Problematic Slope Stability Units in Colorado: Morrison, Mancos, and Pierre Formations

Three of the most problematic units in Colorado for slope stability include the Jurassic Morrison and Cretaceous Mancos and Pierre Formations, which represent slightly less than 10% of the surface and near-surface rocks in the state. All three units are dominantly shale in composition, yet show differences in types of interbeds, mineralogical makeup, and resulting engineering properties. Because of variable properties between thin interbeds, critical properties vary widely within the formations: slake durability ranges from near zero to near 100, compressive strength ranges over an order of magnitude, and friction angles range from 11 to over 25 degrees. This inclusion of extremely weak layers is one contributing element to the vulnerability of these formations, with a second being the high strengths of the flanking formations, which are typically sandstones. As a result, there are numerous cases of significant landslides in these three formations and they match closely with maps of landslide potential that cover large areas. The formations also show widespread small-scale failures, affecting mostly highways and individual structures. Furthermore, they are involved in 25% of the identified landslides affecting large dams in Colorado, a number which increases to 45% if rock avalanches and basaltic blockslides on Grand Mesa are removed from the database.