Engineering geology of the slope instability of two overconsolidated north-central Texas shales

The marine shales of the Eagle Ford and Washita Groups of the Cretaceous System of north-central Texas offer an excellent opportunity for the study of factors influencing development of different landslide types. These shales are exposed within metropolitan areas such as Dallas and Waco and have earned a reputation of being problematic. Although the shales of both groups have similar histories of overconsolidation and postdepositional changes, as indicated by their engineering properties, they can fail along slopes in a noticeably different manner. Types of failures may range from the classic rotational slump to the slow earthflow. Landslide genesis and morphology are found to be largely dictated by locally dominant geologic features, such as stratigraphic position, topographic expression, and specific lithologic and morphologic characteristics. The fundamental causes that dictate the instability of the shales are directly related to these geologic features, as well as to their engineering properties and to the uncontrolled modification of natural slopes.