Slope Failure at Millsaps College, Jackson, Mississippi: There Are No Simple Jobs

Design factors contributing to the failure of a 100 m long engineered block retaining wall include the use of low permeability backfill material, inadequate drainage design, and failure to observe existing natural conditions. A slope composed of Pre-Loess Terrace deposits was excavated and replaced behind a 2 m high retaining wall. The base of the wall was situated at the contact between the Pre-Loess Terrace and the Yazoo Clay, a highly plastic smectite clay. Although deficit spring precipitation masked the direct hydraulic properties of the Terrace deposits during construction, two important indicators of the slope's hydrology were ignored. The base of the slope was lined with well established hydrophytic vegetation and a previously formed slump was still evident on the eastern margin of the proposed wall's footprint. The Pre-Loess Terrace generally exhibits K values from 10^-4 to 10^-5 cm/sec. The excavated and remixed Terrace backfill was essentially impermeable and prevented natural drainage of the elevated Terrace deposits. By late fall, the cumulative annual precipitation had returned to the average value and a slight deflection of the wall was observed. In mid-December 2007, slope failure occurred which resulted in an initial scarp of approximately 1 m and horizontal wall displacement at the toe of 2 m. Continued movement over the next two months resulted in 3.4 m deflection of the wall at the toe and 2.3 m of vertical displacement at the head. A slope indicator survey indicated a failure surface coincident with the base of the low permeability backfill. As the wall was displaced it scraped Yazoo Clay along the leading edge resulting in a soil roll up to 1.4 m in height.