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Sinkhole along the William A Irvin in Duluth, Minnesota (C) Daniel FriedmanMinnesota Sinkholes & Subsidences
Sinkholes in Minnesota Karst & at Seawalls along Lake Superior & Other Seawalls

Sinkholes in Minnesota:

This article describes karst-formation based sinkholes in southeastern and other areas of Minnesota and it also discusses the causes and effects of subsidences and sinkholes forming along seawalls in Minnesota at docks along Lake Superior as well as in other parts of the world.

This article series explains what sinkholes are and why they occur, describes their effects on buildings, and gives building and site inspection advice useful in identifying areas where there is an increased risk of sink holes at properties

. We include the causes of sinkholes, how they can be spotted, sometimes before a sudden collapse, and what to do if you know or suspect that a sinkhole is developing on or close to your property.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?

Sinkholes & Seawall Subsidences in Minnesota

Sinkhole along the William A Irvin in Duluth, Minnesota (C) Daniel FriedmanSome Minnesota sinkholes are large enough to serve as tourist attractions, such as by the Minnesota DNR-constructed observation deck at a sinkhole on the Root River Trail in Fountain MN.

And Minnesota's Fillmore county, billing itself as the "sinkhole capital of the U.S." boasts more than 10,000 sinkholes.

Karst formations in the southeastern part of Minnesota explain a high number of sinkholes and also play a role in the transport of groundwater contaminants.

Other, less famous subsidences and sinkholes can occur along lake Superior, particularly at seawalls subject to the eroding effects of waves and water movement from ships in harbors.

 

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Sinkholes & Waterfront Seawall Subsidence along Lake Superior in Duluth Minnesota

Below, a section of seawall along the harbor walk in Duluth Minnesota. Canal Park's Duluth Lift Bridge, aka the Duluth Aerial Lift Bridge is visible in the upper portion of this photo.

Seawall and dock examples on Lake Superior in Duluth, Minnesota (C) Daniel Friedman

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In 2016 small sinkholes were reported along the Duluth Minnesota waterfront.

These subsidences in Duluth were considered dangerous and a particular hazard to the public along the tourist area of the waterfront near the Duluth Entertainment Convention Center (DECC) where aging seawalls along docks permit wave, tidal, and other water actions that penetrate the original seawall facade to erode filled soil behind the seawall in areas where people may walk. (Nakamura et als. 2008).

Steel & timber seawall at the Duluth waterfront (C) Daniel Friedman

Above is a more recently-constructed steel and timber seawall where during an inspection tour in March 2016 the author (DF) did not observe sinkholes though such subsidences were visible at older seawalls quite nearby.

Even modern driven steel seawalls such as this one can eventually corrode, rust-through, and permit leaks and subsidences. Below we see an intersection of newer steel seawall (photo foreground) with older poured concrete and timber seawalls (background in the photo).

Intersection of two seawall construction types, steel & concrete, Duluth, Minnesota (C) Daniel Friedman

Sinkholes (or earth subsidences) at seawalls are a common phenomenon occurring anywhere that seawalls encounter forces of moving water such as tides, waves, or boat and ship traffic, or where older seawalls or seawall caps have deteriorated permitting water to move behind the seawall where it can attack backfill and infill.

And while karst formations, the most common ingredient in sinkhole formation, are sometimes present where seawalls are also constructed (Gombert et als. 2015), karst is probably not a factor in the Duluth seawall subsidences.

Concrete, wood, & steel seawalls deteroriating in Duluth Minnesota may explain nearby sinkholes and subsidences (C) Daniel Friedman

Above: a closer view of the older concrete (and timber) seawall in Duluth.

Sinkholes and subsidences in Duluth Minnesota along the William A Irvin national monument - ore carrier (C) Daniel Friedman

Duluth, Minnesota sinkholes drew particular attention near the steamer William A Irvin, a retired ore carrier (lake freighter) built in 1938 and now a museum and a national historic monument open to the public at the Lake Superior waterfront.

The Duluth News Tribune reported that 25,000 people visited the freighter in 2015, making pedestrian safety an important concern. (Slater, 2016)

Authorities erected fencing and modest (keep off the grass) warning signs to keep people from walking in the area where subsidence and sinkholes were observed at the the Duluth waterfront.

Fencing and signs keeping public out of sinkhole area in Duluth MN (C) Daniel Friedman

Engineers or other experts also placed small red flags to help monitor the development of sinkholes in Duluth as we illustrate here.

Significant costs will be involved in repairing or replacing seawalls damaged by these forces. Possibly of aid are special products such as steps that break wave actions against the seawall (Nolan 2000).

Flags mark subsidence or sinkholes along the William A Irvin in Duluth MN (C) Daniel Friedman

Below, the huge length of the retired lake freighter occupies a stunning length of dock footage in Duluth. Numerous low spots, wet areas and actual sinkholes were visible in the fenced-off area along the dock.

William A Irvin retired lake Superior frighter museum in Duluth, Minnesota (C) Daniel Friedman

While lake Superior is not tidal its water levels do vary over time but probably more significant as a force in sea-wall wear and resultant soil loss from behind the seawall are the combined forces of wave action and ship traffic or even the agitation of water between a permanently docked steamer William A. Irvin and the seawall (below).

Area of water activity betweek docked freighter and the Duluth Minnesota seawall (C) Daniel Friedman"Sinkholes" that are not verified by a licensed professional geologist or geotechnical engineer to be a true sinkhole are also described in various publications as subsidence incidents.

Readers should see SINKHOLES - IMMEDIATE SAFETY ACTIONS, and also see FOUNDATION CRACKS & DAMAGE GUIDE and CESSPOOL SAFETY WARNINGS.

Additional septic system safety warnings are at SEPTIC & CESSPOOL SAFETY.

The bare minimum that a property owner needs to know about sinkholes or any other sudden subsidence of soils at a property is that these conditions might be very dangerous.

Someone falling into a sink hole or into a collapsing septic tank could be seriously injured or even die.

If a suspicious hole, subsidence, or depression appears at a property the owner should rope off and prevent access to the area to prevent anyone from falling into the opening, and then should seek prompt assistance from a qualified expert, geotechnical engineer, septic contractor, excavator, or the like.

Minnesota sinkhole research

Seawall & Bulkhead Resources


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