Weyerhaeuser project will utilize innovative wall
3 Feb 2023
Community, News, Economic Development
A plan is essentially in place for the Morrison County Historical Society’s (MCHS) riverbank stabilization project at the Charles A. Weyerhaeuser Memorial Museum.
Thursday, Jan. 26, Ulteig Senior Engineer and Client Service Manager Roger Clay presented the latest plan to a handful of MCHS Board members, as well as representatives from the Morrison County Soil and Water Conservation District (SWCD) and Minnesota Department of Natural Resources (DNR) Hydrologist Mark Anderson. By the time the lengthy meeting adjourned, nary a stone was left unturned regarding the specifics of the project.
Those present felt it was a good plan.
“I have to say, the end result is going to be a lot better than the wall,” said MCHS Board member Cathy Adamek, referring to an early option that included a block retaining wall.
“I don’t know about you folks, but I like what I see here,” added MCHS Director Mike Worcester.
In May, MCHS received $140,000 from Morrison County and $70,000 from the city of Little Falls in American Recovery Plan Act (ARPA) funding to pay for the design and development phase. At that time, the estimated construction cost to stabilize the bank was $597,000.
The project is necessary because two major rain events — one in 2015 and another in 2020 — have caused severe erosion on the riverbank. Currently, the museum is about 20 feet from the edge of a straight drop, and further erosion could cause the museum to fall into the river.
Since the group last met in December 2022, Clay said he and his colleagues had done extensive research on different types of vegetative walls. After going through options with various manufacturers, he felt the project would be a good fit for a mechanical stabilized earth (MSE) wall.
The vegetative wall would provide a natural look while also being strong enough to prevent future erosion on the bank of the Mississippi River.
“They were showing it; it was, like, kind of in the foothills of a mountain type of thing,” Clay said. “They were showing this water shooting down this channel at about 25 feet per second. Afterwards, you couldn’t even tell.”
The estimated cost for construction, presented by Clay, was $618,310. That would not include paying contractors to complete the extensive project. MCHS is exploring several avenues for funding, including a request in the state bonding bill and various grants for which it might be eligible.
The MSE wall would be 120 feet long adjacent to the Weyerhaeuser Museum itself and 15 additional feet to both the north and south.
The wall would be built on a 1:1 slope — one foot vertical for every one foot horizontal — near the water line. On the outside, it will consist of environmentally friendly bags that are filled with a mixture of 60% sand and 40% mulch. The sand provides drainage for moisture, while the mulch will give nutrients to the vegetation planted on top of it.
“These bags, which, they’re kind of a fabric but they’re not,” Clay said. “They’re warrantied for 75 years. They think they’ll last longer. They’ve only been around about 20 years.”
Behind the bags will be layers of soil, reinforced with geogrid. The wall will tie into the natural slope of the bank on the south end, as well as to the grading work done on the north end of the property, alongside the parking lot.
About halfway up the bank, the slope will change to 1.5:1, with a transition area in between.
“Where the MSE wall is, we’ll put in, like, willow branches and live stakes,” Clay said.
In terms of the work, Clay said crews will have to dig out the slope, “right up to real close to the building.” They will likely dig all the way down starting on the south end and work their way back to the access point to the north.
The soil that is excavated will be able to be used for fill and in other areas of the project.
The MSE wall will have a riprap (rock) foundation, which provides more slope stability.
“You’ll never see that after it’s built,” Clay said. “That’s going to be hidden from sight.”