Waco Dam Slide

A slide that occurred during the construction of Waco Dam embodied two unusual aspects: marked progressive shear and widespread distribution of pore pressure in the clay shale foundation material. The clay shale had been prestressed in shear as a result of movement on a pair of faults, and subsequent erosion permitted expansion and reduction in resistance to shear. The presence of a thin, relatively pervious layer permitted the pore water pressures to be distributed far beyond the limits of the embankment. The shearing resistance of the clay shale was determined by means of laboratory tests performed under various conditions of loading and rates of strain. Direct shear tests on specimens having a pre-cut shearing surface indicated a strength approximating that determined from an analysis of the slide. A relationship between height of fill and pore pressure was developed, and the pore pressure distribution for the design height of embankment was estimated by extrapolation. Berms were designed to provide a post-construction safety factor of 1.2, and the embankment was successfully completed.