ENGINEERING RECORDS AND CUES FROM THE BUILT ENVIRONMENT AS KARST CIPHERS

Colloquially, the term “sinkhole” covers a wide range of phenomena, ranging from potholes in the street to meteorite craters. Geologically, the term refers to collapse or solution features, most commonly in limestone but also other rock types, such as sandstone, that result from natural or human undermining. Increased precipitation intensity, green practices that infiltrate stormwater, and changes to local base level from dam removal have the potential to increase occurrences of undermining, raising sinkhole risk especially in the built environment. In addition to being a direct threat to life, sinkholes can damage property, disrupt water and sewer infrastructure, obstruct roadways, and serve as conduits for contamination.

Freshwater published a sinkhole risk assessment study for Hennepin County MN in 2021. We (the authors) prioritized shallow bedrock areas near bedrock valleys, because they are more susceptible to sinkhole formation typical of the risks in downtown Minneapolis. We introduced a “new” but inexpensive source of information for investigating urban sinkholes, namely, existing sanitary sewer and storm drain engineering records. These records display geological details in profile view, which can be interpreted by geologists. The records date to the late 19th century and some are illustrated in color.

Our team discovered more than a dozen previously unidentified sinkhole candidates in Minneapolis. Further, we were able to use these records to show that some depressions were not sinkholes. We were able to determine that certain engineering and land-use practices were associated with unidentified geological hazards, including sinkholes. We were able to use these insights to provide Minneapolis with geological guidance to conform to State stormwater regulations dealing with karst. Engineers currently evaluating the removal of the St. Anthony Falls lock and dam are also reviewing these maps because lowering river level has the potential to reactivate karst.