July 12, 2012
Pier pressure: KG makes major investment to shore up Loretta Building’s foundation
Mike Allmendinger jokingly calls it “Geotechnical Awareness Day.”
On Jan. 23, 2012, the general manager of Kilbourne Group received some unexpected news. The downtown revitalization group planned to add a fourth story to the historic Loretta Building to maximize its prime location on the 200 Block of Fargo’s Broadway.
But in the process, Kilbournians learned an additional story would be a problem.
Make that a big, slippery, expensive problem.
The Red River Valley’s notoriously slick and unstable clay soils would not support the additional weight of one more story. In order to meet “live load” requirements for its existing three-story height, Loretta’s foundation needed major TLC.
Blame it on the clay
NDSU Geology Professor Don Schwert blames it all on the smectites – a type of clay that loves to shrink when dry and swell when wet. Smectites extend at least 100 feet below the region’s famously fertile top soil. These highly elastic clays are one reason Red River Valley soils are so rich, but they also cause streets to buckle, basement foundations to crack and railroad tracks to twist.
In order to reside atop this slip-sliding foundation of Jell-O, Fargo-Moorhead residents built what Schwert once described as “a city on stilts.” That’s because all structures of any heft or height actually have tall, spindly legs – either steel pilings or concrete piers, beneath it. These “stilts” thread through 100-plus feet of slick muck until they can be anchored in ice or bedrock. “Fargo’s Water Treatment Plant is supported by at least 350 concrete piers,” wrote Cathy Jelsing in a 2005 interview with Schwert. “The FargoDome rests on more than 300. … Even Fargo’s minimal downtown skyway system stands on stilts.”
But not the Loretta. It was built the same way that most downtown Fargo buildings were at the turn of the 20th century: with concrete footings that reached maybe 15 inches into the ground. Initially, Allmendinger and company thought beefed-up footings were all Loretta needed to bear the load of an extra story. After all, that was more than enough for Kilbourne’s 300 Block, also four stories high, to meet building codes.
But a soil test showed that the clay beneath the Loretta was extraordinarily unstable, even by Red River Valley standards. Consequently, the existing footings weren’t enough to bear the building’s “live load,” its total weight including people and furnishings, even at its three-story height.
If Loretta really planned to stand tall for another 100 years, she would have to do so in heels.
One-hundred-foot-tall heels, to be exact.
To see how Loretta’s foundation was shored up, go to:
The right kind of pier pressure
Kilbourne Group hired RaiseRite, a Wisconsin company, to give Loretta’s foundation the support system it required. (RaiseRite knows a thing or two about working with the Valley’s capricious soils; it is the same company that helped shore up NDSU’s Minard Hall after one wall collapsed in 2009.)
The solution: Excavate down to the footings, then notch the edge of existing concrete footings so special pier brackets could be inserted underneath the building walls.
These brackets would also hold steel piers, which were driven into the ground with a hydraulic ram. Each 4-foot-long steel pier would nest in the pier below it to form a 100-foot-long “stilt.” In the process, the weight of the Loretta was transferred to the stable bedrock down below.
As an added precautionary method, Kilbourne opted to fill each pier with low-density concrete. The overall pier installation was slow and time-consuming, with crews able to install just four to six complete piers a day. In the end, 196 of these concrete-filled piers were added to Loretta’s foundation.
Six months later, Loretta is arguably one of the most unshakable structures on Fargo’s Broadway. And, because this chapter of her renovation wouldn’t normally be visible to the average visitor, Kilbourne Group opted to keep the tops of several piers exposed and preserved in acrylic. So now Loretta’s “pier review” will create one more “teachable moment” for any visitors who choose to tour through this grand, old building.
Those piers, located in the southeast corner of Loretta’s lowest level, will help to tell the story of Loretta. Like any great lady, she’s had a colorful past, an interesting present and a promising future.