The Morrison County Historical Society (MCHS) is preparing to enter the next phase of its shoreline stabilization project.
Members of the MCHS Board of Directors and Executive Director Mike Worcester met with contractors and engineers on the project on October 6 to review the design and development phase of the project. Using this information, he can begin to move forward in the decision-making process for the final plan.
In May, MCHS received $140,000 from Morrison County and $70,000 from the City of Little Falls under the American Recovery Plan Act (ARPA) to pay for the design and development phase. At that time, the estimated cost of the Mississippi River Bank Stabilization Project near the Charles A. Weyerhaeuser Memorial Museum was $597,000.
The project is necessary because two major rain events – one in 2015 and another in 2020 – have caused severe erosion of the riverbank. Currently, the museum sits about 20 feet from the edge of a straight drop, and further erosion could cause the museum to fall into the river.
Roger Clay, principal engineer and customer service manager for Ulteig, the project’s main contractor, said work completed in early July had yielded a “very detailed” topography of the side slope from the south bank of the building to the past where failure ends in the north. of the work.
“That topography is what we needed to do the analysis and design,” Clay said.
Using this topography, Ulteig and Braun Intertec, the engineering company contracted for the project, were able to create a contour map that identified four areas of bank failure. Two of them were actually about 20 years old.
He said ground soundings of approximately 50 feet were taken in three areas: one near the south corner of the building, one near the southeast corner of the driveway/parking lot, and one near the end north of the driveway. Soil samples taken during this process showed that there was “dense to very dense” soil on the bank.
Soil density also helps determine what types of projects can be done. Bryan Ripp, senior geotechnical investigator at Braun, added that there was no bedrock encountered.
“Is it an advantage to rebuild?” asked MCHS board member Camille Warzecha.
“Yeah,” Ripp said. “For the bearing, absolutely. It is an excellent soil for laying foundations etc. From a bearing capacity point of view, very good. From a slope stability point of view, very good. Looking at the alternatives, we pretty much have to look for something that can rest on the ground to provide slope stability. »
Thus, the recommended design was created in two sections.
The first covers about 120 feet from 15 feet south of the building to 15 feet north of the building. Due to the side restrictions, Ripp said there will have to be a retaining wall included in the plans. He recommended making a gravity wall, which would have a vertical face of approximately 12.67 feet facing the river.
At the back of the individual 16-24 inch blocks, soil will be placed inside and on top of the blocks.
“It acts as a buttress to hold the blocks in place,” Ripp said. “There’s no grid that provides, like, an anchor. It’s just the weight of the ground resting on it.
The top of the excavation will be approximately two feet from the foundation of the building at the point where it is closest to the slope. The wall would rest on a five-foot foundation filled with a lean concrete that would extend below the predicted frost line
In the other section, near the museum parking lot, Ripp said there was room to level the slope rather than put up a wall, which would cost a lot less. One thing the MCHS board will have to decide, however, is how steep a slope they want to put in that area.
A 2:1 slope – two horizontal feet for every one foot vertical drop – has a high safety factor at 1.7. In this case, the top of the slope would butt up against the parking lot, maybe even a little into it. A slope of 1.5:1 would only have a safety factor of 1.3.
“We wouldn’t consider that acceptable for the building, for critical infrastructure like that,” Ripp said. “However, if the board is comfortable with taking additional risks and some minor slippage that might occur, we might go for the 1.5:1 outside of the construction zone. 1 .5 is as steep as possible.
This would equate to savings on the area to be cut. Nor would it encroach on the existing parking lot. The trees along the shore will probably be lost anyway.
Clay added that the Council could opt for an intermediate slope between the two, perhaps with a safety factor around 1.5.
In slopes, a 2 inch high geographic grid would be included. Topsoil and seed would be added to the grid cells.
“I got some native plant seed mixes from Mike,” Clay said. “One for the lower half of the hill, closer to the river where it’s wetter, one for the upper half of the hill where it’s drier. We would use native plantings there. In time, the trees would come back.
“These native plantings, I assume the plants are chosen with a good root system that would eventually grow and help hold on?” said MCHS board member Cathy Adamek.
Warzecha said it would, as it’s also a big factor in some of the grants they plan to apply for to pay for the rest of the project.
Riprap would be added to the base near the shore along both sections. Clay said this was necessary to protect the lower slope from river erosion. They could also use things like willow wattles, which will help anchor the riprap and “re-green” the area.
Warzecha asked MNR hydrologist Mark Anderson what his organization was looking to see along the river in terms of reconstruction or regeneration.
“Roger and I have talked about this before that if a wooden tow bench project isn’t feasible in this area, rockfill is sort of the go-to alternative,” he said. “Roger and I have talked about making sure we incorporate vegetation such as willow wattles into the riprap itself. It offers a number of benefits.
He said these benefits include stabilizing the slope and creating a habitat corridor for terrestrial and aquatic species.
“You might have amphibians popping up and using some of the shade; things of that nature,” Anderson said.
John Thomas, river geomorphologist for Ulteig, also discussed bends in the river that can be created as part of the project. They would seek to create rock jetties that would throw themselves into the current of the river, but the tops of these would remain level with the low flow of the water.
“The idea is that at low flows, the water is prevented from constantly, slowly, gradually eroding the tow of the bank,” he said. “At high flows, the water has to reorient itself.”
The area between the bends will silt up over time and the river will form its own floodplain bank.
Due to the curvature of the river, Thomas said they would only need two or three bends spaced about 60 to 100 feet apart.
“We’re trying to redirect and resist the energy of the water flowing downstream,” he said.
In terms of alternatives, Ripp said another option instead of the gravity wall would be a grid-reinforced wall. This would use a geographic grid, which is similar to a snow fence but has greater structural capacity.
The geographic grid would be placed in layers and attached to the front of the wall with pegs. Each of these layers acts as anchors to hold the wall together, with the weight of the floor preventing the grid from retreating.
“There’s the added cost of that,” Ripp said. “There would be an additional cost for the fill placed and compacted behind. It’s probably not economical, but for the sake of argument, it’s another type of wall that could be used here. It’s just going to complicate things, I think.
Warzecha asked Clay if he could provide the cost for each different scenario. He said the gravity wall would be less expensive than any alternative for this section. In the area away from the building, he said the cost would depend on the slope.
Knowing they have to meet the 1.5 safety factor near the building, he asked the board members what they thought of the sloping area.
“We don’t want to increase the risk too much,” said MCHS board member Ron Jones. “We hope it’s a one-time deal, obviously.”
“I think we’re all in agreement on 1.5 here,” Adamek said.
The MCHS board has posted the plans on its website and is hoping to get public comment. These details can be viewed for the next four weeks by visiting morrisoncountyhistory.org/?p=9951.