BEDROCK COLLAPSE IN ST. PETER SANDSTONE

Bedrock collapse is a low-probability, high consequence risk in Hennepin County MN. Most parts of the county are covered with thick (more than 30m) layer of glacio-fluvial sediment that limit sinkhole forming processes that are found in the region. However, there are areas where this cover is thin or absent, and these areas happen to be the most developed parts of the county where bedrock collapse would have the most severe consequences. There are historical examples of bedrock collapse, where in 1869 and 1880 tunnel failures caused bedrock to collapse in Minneapolis, which disrupted the regional economy.

Sinkhole risk in Hennepin County is related to sandstone, not carbonate rocks. This is due to the occurrence of weathered rock that has been buried by Pleistocene glaciation, fractures, and the lack of cement in the sandstone itself. This project identified areas in the county where sandstone has less than 15m of sediment cover and is first bedrock. We have started to investigate these areas for bedrock collapse risk. We are using historical archives, construction records, well and boring records, technical publications, and limited site investigation in areas where bedrock has little cover to produce a hazard map similar to FEMA flood zone maps. Mapping is still in progress, but once complete it will allow Hennepin County and other stakeholders to take action to mitigate risk.

Our investigation has provided insight on the St. Peter Sandstone, noting that massive portions of the unit are better cemented than the sandstone with visible bedding. We traced these units within the St. Peter Sandstone along the Mississippi River bluffs ~20km from the southeastern tip of Ramsey County northwest to the University of Minnesota campus in Minneapolis. We propose that the bedded and cross-bedded zones in the St. Peter Sandstone are more prone to collapse than massive zones of this sandstone, and presents an opportunity for future research.