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A project to replace retaining walls near City Hall and the Minot Municipal Auditorium received the green light at a recent Minot City Council meeting.
The Council approved an engineering proposal for the $4.7 million project after expressing concern that the cost of the project had risen dramatically from the original $2.8 million estimate.
An engineer report from KLJ highlighted concerns with the retaining walls, including a lack of drainage behind the slope face, a loss of bond between grout and rocks, and inadequate restraint between the slope face and the backfill. Life expectancy for these types of walls is typically no longer than 10 years, according to Cassie McNames a structural engineer with KLJ, who presented the report to the City Council.
The walls near City Hall were constructed in 1954 and have been repaired and patched numerous times since then.
KLJ evaluated several areas related to the project, including safety, minimizing impact to adjacent infrastructure, cost-effectiveness, maximizing useable space, and aesthetics.
The report presented options to the Council, including repairing the existing grouted rock slope and replacing the grouted rock with grass slopes. Each option has drawbacks. Repairing the existing grout would require continued maintenance for as much as 10 years, and have a larger cost investment when compared to the life expectancy of a new retaining wall.
Creating slopes with native grasses also created concerns, since the slope would have to more gradual in order for the slope to be structurally sound and to allow it to be regularly maintained. The slope would use much more land than the current slope, and would impact Third Avenue near the Auditorium, eliminate the narrow access lane just west of Municipal Court, and eliminate fire lanes and parking spaces in the Auditorium parking lot.
The recommendation was to replace the existing walls with one of three options: large block gravity walls, mechanically stabilized earth walls, and soldier pile walls. Each alternative had benefits, but ultimately the Council settled on using soldier pile walls to replace the current grouted rock walls.
The grouted rock walls are similar to a wall on Minot’s Hillcrest Drive that failed in 2015 and had to be replaced.