Engineering geology study of the instability of the South Bosque Shale and the Del Rio Clay in the Waco area

The fundamental cause of slope instability in the Waco area is the response of the local strata to natural and man-made disturbances. Most slope failures occur during or shortly after periods of heavy rainfall, where natural or artificial slopes are oversteepened, overloaded, or excessively high. Thus, in general, disturbance of natural slopes and improper design of man-made cuts constitute the immediate causes of instability. Certain aspects of the local geology exert a decisive influence on the unstable behavior of the South Bosque Shale and Del Rio Clay in the Waco area. Their particular failure style is largely dictated by stratigraphic position and specific lithologic characteristics. The South Bosque Shale fails because the top load tends to force the shale outward from the slope's face. The highly expansive characteristics of the loaded, wet shale contribute to the process, as the shale is able to more freely expand in this direction. Failure typically assumes the form of mass flow with no single, well-defined, cylindrical slip surface. The chalk caprock prevents the opening of tension cracks on top of the slopes and inhibits the initiation of a single slip surface at this end. Under these conditions, it is easier for the shale to deform along a system of partial slide surfaces, rather than to move along a single surface. The blocky character of the shale provides it with an already existing system of surfaces along which sliding can take place, thus facilitating flow. Failure of the overlying, highly fractured Austin Chalk caprock occurs as huge blocks are transported down the slope by the shale flow, or in the form of rock falls. These rock falls result from the failure of the underlying shale, or simply from the undermining of the caprock due to rapid erosion.