Slough slides in embankments with high plasticity clays

A slough slide generally occurs in compacted embankment slopes consisting of high plasticity clays (CH). A typical slough slide occurs entirely within the slope and has a slide plane 4 ft to 7 ft below the surface. A slough slide does not generally provide an immediate threat to the safety of a dam or levee. However, a slough will progressively enlarge if not repaired in a timely manner and can eventually endanger the dam or levee. Slough slides in compacted high plasticity clay embankment slopes have long been recognized as a serious maintenance problem for dams, levees and roadway embankments. Fleming et al. (1) reported that the U.S. Army Engineer District, Vicksburg, repaired 200 to 225 slough slides in mainline Mississippi River levee slopes between 1964 and 1982 and as many as 41 slough slides during a single year. McCook and Shanklin (2) reported that, over a 25-year period, the Natural Resources Conservation Service (NRCS) repaired slough slides at about 35 dam sites in Texas, six in Alabama, one in Georgia and two in Mississippi. Other agencies that have reported an extensive number of slough slides include the Texas Department of Transportation and the Louisiana Department of Transportation and Development, Perry. A significant amount of research has been performed on the development of slides in high plasticity clays over the past 50 years or more. Much of the earlier research was performed in connection with slope instabilities in natural or cut slopes of heavily overconsolidated clays. Terzaghi was one of the first to recognize that the slope instabilities in high plasticity, stiff fissured clays occur at shear stress levels considerably below the peak strength. 15 pp., 18 references, 2 tables, 6 figures.