Member Login Menu
Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

Building Underwater Poses Unique Challenges

By Jean Thilmany

The design firm charged with lowering the end of Montreal's Alexandra Pier for riverfront access says constructing a wharf wall underwater isn't quite like building blind but it's close.

featured image
Engineers at WSP lowered the end walls of the Montreal’s Alexandra Pier and added steps for pedestrian access to the Saint Lawrence River. The project is part of a comprehensive overhaul to make the pier more tourist- and pedestrian-friendly. © Provencher Roy, courtesy of v2com

July 26, 2016—Building an underwater structure isn't something you learn strictly from books. The know-how comes with experience, says Tony Mailhot, the technical director for special projects and port structures for WSP Canada Inc., a subsidiary of WSP Global Inc. (which deliver projects globally under the WSP and WSP | Parsons Brinckerhoff brands). Mailhot is drawing upon his own experience as the firm works to stabilize and lower the wharf at the end of Montreal's more than 100-year-old Alexandra Pier, which—along with the 1940s-built Iberville Passenger Terminal there—is undergoing a CAN$78-million (U.S.$60-million) redesign and reconstruction this year.

While the Montreal-based architecture firm Provencher Roy designed the updated wharf, the terminal upgrade, and a lighthouselike tower, the Montreal Port Authority turned to WSP for the wharf engineering. The port authority asked WSP engineers to lower the end of the 350 m long pier to give pedestrians and small boats riverfront access in an effort to attract more tourists to Montreal. The upgraded terminal will include a rooftop terrace with access to an esplanade that runs alongside the pier and leads to a riverside park. The terminal's passenger loading and unloading area will be moved from the first floor aboveground to the ground floor, and the architects have designed a connection to the pier's street-side entrance as well, according to a written statement provided by Provencher Roy.

The project includes a new observation tower overlooking the St. Lawrence River, to be built by 2019. The rest of the work is slated for completion in time for the 2017 cruise-ship season, according to the Montreal Port Authority.

The designs call for a small park at the pier's end with steps that lead to the Saint Lawrence River to give pedestrians direct access to the river. Water will sometimes cover the bottom few steps, depending on the river's level, Mailhot explains, so the structure had to be designed with that in mind.

The changes to the pier come with a set of challenges that includes stabilizing the wharf and drilling through an existing wooden crib that is original to the late-1800s construction. To lower the wharf, engineers needed to plan for the removal of the lateral wall and part of a sidewall at the end of the existing pier. The walls comprised large wooden cribs filled with miscellaneous granular fill and topped with an unreinforced concrete coping wall, Mailhot says.

Contractors will then build a new wall that gradually slopes, as does part of the east sidewall, toward a lip at the northeast point; the riverfront steps will be fit into the lip, he says. A light pole used as bollard will prevent boats from striking the wharf when water levels rise, he adds.

After winning the project, WSP completed 3-D designs and renderings of the site, which enabled all of the team members as well as the port to see how the wharf would look at various stages of construction, Mailhot says. "It's kind of new for the owner to build a wharf very close to or below water, so we helped them plan the whole thing," he explains. "We showed a lot of detailed construction methods in our plans to help the owner understand how construction will take place."

WSP knew they'd face at least one big challenge: wharf excavation would reveal the heavy deadmen and the concrete cylinder anchors below, making the wharf unstable. The mooring mechanisms that allow the wharf to remain steady even as the river flows around it had been buried during a reconstruction project in the 1940s. "The anchors were buried in the soil and if you remove the soil beside them, they become unstable," Mailhot explains. "By lowering the site, we were digging out the anchors; so we had to stabilize first and start digging after that."

So the engineers designed and installed rock anchors to stabilize the walls that would remain above water. The iron anchors expand, wedging themselves against the wharf walls, and the stronger the pull on them, the tighter they get, Mailhot says.

Following excavation and the installation of the new anchor and deadman system, the next order of business will be constructing a below-water wharf wall, Mailhot says. He had to design the wall so it could be constructed literally sight-unseen, underwater, using specialized equipment. "You're working blind when you're working underwater," he says. "Imagine yourself at the beach, trying to work something out below water, especially when the water visibility is poor; it is not easy." So the construction methods were precisely determined during the design stage.

The wall, which is currently being constructed, will comprise caissons socketed into bedrock. Sheet pile will be driven into the bedrock between each caisson, Mailhot said.

Rock anchors will moor the end of the wall that is below water in place. To reach rock, those anchors have to be run though the original, 1800s-era caissons, which were constructed from 1 by 1 ft hardwood posts. "The contractor managed to find the right coring drill machine to make that happen," Mailhot says.

The contractor, Hamel Construction, Inc., of St-Édouard, Quebec, turned to an 18 m long heavy-duty drill made especially for underwater work. The cab of the machine remains on land while the diamond-tipped bit that cuts through the heavy wood is dropped below water. "The thing was specially brought in to go through the big pieces of wood they'd used to build the very first wharf," Mailhot says. "The full wall of old wood was hard to go through, but the contractor used the right equipment to go through the material."

But the right equipment is also large, noisy, and difficult to transport.

"The work is basically in the downtown port of Montreal, so planning is different than if you build on the Saint Lawrence River, where there aren't too many buildings around," Mailhot says. "This is an urban area that has to be accessed through the downtown, so the drawings specified the noise-control criteria that needed to be respected during construction."

The wharf reconstruction is well under way; in the next few weeks, the river water level is expected to recede, allowing the contractors to work on the cast-in-place concrete stair system. If that doesn't happen, WSP has a plan B in its back pocket: engineers would mount a prefabricated stair system.

"With the advantageous water-level conditions and the capacity of the contractor to work underwater or close to water, we fortunately [won't] have to use that plan B," Mailhot says. "Our client, the port authority, also wasn't very open to plan B because of the precast yard we'd need to set up and the large-size pieces to handle. Luckily, we won't have to use that plan."

By next summer, cruise-goers, tourists, and Montreal residents alike will have the direct riverfront access they've been long awaiting. 

related

Read Civil Engineering magazine on your smart device: download our apps.

app store play store