Rumors that a sinkhole might have contributed to the collapse of a structural pillar at Red Ginger in downtown Traverse City last Friday night appear to be unfounded, according to city officials who surveyed the building.

But the rumors stemmed from a reality that does plague the alley running behind the restaurant on the 200 block of Front Street: a decades-long problem of recurring sinkholes, or "differential settlements," that are now prompting city staff to renew an investigation of their underlying causes.

In the last month, the city's streets department fixed two sinkholes in the 200 Front Street alley: one close to Cass Street on May 7, and one just outside the access drive to 101 North Park on May 28. A current puddle forming in the alley near 101 North Park is an early indicator a third sinkhole might be developing in the area.

City Engineer Tim Lodge, who performed a quick assessment of Red Ginger Friday immediately following the accident and found “no evidence” to suggest a connection between the pillar collapse and the recently repaired sites, emphasizes a distinction between the technical term “sinkhole” and its sensationalized association in national news with mammoth holes that swallow cars or buildings.

“These differential settlements...are usually about the size of potholes,” Lodge says. “They've never translated to private property damage downtown. It's a low probability they ever will...but we need to do our due diligence.”

A 2010 report from Gourdie-Fraser, Inc. commissioned by the city concluded that “differential settlement that is occurring in the alley is likely the result of poor initial backfill and aging underground utilities.” To address the issue, the city undertook a major improvements project in 2012 to fix all of the alley's underground pipes, sanitary sewer leads, storm sewer structures and power conduits, anticipating the repairs would eliminate the alley's sinkhole problem. After the projected was completed, however, the settlements continued to appear.

“That made us realize we have to take a different approach,” says Lodge. “Our next step is to look at the relationship between the retaining wall and the (Boardman) river.”

Lodge theorizes the moving water in the Boardman is resulting in a "scour phenomenon" in which river action displaces soil, soil behind it moves in to take its place and voids are created behind the retaining wall. In the early 1980s, a section of the wall – originally constructed in the 1940s – failed and collapsed into the Boardman River near Cass Street.

The wall was shored up with steel sheet piling, and has not failed since. But Lodge would like to hire a professional diver to evaluate the wall at its depth and its interface with the river – particularly since the settlement phenomenon has also recurred in the 100 block of Front Street.

“At this point, doing nothing is not an option,” says Lodge, who says the danger posed to surrounding buildings in the area is “very small...but there's always a risk.” He adds: “First we need to identify what the need is, then costs, then funding sources. We need to take it through the process. It will likely have to go through both the city and the DDA (Downtown Development Authority).”

While a professional assessment team is still working at Red Ginger to determine the cause of Friday's pillar collapse, it's possible one commonality could emerge between the two investigations: a need to evaluate how the city's aging infrastructure interacts with the newer development surrounding it.

“Is the structural integrity of a building three stories tall dependent on the soil underneath it?” says Lodge. “Absolutely. But do property owners along that stretch have foundation issues? I can't make that leap. All we know for sure is that there is a recurring settlement issue in the alley, and we believe the risk it poses is minimal. But continuing (to investigate it) is our most prudent option.”