Characterization of the 2020 US-231 landslide near Lacey Springs, Alabama and the importance of nonlinear strength envelopes

Abstract A large landslide occurred at milepost 35.9 on US-231 near Lacey Springs, Alabama on February 13, 2020. This 300-m section of highway was constructed in an area with a history of movement dating back to before the current highway was built. As a result of the landslide, US-231, a heavily used corridor for Huntsville commuters, was closed in both directions for more than 7 months, leading to significant detours. Subsurface characterization of the landslide was performed using borings and electrical resistivity tomography surveys. The subsurface data and inclinometers installed before the failure indicated that the movement likely occurred in a weathered shale layer. Index testing, X-ray diffraction, and ring shear tests were conducted on this layer, which was identified as a degraded illite. Similar soils have caused stability problems in other regions of the USA and the world. The ring shear results were used to fit a nonlinear residual strength envelope, which was used in limit equilibrium analyses of a representative cross-section of the landslide. The limit equilibrium results demonstrate that the site had marginal stability (factor of safety near 1.0) for lower groundwater levels, but the site would be expected to be unstable when the groundwater level rose, as it did in February 2020. The measured nonlinear strength envelope for the degraded illite is consistent with strengths measured on similar soils in previous studies but shows more curvature than existing residual strength correlations. The results of this study demonstrate the importance of considering this nonlinear residual strength envelope when analyzing potential failures in deposits with degraded illite.