Landslide Investigated

An earth slump landslide occurred within materials on a bench of limestone bedrock in Grafton, Illinois. An ISGS engineering geologist responded to a request to evaluate the setting for the Illinois Emergency Management Agency (IEMA), since the Grafton mayor closed State Route 100 until the threat of potential landslide onto the road was assessed. Material from high on the slope flowed down the slope, as did the thin soil on the bedrock face below the bench. Part of the lower part of the slide pressed up against the back of a house. The geologist supplied information to the city engineering firm and IEMA regarding the setting, diagrams, and borehole information and provided a list of possible mitigation issues to investigate.

The engineering geologist also provided pictures, locations, and descriptions of Illinois landslides to the U.S. Geological Survey Landslide Centerafter their staff saw press coverage of the Grafton landslide. Among the described images was a rock fall event along the Mississippi river north of Savanna and historic pictures of rock slumps in the same area in the 1920s. Also, landslides that damaged houses west of Peoria, Illinois, were provided.

While mapping in Calhoun County, Illinois, for the STATEMAP program, two ISGS geologists visited a landslide located on a property just below Tara Point Inn in Grafton, Illinois. A scarp developed in the Hannibal Shale, which is about 50 to 60 feet thick in this area of the Grafton Quadrangle and overlies Silurian age dolomite. A perched groundwater table, due to a large amount of rainfall and a small pond, contributed to the slide. Jointing of the carbonate rocks above the Hannibal Shale yielded a lot of water, which further exaggerated the slump. The Hannibal Shale can turn into a greenish claystone and become very unstable.