Geologists blame NDSU building collapse on weak clays
Days before Minard Hall collapsed, a North Dakota State University geosciences professor looked at the excavation work to see if the hole revealed interesting geological formations.
Allan Ashworth said he and a colleague scratched their heads at the sight of Minard's exposed basement and a hole that at the time was 12 feet deep.
"I was very surprised to see them digging a deep hole so close to a building like Minard," he said.
Ashworth said he saw potential for a problem, considering the weak clays beneath Minard and a hole that provided them a place to flow.
Prior to the collapse, the excavation work went even deeper, making the hole 25 feet deep and exposing damp clays.
The footings of a pillar on a northwest wall of the 108-year-old building failed early Dec. 27.
Now, officials are monitoring adjacent Morrill Hall, which experienced vibrations during the work going on at Minard.
Determining a cause for Minard's collapse is weeks out, but the focus of the investigation will be on the soils, said Bruce Frantz, NDSU facilities management director.
Ashworth and Donald Schwert, a geosciences professor who has lectured on Fargo's earthen foundation, each shared theories on the collapse with President Dick Hanson.
Neither professor has detailed information about the construction project. They base their hypotheses on their knowledge of the subsurface of Fargo and engineering problems faced in the region.
Both believe the weight of the building squeezed some of the clays toward the excavation.
Prior to the collapse, Minard was subjected to vibrations when pilings were pounded into the ground to protect a steam tunnel. Those vibrations likely weakened the clays further, Ashworth said.
In addition, the excavation may have been deep enough to expose water-laden silts, Schwert said.
Once the clays flowed toward the hole, the unsupported basement cracked and the weight of the building caused it to collapse, Ashworth said.
Assigning blame for the collapse depends on what discussions went on among architects and engineers about the potential problems of excavating a deep hole adjacent to Minard, Ashworth said.
Schwert said he attempted to photograph the pit after the collapse to see if it was a result of soil failure. But by that time, crews had used fill to stabilize the building.
"A lot of the evidence that I would need to see, or others would need to see, right now is lying buried," Schwert said. "This could be difficult to litigate."
Crews plan to remove some of the fill and debris to investigate the cause, Frantz said.
But first the roof needs to be stabilized and floors jacked up, which will take about two to three weeks.
Weak clays have been the reason for many building collapses in Fargo-Moorhead, with the most famous being a 1955 grain elevator collapse, Schwert said.
The clay-rich sediments from the Lake Agassiz basin are too weak to hold up massive structures, Schwert said.
That's why heavier structures such as the Fargodome and the Radisson Hotel are supported by hundreds of concrete piers that extend through more than 100 feet of unstable clay to stronger materials.
Schwert, who calls Fargo a "city on stilts," said the NDSU high-rises have such supports beneath them.
Officials monitoring Morrill haven't found anything that gives reason for concern, Frantz said.
Hairline cracks in the plaster have been noticed on some walls, but no one knows when they appeared. The cracks, which crews painted over this week, were about 3 feet long from the top of display cases to the ceiling.
Frantz said paint crews have been repairing such cracks in that building for years, and they're caused by changes in the humidity.