The new span is slated to open in 2015, and will replace the current green landmark bridge, which was built in 1914. But how exactly does a bridge get built? What is the process? And how will this bridge be different than the old one? Here's a primer:

For several months now, crews have been busy preparing the site for the new Walterdale Bridge, which will cross the North Saskatchewan River at 105th Street. Trees have been taken down, Queen Elizabeth Park Road has been realigned, and large steel pilings are being drilled in the river banks.

Design phase (2008-2012)

In 2008, the city decided the Walterdale Bridge had reached the end of its life, and began researching a replacement. In 2010, four designs were proposed, and after several consultations with the public and stakeholder groups, the city decided to go with the most ambitious proposal – the arch bridge, a design not seen before in the river valley. It was chosen over more simplistic styles, such as a girder bridge like the High Level Bridge, due to several factors, including aesthetics, environmental concerns and lifecycle costs. In April 2011, city council approved the design.

At that time, a concept-level proposal for the three-lane arch bridge was estimated at $132 million, which was $43 million more than a two-lane girder bridge. After receiving bids from four contractors, the city selected the lowest offer, awarding the project to Vancouver-based Spanish company Acciona/Pace Joint Venture. However, the bid was still 14 per cent more than earlier estimates, bringing the price tag of the Walterdale Bridge up to $155 million.

Pre-construction phase (2013)

Construction crews needed to get the area ready in order to facilitate construction over the next two years. The process started in January 2013, and included removing trees, building access roads and clearing a large area for the necessary building materials and supplies. From July to October 2013, Queen Elizabeth Park Road was closed so that it could be realigned to give better access to the future bridge.

The arches The arches and cables will be the first parts of the superstructure to be installed. If all goes according to plan, the crews will begin installing the arches in May 2014. The two arches for both sides of the bridge will be brought to the construction site in sections, and lifted into place connecting with the thrust blocks. The arches are 206 metres wide, and incline towards each other at an angle. The arches will then be secured to the deck by steel cables. At the highest point of the arch, the cables will stretch 43 metres high. The deck The deck will be installed once the arches and cables are secured. The new bridge deck will be quite different from the old Walterdale Bridge, which has a steel waffle-patterned frame that makes a distinctive noise as cars drive over it. With the new bridge, a framework made of steel will be put in first, then cast in concrete and finally protected with a rubber-like waterproof sealant to protect it from the harsh winters. Once the sealant is in, asphalt will be laid out making for a more quiet drive across the river. Abutments Work will start sometime this winter (late 2013 or early 2014) on the abutments, which are the retaining walls at either end of the river supporting the bridge. Constructing abutments is similar to building thrust blocks. The crews will bring in piles of clay, excavate deep into ground and fill it with a steel frame (piles) before casting it with concrete. Thrust blocks The stress from the bridge load will be carried up through the cables and into the arches, which are supported by what is known as a thrust block. These are currently being constructed. Located about 12 metres away from the abutments closer to the river, the 10-metre by 10-metre concrete blocks will go about 20 metres deep into the ground and connect with the arch. Driving by the construction site today you can see steel beams sticking out of the ground. These beams are part of the construction of the thrust blocks. They are referred to as piles, and are driven into the soil beneath the bridge until it either contacts bedrock or is deep enough so the friction between the pile and soil will support the bridge load. The thrust blocks are one of the most crucial parts of the bridge, holding both arches in place as well as carrying the load from the arches. The thrust blocks will take several months to construct. After they are done, the abutments will be built. Superstructure fabrication After the substructure is built, then the superstructure can be fabricated and installed. The superstructure is all the components that span the obstacle the bridge is intended to cross. The construction of the steel frame of the superstructure will likely begin in May 2014 with the installation of the arches. The arches will be constructed outside of Edmonton, with the steel for the frame supplied by South Korea’s Daewoo International Corp., a company working with Acciona/Pacer Joint Venture. Daewoo will provide the steel that makes up the main components of the superstructure, including the arches and the deck frame. These parts of the superstructure will likely be brought down the river by barges in pieces, and lifted into place by two cranes, one on the river, and one on the river bank. The road During the construction process, either the Walterdale Hill Road or Queen Elizabeth Park Road will always remain open to traffic. The Queen Elizabeth Park Road was reopened in October after being closed for several months, and the Walterdale Hill Road will be closed during the 2014 construction season for realignment. Once the road realignments are finished and the superstructure is complete, the bridge will open to the public. The projected time for the official unveiling is the fall of 2015.

Hover over any of the white circles on the bridge blueprints to see details of the bridge's construction. Interactive by Lucas Timmons

Post construction phase (2015-2115)

“The new Walterdale bridge is a different animal altogether. It's a very complex signature structure, but will serve as a unique and beautiful gateway into downtown Edmonton.” - Shiraz Kanji, City of Edmonton bridge engineer

The new Walterdale Bridge is designed to have a lifespan of 100 years, but with ongoing maintenance, the city expects it to last much longer than that.

Every spring, the city cleans all its bridges, especially the bridge decks, which can often be heavily damaged due to salt, sand and other winter road work. Thanks to all that freezing and thawing, inspectors must regularly monitor the bridges across the city for concrete cracks. Nonetheless, the Walterdale Bridge's new concrete deck will require less maintenance than the old steel deck, which required constant repairs due to welds breaking.

Another concern will be sliding snow from the arch ribs. The city plans to install extrusions, similar to those on the roof of the Edmonton Art Gallery, to mitigate the problem. The city is also looking into additional measures to protect the bridge's steel cables from icing over, a serious issue on the Port Mann Bridge in Vancouver that has raised some concerns in Edmonton.

As for the old Walterdale Bridge, the city plans to demolish it. Crews expect the demolition process to take at least four months, as there are several environmental and safety concerns to consider in the process. That work should begin after the new bridge is completed, in the fall of 2015.

asiekierska@edmontonjournal.com