Transportation: A Tunnel and a Tower
In 2007, when engineers were doing concept planning for the northern extension to Edmonton’s light rail transit system, they came up against a problem. A twin tunnel that is part of the North LRT Extension was in potential conflict with...
In 2007, when engineers were doing concept planning for the northern extension to Edmonton’s light rail transit system, they came up against a problem. A twin tunnel that is part of the North LRT Extension was in potential conflict with the first new tower on the Station Lands redevelopment area downtown.
The Epcor Tower was approved for fast-track construction, but the new tunnel was aligned below it. The line passed very close to the existing CN office tower as well.
To avoid potential structural conflicts between the tower and the tunnel, it was decided to design and build the Epcor Tower and tunnel in parallel. So while the LRT line project is not due for completion until the end of 2013, the tunnel and tower were completed in 2011
By designing and building the tunnel and tower at the same time, the project is estimated to have saved the city of Edmonton $140 million (the current total cost of the LRT extension is $755 million).
If the designs had not been coordinated, either the tunnel would have had to be realigned in a new location, or it would have had to be dug much deeper to avoid the tower’s foundations. A deeper tunnel would have needed to be longer and would have had steep grades. The new MacEwan station would have had to be built underground, and a link to Edmonton’s existing 1970s-era LRT system at Churchill Station might not have been feasible.
Once LRT trains start running on the extension in 2014 they will carry people from the existing Churchill Station through the new 700-metre underground tunnel below the Epcor Tower, then climb to grade at 105 Avenue, near Grant MacEwan University. The line proceeds at street level from here to the Northern Alberta Institute of Technology (NAIT), 3.3 kilometres to the north.
Two owners, two design firms, two budgets
The partnership to build and design the tower and tunnel required extraordinary cooperation between the players. There were two owners, two design firms and two budgets. UMA was prime consulting engineer for the tunnel and Earth Tech was the structural engineer for the tower. As it happened, by the end of the project both these companies had become part of AECOM. Ledcor managed the construction of both projects.
The tunnel costs had to be allocated two ways: costs of the tunnel itself of $18 million ($7 million below the original $25-million budget), and incremental costs to the tower because of accommodating the tunnel of $13 million ($2 million below the original estimate).
Combining the tunnel and the tower projects made things complex, but it also provided benefits for the tower. The designers took advantage of the deeper raft slab now required for the tower and used this “found” space to accommodate a system of earth tubes. The tubes help to lower the building’s heating and cooling costs and reduce its environmental footprint. In addition, extended core walls were modified into a water tank to store rain water for use in the building’s sanitary system.
The Epcor Tower has 28 floors above ground and is surrounded by a plaza. The plaza sits above four levels of underground parking and is designed to eventually support as many as eight floors above ground.
The office tower’s columns are on a 9.144 metre (30 feet) square grid founded on spread footings. The tower structure has a central core founded on a raft slab. The lowest parking level at P4 is approximately 14 metres below grade.
The LRT structure is a cut-and-cover style box structure immediately below the P4 parking level. At its lowest point, the excavation for the base slab of the LRT tunnel is approximately 24 metres below grade, placing it well into a sand and gravel layer known as the Empress Formation. This formation extends to bedrock at approximately 35 metres.
Coordinating the designs
To coordinate the designs, the 30-ft. grid system set up for the Epcor Tower was adopted for both projects.
From gridline 1 on the west side of the building to gridline 9 located towards the middle, the LRT structure is aligned with the building above. This makes a somewhat larger tunnel cavity than would ordinarily be required for the LRT but it allows column loads from the parkade and plaza to be taken directly on the tunnel walls.
East of gridline 9 the curving LRT alignment made it impossible to align the cavity structure with the columns above. Support for the parkade and plaza columns is provided by 2.2-m deep transfer girders.
From the perspective of the LRT tunnel, the major design issues were having to deal with (1) loads from the tower core foundation; (2) loads from the parkade and plaza columns within the LRT right-of-way; and (3) potential delays to the Epcor Tower project due to the LRT construction, which could have resulted in claims against the city.
Accommodating the tower foundation loads
Several approaches for handling the tower foundation loads were considered. Carrying the loads on the LRT would require a substantial structure.
The innovative solution chosen was to avoid the problem by deepening the tower raft foundation and footings so that their bearing would not load the side walls of the tunnel. For the raft foundation supporting the tower core this was done with a combination of strategies. The raft foundation was lowered by 3.5 metres. In addition, an unreinforced concrete wedge running beneath the edge of the raft foundation and adjacent to the LRT base slab brought the foundation bearing of the tower to the same elevation as the LRT base slab. Column footings were also lowered, the amount depending upon their proximity to the LRT tunnel.
The LRT base slab serves as a foundation for both the LRT and a number of parkade and plaza columns from the tower. Reinforcement for the LRT base slab made use of prefabricated cages complete with shear reinforcement. As a conventionally reinforced one-way slab, the thickness of the base slab, governed by shear, would be in excess of 1.5 metres. With the cages, the thickness of the base slab was reduced to 1.2 metres.
Modular reinforcement was also used for the LRT roof slab and transfer girders. The modular nature of the reinforcement saved approximately eight weeks of construction time.
Coordinating the structures east of gridline 14 was a major challenge so as not to compromise future developments on the Station Lands site. Critical for the tower in this area is a truck ramp for delivery access to the first parking level, P1. Maintaining delivery access during future development requires that the truck ramp be on structural supports to the full depth of the parkade. The design was complicated by the depth of the excavation, approximately 24 metres below grade, and the skew of the LRT alignment relative to the tower structure. This area is also where the LRT alignment has its nearest approach to the existing CN Tower.
A number of other modifications were made to accommodate both structures. For example, the grades at the west end of the lowest parking level in the tower were adjusted to make it possible for the LRT alignment to reach grade at the new MacEwan Station. Also, the walls and roof of the LRT tunnel cavity were insulated to protect the P4 level from cold air within the tunnel.
To see a video of the project, visit http://www.edmonton.ca/city_government/news/north-lrt-to-nait-video.aspx
Name of project:
Station Lands Tunnel Cavity,
North LRT Extension, Edmonton
City of Edmonton
AECOM (Art Washuta, P.Eng.
Scott Alexander, P.Eng.)
Kasian (architect/prime consultant); AECOM (structural engineer), Thurber (geotechnical)