Provided in public service by the:

Department of Geosciences
North Dakota State University
Fargo, North Dakota 58108-6050

The clay-rich sediments deposited in the central basin of Lake Agassiz in North Dakota and Minnesota have inheritantly weak properties. The clays, themselves, are expansive: capable of absorbing great quantities of water. The more water that is absorbed by these clays, the weaker they become. The clays have sufficient strength to support low-load buildings in Fargo, such as houses, shopping centers, and small businesses. But they are incapable of supporting larger load structures, such as high rises, water towers, bridge supports, etc. Should such a high-load structure be placed onto these clays, it risks sinking or toppling.

And so how can high-load structures be supported on these clays? The answer is that they can't. Instead, the load of the structure must be transmitted through the Lake Agassiz sediments to firmer materials >100 feet beneath the surface. These stronger materials are usually glacial drift: till, outwash, and other sediments deposited by glaciers of the past ice ages.

In part, then, Fargo is a city built on "stilts." Heavier structures are supported by concrete piers (caissons) or steel pilings that transmit the weight of the structure down to depths of 105 to 130 feet. The FargoDome rests on >240 caissons, some with diameters of >5 feet. The Fargo Water Treatment Plant at 13th Avenue S. and 4th Street rests on >300 caissons. Even the skywalks in downtown Fargo are supported by caissons, so as to assure that their weight won't cause them to drift in the weak clays underneath.

Each of the steel I-beams of NDSU's Bison Sports Arena is supported on a caisson (arrow), each of which extends >100 feet below the surface.

(Click on image for enlargement).

All of this, of course, adds to the "cost of living" here in the Red River Valley. (The author notes that it would have been a far better idea to have placed "Fargo" onto the firmer, higher ground near Tower City, than onto the flood-prone, weak clays in the central portion of the Valley . . but it's hard to change that history now!).

Watching the construction of caissons is fascinating. Massive augers bore downward through the weak clays. Because the adjacent weak clays would squeeze back into the hole, temporary iron casings are emplaced to hold the hole open. A pre-constructed cage of steel rod is lowered into the cased hole, and then cement is poured into the hole (generally, about 7 truckloads of cement are required per caisson). As the cement is poured into the hole, the casings are lifted out. Within a few days, the cement will have solidified to concrete within the hole. One now has a solid, steel-reinforced concrete caisson extending through the entire pile of Lake Agassiz sediments. Onto to this caisson will go the I-beams and other supports for the structure above.

Augering caissons for downtown Fargo skywalks (1984). Note giant augers (left). Cylinder-shaped device at right is for "belling" the base of the caisson hole.

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Each of NDSU's highrise dormitories rests on 36 caissons, transmitting the weight of each structure >100 feet downward to stronger materials. Likewise, the water tower (distant-center) is supported on steel pilings that extend down this depth.

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Failure of one or more of its caissons caused Neumaier Hall at Minnesota State University-Moorhead to start rifting. (Note developing fissures [arrows] in the brickwork (1991 photo).

(Click on image for enlargement).

Using at the completion of the augering of the hole, the base of the hole (where it is within glacial drift) is "belled": a special auger is used to widen the base of the hole (and caisson), so as to establish a larger surface area onto which the caisson will be supported. In effect, then, the function of the "belled" component is to act much like a foot does on a human's leg.

When one or more caissons fail, the integrity of a structure is endangered. Such was apparent case of Neumaier Hall, a high-rise dormitory at Minnesota State University - Moorhead. Engineers now suspect that the concrete in one or more caissons deteriorated over time, causing the building above to shift and then rift. In 1999, Neumaier Hall was evacuated due to its precarious structural situation. It was razed by implosion.

Donald P. Schwert


(Click on each photo for an enlargement)

Related Sites:

[ More Brief Insights on Fargo's Geology ]

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This web site represents the views of the author and not necessarily those of North Dakota State University. NDSU is not responsible or liable for its contents. Copyright © Donald P. Schwert.