GEOLOGIC IMPACTS OF LANDSLIDE STABILIZATION IN SOUTH DAKATO

An Engineering Geology Analysis of the Forest City Bridge Landslide in Pierre Shale, Lake Oahe, South Dakota Martin Patrick McDermott, P.G. & Roy E. Hunt, P.E., P.G.

The South Dakota Department of Transportation (SDDOT) contracted for a multi-task investigation to analyze the Forest City Bridge and the active landsliding conditions impacting the bridge and approach roadway. The purpose of the investigation was to determine if the landslide could be feasibly stabilized and the present bridge crossing maintained; or, relocate the bridge crossing at an alternate site. Bridge construction was completed in 1958 to provide a new crossing for Route 212 over the impoundment waters behind the Oahe Dam. The dam was completed at about the same time some 50 miles downstream of the site near Pierre, South Dakota. An engineering geology and geotechnical investigation of the Forest City Landslide has been conducted to formulate a remediation scheme to stabilize the massive landslide adversely impacting the 4600-feet long bridge structure. Landslide activity has also deteriorated the mile long approach roadway to the bridge, especially the 40-foot high southern approach embankment. Landslide movement has been detected along a deep common slip plane for the overall slide and also local shallow failures are associated with the approach embankment fill. Geologic impacts of the slide will be presented.