Mackinaw Straits State Harbor - Project Update

By Jason Edberg, P.E., Chief Business Development Officer 

In late 2022, NTH was selected by the Michigan Department of Natural Resources (MDNR) to inspect and assess the condition of the shoreline structures and adjacent pavements at the Straits State Harbor, a facility constructed in the 1920’s and 1930’s as a passenger and car ferry terminal for the Mackinaw Straits Ferry, prior to the construction of the Mackinac Bridge. The reason for this inspection/assessment is that the MDNR is required by state law to operate and maintain the facility as an emergency route, should the Mackinac Bridge be taken out of service. The overall scope for this project also involved historic document review and digitalization, underwater inspections, field investigations, evaluations, and recommendations.

NTH sent a team up to Mackinaw City to perform underwater inspections, using our Remotely Operated Vehicle (ROV), of the various docks, piers, and shoreline protection systems, and assess the condition of the main structural elements (seawalls, cofferdams, and caissons). In addition, NTH also performed geophysical investigations to determine the cause of sinkholes that had appeared across the site. NTH performed geophysical exploration using ground penetrating radar (GPR) scans (also referred to as “lines”) along the various segments of the main harbor peninsula, adjacent to the lake, as well as using Multi-Channel Analyses of Surface Waves (MASW) on the North and South alignments of the main harbor peninsula. NTH also used non-destructive testing and evaluation of the pavements across the site to determine their load rating and remaining service life, with Falling Weight Deflectometer (FWD) testing performed by Applied Research Associates, Inc..
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Overall, NTH concluded that the facilities were in good condition, especially considering their age. The condition assessments revealed only minor defects in the various shoreline protection systems, primarily consisting of holes in the steel sheet pile elements and gaps in the timber cribbing structures that allow portions of the internal aggregate fill to “leak” out into the lake.
 
NTH recommended patching holes and gaps, performing test pits to investigate the buried anchorage elements of the deflected portion of the East Face Seawall, backfilling areas of surficial subsidence or “sinkholes” to original grade, replacement of bolts, replacement of lost aggregate fill within the structures, and repairing the damaged portion of the North Armored Shoreline.
 
In addition, based upon the pavement evaluation, NTH recommended sealing existing cracks to help reduce water infiltration, inspecting storm sewers via CCTV to identify soil loss, and sawcut and patch pavement depressions if deemed necessary by the CCTV inspections. If the site is to be used for commercial applications moving forward, NTH recommended that there be limitations on the vehicle load and/or frequency, and that there be an annual fee for access established, specifically for maintenance and future rehabilitation of the pavement.
 
Once the repairs are completed, it is reasonable to expect the structures to continue to perform over a 50-year remaining service life.