A stress crack has formed in the new levee under construction behind Pleasant Avenue, and as a result a section of the new levee as much as 350 long won't be completed along with the rest of the current phase of the Sampson's Addition work, and will have to be finished sometime next year as part of a separate contract.
"It's significant, but we won't know for some time exactly how significant it is," said Community Development Director Mike MacDonald, the city's point person on flood control.
Flood control work in the neighborhood in general won't be affected by the series of cracks, which were observed last week in a 425-foot section of the dry side of the new levee's center line, just as workers from Rachel Construction of St. Michael, Minn. were bringing the levee to its final dimensions. MacDonald said the rest of the current phase's components will still be finished this year and, in early 2010, bids will go out for the final phase of Sampson's Addition work. But now, in addition, a separate contract that may require "specialty work" to modify the section of levee that cracked will also be necessary.
In the meantime, dirt is being removed from the entire section of new levee to reduce the weight and subsequent stress level as much as possible, and the cracked area is being removed.
As a result, the removal of the dirt might affect the city's response to the 2010 spring melt and rise of the Red Lake River, MacDonald said. While areas with flood control work that aren't affected by the stress cracks will still be protected to just under 27 feet – around the level of a 100-year flood event – a 300 to 350 foot stretch behind Pleasant Avenue will be protected to around 24 feet, 3 inches. That means, MacDonald explained, that if spring flood outlooks indicate a significant rise of the Red Lake River in Crookston, the area where dirt has been removed because of the cracks will need to be sandbagged.
Too much weight, stress
So what happened? When new levees are being constructed, MacDonald said, they're most unstable during the construction process than they are over the long term after they're completed. Soil borings taken prior to the start of the project didn't indicate a stability problem, but he said new borings will have to be taken before the most effective and financially feasible solution is found.
"When you're doing a project like this, you're changing the load profile rapidly and it tends to destabilize things," MacDonald explained. "Obviously, something's going on in there that's different than what the borings indicated would happen and it couldn't perform under the conditions of the project."
The repair could for the most part involve "putting the dirt back," he added. "But it's also possible that the section of the project will be modified significantly," MacDonald said. "We'll have some answers in early 2010."
