A city on stilts
Standing beside a slick, black pit in the middle of Fargo, North Dakota, the veteran engineer from New Orleans is amazed. He's engineered major construction projects in the wet, weak soils of the Mississippi Delta -- supposedly the most notoriously unstable geology in the nation. But this Red River Valley mire is something else. "I've never seen anything like this s***," he says as he views the future site of Fargo's Water Treatment Plant.
The North Dakota State University geology students clustered around the Southerner laugh. So does their professor, Donald Schwert, but for different reasons. Schwert knows exactly why this engineer's expertise is required so far from the Delta.
It's the smectites. Clays that love to swell and love to shrink. Clays that in the Red River Valley extend more than 100 feet beneath the region's famously rich topsoil. Clays that mean it's impossible to erect so much as a water tower or a highway bridge without standing it on a steel piling or concrete pier sunk through 105 feet of muck until it hits a firm layer of glacial drift.
The story behind the smectites dates back about 12,000 years, when the region was covered by Lake Agassiz, the largest freshwater lake in the history of the world. Formed by melting glaciers, Lake Agassiz receded into Canada over a 3,000-year period, leaving behind deep layers of sediment -- made up mostly of smectites. The smectites were delivered into Lake Agassiz from muddy, glacial meltwater rivers cutting in Cretaceous shales, themselves rich in smectites. Exposed examples of these shales can be found today in the rolling hills near Valley City, 60 miles west of Fargo. Here hillsides edged by twisted railroad tracks testify to the shale's instability. Because the shale is packed with smectites, when hydrated it becomes plastic and loses strength.
It's the smectite-rich sediments that help make the Red River Valley's cropland so productive. They also are the materials upon which homes are built and streets are laid. It's where water mains and gas lines are buried. The ground appears solid, but as the clays gain and lose water, the land rises and falls, causing pavement to buckle, water mains to break and basement foundations to crack and separate from the surrounding dirt.
Every city built on the banks of the Red River today is affected by Lake Agassiz's soil legacy. Cities and towns up and down the North Dakota-Minnesota border -- Fargo-Moorhead, Wahpeton-Breckenridge, Grand Forks-East Grand Forks and on up to Winnipeg, Canada -- all ride on top of this problematic soil. "Geographically it was inevitable that cities would be established where railroads crossed the Red River. But geologically," Schwert says, "it's been a mistake."
Schwert has spent 26 years being alternately fascinated and frustrated by the substrate upon which these Red River cities are built. He's fed his curiosity through study and by crawling into every big hole he can find. And when he was invited to give NDSU's prestigious Faculty Lectureship, he chose "A City on Stilts: The Geology Under Fargo" as his topic.
It's quite an image. A city on stilts. But if Fargo could be hoisted up for a view, every structure of any weight or height would look like it had grown legs. Lots of legs. Fargo's Water Treatment Plant is supported by at least 350 concrete piers. The FargoDome rests on more than 300. Work on the massive, multi-storied Scheels All Sport store -- now under construction on Fargo's 45th Street -- couldn't begin above ground until 103 concrete piers were installed below. Even Fargo's minimal downtown skyway system stands on stilts.
Drawings of concrete caissons, photos of compromised buildings, maps and graphs flash on a screen in NDSU's Century Theater, mesmerizing the standing-room-only audience who came for Schwert's lecture. Who knew geology could be so interesting? Obviously Schwert did. The award-winning teacher, and interim dean of the College of Science and Mathematics, has the crowd on the edge of their seats. Having made the case for "stilts," Schwert shows what can happen without them. The year is 1913. The place is Winnipeg, Manitoba. The project is the Transcona Elevator. It will be Canada's largest grain elevator. The structure is simple: several tall cylinders erected on a raft of concrete. The result is magnificent. The day finally arrives when it's time to fill the elevator with grain. First bin No. 1, then No. 2, then No. 3. "And somewhere around lunch time," Schwert says, "Workers hear this terrible groan, this moan, and the elevator starts to topple over and then stops. There's 23 degrees of tilt." It's funny now; it wasn't so funny then. To salvage the elevator, laborers had to tunnel under the structure and construct caissons. Crews then excavated around the elevator and finally eased it down onto the caissons. The elevator still stands today, 12 feet below its original level.
The near catastrophe in Canada was big news in soil mechanics literature of the day. But somehow the lesson got lost. In 1955, investors built the same type of elevator at what is now the intersection of Main Avenue and I-29 in Fargo. This time, as they began to fill the 125-foot-tall bins, the elevator collapsed beyond rescue. So much for learning from others' mistakes.
These days engineers, architects and contractors know big and tall projects in the Red River Valley require special underpinning. Even so, there can be problems. A few years ago, engineers didn't like what they were seeing in Neumaier Hall, a 15-story residence hall in the center of the Minnesota State University Moorhead campus, across the river from Fargo. Cracks in the brick walls led them to believe the 29-year-old building was failing. In 1999 Neumaier was razed by implosion. Engineers now think one of the caissons shifted or that concrete in the caissons supporting the dormitory had deteriorated over time, causing the building to shift and begin to break up.
Enjoying the view from a tall riverbank in north Fargo, one tends to forgive the region's geology for its tricks. Silhouettes of purple thistle, yellow clover and cockleburs are sharpened by a brilliant blue sky. Goldfinches dart along the russet-tinged current as it shimmers by green trees on the opposite shore. It's an idyllic scene, and a perfect example of how nature has its way with man.
No one knows for sure how far this bank once extended toward the river. What is known is that there used to be a poor farm nearby. And there was a pauper's cemetery. In 1985, people began seeing exposed coffins and human bones jutting from the bank. The river's current was gradually unearthing the graves. Schwert arrived to document the scene, parting sheets hung by workers to give the dead their dignity before relocating their remains to a new county cemetery.
Twenty years later, there are still some bodies buried there, but precautions have been taken to stabilize the area. A million dollars worth of rock has been dumped along the outside edge of the u-shaped meander loop of the river to impede erosion. Trees have been removed and the grade of the bank made less steep. A road has been closed, partly because there's no ground to support it and partly to reduce pressure on this vulnerable geology.
Unfortunately, seemingly sublime settings like these have lured many Red River Valley homeowners to the riverfront. The view from tall banks. The romantic bend of the river. The distance from the flood plain. But houses bordering the outside edge of the Red River's meanders often do not fair well. The combination of river dynamics, unstable soil, the extra weight of the house and the fill used to develop the lot, plus extra moisture from watering or septic systems can be a recipe for disaster. "When water increases the plasticity of the clays, the weight of the house itself simply adds to the overall problem," Schwert says. Ultimately everything starts slipping toward the river. Schwert -- often called upon to share his expertise -- has seen river property go from perfect to perilous in as little as two years. "By the time I'm called in," he says, "it's usually too late to do anything about it."
Even though as a scientist he's interested in viewing evidence of what he calls geo-vandalism -- like bike trails mutated by the Earth's heave -- in his heart of hearts Schwert is on the side of the humans who live in the Red River Valley. The catch is not everyone is thrilled with his prescription for living well above ancient Lake Agassiz's silt.
"If I were the Czar of Fargo," Schwert says, standing on the banks of the Red, "I wouldn't allow anyone to build basements. Basements here are prone to seepage, shifting and flooding, plus they often have elevated levels of radon." But Schwert knows people want basements -- his house has a basement -- so they'll probably build them anyway.
A more crucial edict -- if Schwert had the power to make one -- would require Fargo builders to follow setback guidelines that he and city and county officials have developed. The guidelines are under review, and some real estate developers are not pleased. There's a market for riverfront property and these guidelines would impact sales.
So, Schwert cloaks consumer cautions in amazing tales of "a city on stilts," in hopes of helping people think twice about where they decide to build their dream homes. In his mind, an ounce of prevention is worth a thousand times the cure.
"In the end," Schwert predicts, "mortgage lenders will be the ones who put a stop to this kind of development. All it will take is for one homeowner to walk away from one of these properties and for the mortgage owner to be left holding the bag. That would lead to some very sudden education on the part of the mortgage lending community."
With some wise planning, and a little more green space, homes in the Red River Valley can be nearly as safe from the sway of the smectites as those structures standing on stilts.
-- C. Jelsing