Erosion and scouring can undermine foundations
In 1986, the Northern Ontario community of Winisk was washed away when a spring ice jam on the Winisk River caused flood waters to reach as far as six kilometres inland, sending every structure but two into Hudson Bay. In 1997, the most severe flood of the Red River in over 150 years reached throughout the Red River Valley, inundating Grand Forks, Minnesota, and causing over $500 million in damages in Manitoba. In 2003 two lives were lost and bridges were washed out in a “storm of the century” when torrential rains and huge amounts of melt water from the Pemberton Icecap combined to flood the Sea to Sky region of British Columbia. In 2008, rapid snowmelt after an unusually severe winter caused New Brunswick’s Saint John River to reach its highest level ever recorded. The 2009 Red River Flood inundated parts of North Dakota, Minnesota and Southern Manitoba. A “1-in-300-year flood” of the Assiniboine River in 2011 affected much of Western Manitoba. Devastating floods caused by excessive rainfall or snowmelt are happening ever more frequently across Canada, and “hundred-year” floods are becoming much more common.
“About 15 years ago, a ‘100-year event’ was the hugest event,” said Ruth Eden, director of the Structures Design and Construction Branch of Water Management and Structures at the Manitoba Department of Infrastructure and Transportation (MIT). “In 2011, we had a ‘one-in-300’ on certain portions of our major rivers, on the Assiniboine and Souris.” Redefining what’s usual and unusual will be the job for hydrologists and hydraulic engineers, she said, “but it’s definitely a moving target.”
Manitoba has been using large riprap for at least 40 years to prevent flood erosion around its bridges, with the rocks sized for the expected flood velocity, said Eden. Since ensuring the safety of major transportation infrastructure is one of MIT’s principle responsibilities, “we have to go back in on a regular basis to make sure the rock hasn’t eroded away and to replace it if we need to,” she said.
Most bridges in Manitoba are on deep piled foundations, according to Eden. “If we can, they’ll go to bedrock or to a hard till layer to give them the load capacity.” But for the bridges in the province that have spread footings directly founded on a hard till layer near the ground line, erosion and scouring caused by flooding can undermine foundations. In Manitoba scouring has caused severe damage even to bridges with driven pile foundations, and in both situations the MIT must do an immediate assessment to determine whether the bridge still has some capacity or must be closed. Is the piling end-bearing? Has there been lost shaft resistance? In 2011, when floodwaters exposed eight to twelve feet of piling under the Assiniboine River’s Grand Valley Bridge, just west of Brandon on the TransCanada Highway, the westbound span had to be closed for four months.
To repair the bridge a permanent cofferdam was first built around the footing and the exposed piles. Said Eden, “We came in and poured structural concrete, added more piling and enlarged the footing to account for the loss of capacity on the existing piles, new piles and the increased dead load, and redesigned the entire pier to bring it back to its original capacity.” Today the bridge is back open and functioning well. “We’re going to be doing modifications on two bridges in the next year,” she said, “specifically to the foundations, to change how they are carrying the load or functioning.” She notes that some bridges over the Assiniboine and Souris River in Manitoba are fairly large, and that at one bridge four or five piers need to be modified. Rock-socketed caissons going down into the till layer will be installed, “so it won’t matter if we have erosion at the spread footing ‒ we’ll have those caissons carrying the load.”
“You have to modify, improve and add to what’s already there – without damaging what’s already there while you’re doing the work.”
Equipment access will be one challenge, so the work is planned for the winter to take advantage of ice and low water levels. “Contractors will build cofferdams to create a dry area and work within that area,” she said. Another challenge will be working with and around existing foundations, she noted. “You have to modify, improve and add to what’s already there – without damaging what’s already there while you’re doing the work.” The intent will be to keep the bridges open while work proceeds.
“In Manitoba we’ve been using a lot of PDA (Pile Dynamic Analysis) testing,” measuring the energy through the piling to verify capacity, short-term and long term impacts, said Eden. “We’re doing testing before we go into sites and then during construction, to confirm that we’re actually getting in the construction process what we assumed during the design process.” A number of Manitoba bridges now have permanent structural “health monitoring” equipment installed that will transmit information about how they are performing under loading for ten years or more.
And on bridges on the Red River, where embankments tend to move towards the river and take the bridge foundations with them, inclinometers have been installed on piers, “just to be sure they’re staying vertical,” said Eden. “It’s a large issue for us. We’ve done a lot of stabilization and we understand it, but it is an expensive problem to solve.” Soil, clay and cohesive material on the embankment must be replaced with gravel, “interlocked so that it doesn’t move and slide.”
For overland flooding where the flows aren’t that great, sandbag dikes and other flood protection methods can be very effective, said Eden. “Some are better in flow situations, some are better where it’s overland flooding, where the water is rising but you’re not getting wave or wind action.” MIT uses a number of different flood protection devices, she said, including aqua barriers, flood tubes, metre-square “super sandbags” and geotextile baskets with wire mesh on the sides and bottom that can be filled with sand that are called Hesco bastion dikes or barriers.
“You have to plan and anticipate what the water level’s going to be,” said Garry McFarlane, product manager with Brock White, which sells sand bags, erosion control blankets and matting products and geo-grids along with concrete supplies and other construction materials in Western Canada. “Everyone has a good idea of what’s going to happen,” he said, noting that in his region the annual flood forecast was based on anticipated snow melt from the watershed to the west through the Assiniboine Diversion and the Red River from the south.
“I’m not sure if there are many jurisdictions that have had to contend with these situations,” said Eden. “Prairie people are very hardy,” said McFarlane, “and we’re prepared for it.”
The Destructive Force of Water
Water, mud and mold can wreak havoc on the exterior and interior finishes, wiring and contents of a building. But according to Craig Rogers of Rimkus Consulting Group, a forensic engineering company headquartered in Charleston, South Carolina, the physical forces of the hydrostatic, hydrodynamic and impact loads of flooding can be even more devastating:
- Hydrostatic forces can cause permanent deflections as well as structural and foundational damage in buildings. When floodwater presses inward against a building’s exterior walls, or when indoor floodwater tries to escape, it exerts a lateral pressure of approximately 310 kilograms per square metre for every metre of differential (the difference between water levels on either side of the wall). In addition, when buildings are pumped out too quickly, lateral pressures exerted by still-saturated soils may exceed the structural capacity of basement walls, causing them to collapse inward.
- Water flowing around a building also creates hydrodynamic loads ‒ frontal impact loads on the structure from the upstream flow, drag on the sides of the building, and suction on the structure’s rear face as water flows around it. The magnitude of these loads depends on the velocity of the floodwaters and the shape of the structure, but structural walls may collapse and a building can be shifted from its foundation.
- The forces associated with waves or floating debris can be much higher than the hydrostatic and hydrodynamic forces of flowing or still water, and especially destructive.
And when water’s buoyant force exceeds the weight of a building’s components and there are few or no mechanical connections between the floor system and foundation, a building can actually be lifted by the floodwaters and float away.
Flood damage is often made worse by the effects of fast-moving water scouring soil from around and below foundations. Sandy and silty soils are more prone to scour than stiff clay soils. Reduced foundation capacity may lead to a shifting of the building, a partial or even a complete collapse of the structure.
SOURCES:Ruth J. Eden, M. Sc., P. Eng. Director of Structures Design and Construction Branch Water Management and Structures Manitoba Department of Infrastructure and Transportation 600 – 215 Garry Street Winnipeg, Manitoba R3C 3Z1 Phone: 204-945-5058 E-mail: firstname.lastname@example.org Garry McFarlane Brock White Canada Phone: 204-631-3124 Cell: 204-791-2596 E-mail: email@example.com Craig D. Rogers, P.E. Eastern Region Property Division Manager Rimkus Consulting Group, Inc. 1525 Sam Rittenberg Blvd # 103 Charleston, SC 29407 USA Phone: 843-766-8755 Cell: 843-276-6802 E-mail: firstname.lastname@example.org