ENGINEERING PROBLEMS OF GYPSUM KARST ALONG THE INTERSTATE 90 DEVELOPMENT CORRIDOR IN THE BLACK HILLS, SOUTH DAKOTA

Approximately 80 km of Interstate 90 between Rapid City and Spearfish, South Dakota, is on outcrops of the Triassic Spearfish Formation. The Gypsum Spring Member is near the top of the Spearfish Formation and consists of up to 10 m of massive gypsum. This member has formation status in Wyoming and forms a prominent ridge that is continuous from the Wyoming-South Dakota border southeastward for more than 50 km along I-90. Other scattered gypsum layers are present throughout the 100-m to 200-m thickness of the Spearfish Formation in the Black Hills. Residential and industrial development is accelerating along the I-90 development corridor in the Black Hills, and much of the area is becoming urban or suburban. Although the white gypsum layers typically are prominent and recognizable, new housing and other construction often is built on or near unstable gypsum in the Spearfish Formation, including the Gypsum Spring Member. Because of increasing development pressure, it is critical that geologists and geological engineers participate in land-use decisions where karstic gypsum is involved. Current initiatives have proposed engineering hazards mapping for the entire I-90 development corridor in the Black Hills, in order to inform the public and local government about potential problems.