Case History of Two Landslides in Lithostratigraphic Equivalent Formations

The Upper Cretaceous Mancos Shale is found in the Colorado Plateau physiographic province west of the Rocky Mountains. The Upper Cretaceous Pierre Shale, an approximate lithostratigraphic equivalent to the Mancos Shale, is found in the eastern Front Range of Colorado and extends into the Great Plains physiographic province. Both formations are composed of a thick succession of marine sediments, and are susceptible to sliding on moderate to steep slopes. Geotechnical investigations were conducted for two landslides: one failure in the Mancos Shale on the Western Slope of the Rocky Mountains in Grand Junction, Colorado; and one failure in the Pierre Shale on the eastern Front Range in Colorado Springs, Colorado. Geotechnical investigations included subsurface exploration, installation and monitoring of geotechnical instrumentation, geological site reconnaissance, aerial photograph review, geotechnical laboratory testing, and slope stability analyses. Due to the large amount of strain experienced by the shale in each failure, the residual shear strength of the bedrock and slide materials were used in slope stability modeling of the failures. Values of residua