A five-kilometre sheet-pile dike wall was accomplished in three-and-a-half months
By late April of 2019, a cold, snowy winter, a late spring melt and an unusually rainy spring led to extremely high water upstream of the St. Lawrence River system in areas of Quebec east of Ottawa and Cornwall, Ont. On the evening of April 27, 2019, in the region of Sainte-Marthe-sur-le-Lac, an off-island suburban community of 18,000 about 40 kilometres northwest of Montreal, a 20-metre breach in a natural earthen dike quickly became wider as the high, rushing water from Lac des Deux-Montagnes coursed through the weakened structure, which then completely broke down.
The quickly rising water, in some cases high enough to submerge cars and strong enough to snap trees, resulted in the widespread flooding of dozens of streets and damage to about 2,500 properties. A state of emergency was declared by the government. More than 6,000 people – a third of the community’s residents – were ordered to evacuate to higher ground, with teams from the Sûreté du Québec, the Canadian army, local fire and police departments and volunteers from neighbouring municipalities deployed to provide assistance to the victims. The next morning, as the water level continued to rise, flooding extended a half-kilometre inland and another 100 properties had to be evacuated. By the time the waters receded, almost a third of the town’s homes had been flooded, according to news reports. Sixty of them were damaged so severely that they had to be demolished.
Unfortunately, extreme weather, including “millennial flooding events,” have become increasingly common. The Quebec government offered up to $200,000 to flood victims who wished to relocate elsewhere in the aftermath of the Saint-Marthe-sur-le-Lac catastrophe, but many residents chose to stay in the community they loved and to rebuild. To ensure future safety and allow residents to regain some peace of mind, the work of designing, repairing, reinforcing, raising and waterproofing the dike would have to be accomplished as quickly as possible, before 2020’s spring thaw.
Engineers determined that to protect the entire Marthelacquois municipality against spring floods for the next 100 years, a dike 26.5 metres above sea level, along the entire length of the shore would be needed, which would be about 1.4 metres higher than the previous earthen dike. The federal and provincial governments would pay 80 per cent of the project costs and the City of Sainte-Marthe-sur-le-Lac 20 per cent.
Protecting against extreme spring flooding for 100 years
By mid-June, borehole drilling had begun to determine the nature of the soils inside and under the dike. The general contractor, DuroKing Construction, began pre-construction work in August. Its subcontractor, ETPO Géodex Inc., which specializes in piles and deep foundations and maritime work, started the sheet piling installation in September, only a few weeks later.
Made from more than 2,000 pairs of AZ 14-770 sheet piling – from six to eight and even 10 metres long – driven in impermeable soil, the new waterproof dike includes a sturdy, calibrated riprap up to the crest of the dike on the lake side to provide the strength and durability to cope with floods and strong waves, and an obstacle-free path for effective maintenance. When Phase 1 of the job was finished, the sheet piling portion of the dike was five kilometres long.
“Some portions of the new sheet-pile dike are located in the same places as the failed natural dike, but some others are at new locations, following the experts’ recommendations,” said Alex Gravel, sales and marketing manager of Gilbert Inc., manufacturer of the Grizzly MultiGrip vibratory pile drivers used to install the sheet piles.
“The soil conditions are very inconsistent,” he said. “In some places, the ground is very soft, while in others, there is an extremely hard layer of boulders.” He notes that both the model MG-60, mounted on a CAT 330C, and the MG-90, on a John Deere 380, offered 360-degree rotation for extreme maneuverability and precise handling when compared to conventional crane and vibro hammer setups, and their SafeGRIP system ensured that the jaws would remain tightly closed in case of a broken line.
ETPO Géodex, which already owned one Grizzly vibro, had used and appreciated its speed and versatility on several other jobs, says Serge Riel, ETPO Géodex’s general superintendent.
“Usually we’d do sheet piling with a crane and a suspended vibro, but with that method, first you have to lift the sheet, slip one into the other like tongue in groove, and then you can vibrate it,” he said. “With the Grizzly Multigrip, we can perform both operations in one step,” enabling quick handling of such a large number of sheet piles while reducing ground vibration and minimizing inconvenience to nearby residents.
“Because of the narrow streets where we could not go with big trucks, and the streets where you cannot back up, we also had to find a way to bring the materials to our equipment and our teams,” Riel said. Phases 1 and 2 required extensive planning and preparation, adding that on the site itself, organization of both materials and crews was key.
“If you’re loading and unloading fast, everything goes better after,” he said.
The sheet piles were hauled to the site by trailer, unloaded and then positioned to be driven.
“We had a groundman in front of every machine, an operator, also a lot of workers in Skytracks,” and the welders who welded the sheets together once they were in place to waterproof the wall. Altogether a crew of about 15 workers completed the job.
Although the ground had been surveyed prior to construction, “You never know what’s under your feet 30-feet deep,” Riel said. “Frozen soil can also slow the process down a little bit, and we were doing things on the run – that’s the art. On good days,” depending on the compaction of the soil and the weather, “we could drive up to over 20 pairs of sheet piles a day, and on bad days we’d go down to about a dozen pairs of sheet piles.”
Equipment offered better maneuverability and quicker handling
Some of the owners of waterfront houses were unhappy that dozens of trees lining the waterfront were removed for the new dike and that the new, higher structure blocks their water access and view of Lac des Deux-Montagnes. Other residents of the community – many of those whose homes had sustained extensive damage or had to be completely demolished – wanted the dike repaired as soon as possible and are eager for the bigger dike to be complete before the 2020 spring melt. Riel is philosophical about the controversy.
“Any project you do, there’s always 50 per cent that are happy and 50 per cent that are not happy,” whether because of noise or other factors, he said. Even with those problems to solve, he says Phase 1 was a great success and because it went so well, Excavation Loiselle Inc. awarded ETPO Géodex the contract for Phase 2: the installation of an additional 1.8 kilometres of sheets piles. Phase 2 began early in January 2020, and had to be completed by the end of February. The surface of the ridge will be coated and the residential side covered with turf once the most urgent floodproofing work is complete.
“I’ve been in the piling business for 30 years, but having to install that quantity of sheet piles in a short time frame, I don’t think I’ve ever seen that before here,” said Riel. “It’s been a huge project involving lots of different players and resources, and the greatest challenge has been to keep the steady pace and respect of the deadlines. I’m very satisfied with how it’s going.”
Photos courtesy of Gilbert 