractors for the Epcor Tower (completed in November 2010) were doing their excavations the NLRT project was able to see the ground conditions before any of the tunnelling work started (November 2011). "When they excavated the Epcor Tower the sands stand stood up fairly well, so it alleviated some of our concerns," Stebelsky says.

The design brie_ y looked at the possibility of using a TBM, but due to the short length of the tunnels and the tight working space in downtown Edmonton it was not feasible.

"One of our big concerns was the pillar between the two tunnels. At the narrowest it was down to about a meter-andhalf at Station Lands," Stebelsky says. "It widened out as you got out of there both ways. But that was a really tight area, and we did have some concerns that the pillar wouldn’t stand up or maybe we’d have some problems with _ uf_ ng because some of it was in sand but it actually held up very well."

CHANGING CHALLENGES

The design provided three categories of support for the SEM tunnelling. Flatiron chose to use a Liebherr 924c for the tunnel work, mostly working from one heading. It was originally intended for roughly 90 per cent of the tunnel’s length to be done as a full-faced excavation. But in end Flatiron ended up doing 100 per cent of the excavation as a benched tunnel, with one-meter advances. Crews installed lattice girders every meter with wire mesh shotcrete. Shotcreting was done in two phases, a minimum of 50mm, followed by the wire mesh and lattice girders, which was followed by another 100 to 150mm of shotcrete for a total of 200mm at minimum.

"The challenging part was that the geological report indicated full faced construction would be possible," says Bob Newland of SNC Lavalin, and construction director for NLP.

"Our subcontractor found that to be unsafe and they ended up doing staged heading and bench excavation for the entire tunnel; they never attempted full face." He adds the change became a contractual issue with City of Edmonton, and there was a claim for extra costs for the additional excavation and shotcrete to complete the work.

According to information released by Flatiron, crews working 24-hour shifts were able to complete 12m of invert in about 24 hours, and 12m of arch concrete in about 48 hours. Tunnelling made a breakthrough at Churchill Station in November 2012.

"When we did the breakthrough there was an existing concrete wall and we used Dexpan, a non-explosive concrete grout," says Dan Martinson, deputy project manager with Flatiron. He explains, after drilling holes through the concrete wall, the Dexpan, mixed with water, is applied and it slowly expands to break apart the wall.

When the existing line was built in the 1970s the work included building block outs for a future tie-in, Stebelsky explains, so the breakthrough was off a ways from the main line.

"The block outs were a little bit thicker than we expected for block out walls. They were about 3ft [90cm] of concrete."

After tunnelling broke through, Flatiron built bulkheads and installed fans, to block off the work from the active station. The 250mm thick reinforced concrete final liner comprises a layer of waterproofing over the support shotcrete, rebar and a final layer of concrete. The dimensions of the finished tunnel are 5.2m wide by 5.8m high.

Another challenge to construction was the logistics of only working from one end of the tunnel, since the breakthrough came at an active station.

There were also a number of significant local structures nearby. "We had some concerns because we were breaking into an existing tower, we were going next to City Hall and there is also the Law Courts building right next to it," Stebelsky says.

Settlement monitors were used on the surface along the full tunnel corridor and took geotechnical measurements continuously. "In the end we learned that there was virtually no settlement on the surface," Newland adds.

Moving on

Tunnel work is now substantially complete and crews started installing rail in one of the tunnels in late August. As Tunnels went to print NLP planned its first cautionary push through of a train for 4 September to confirm clearance measurements. Test trains are expected to start running in October and November. Construction is scheduled to end this year, with service starting spring 2014. NLRT is the first segment of a planned LRT expansion to Edmonton’s city limits by 2040. The Valley Line extension (Southeast to West LRT) will connect the downtown area to Mill Woods primarily at grade, though a small portion will require tunnelling to cross the river valley. The project will bid as a P3