BACK ANALYSIS OF OLMSTED LANDSLIDE USING ANISOTROPIC STRENGTHS

An analytic model of existing landslides on the Illinois shore of the Ohio River at the proposed Olmsted Locks and Dam site has been developed. This model is based on field and laboratory investigations that characterize the geometry, material distribution, and groundwater levels at the existing slides. Back analyses of landslide stability were used to determine the soil strengths, which were necessary to complete the model. Trial residual friction angles for the back analyses were based on the results of laboratory shear tests and on a correlation with the plasticity index. However, because of difficulties in performing the laboratory shear tests, greater reliance was placed on the correlation with the plasticity index. One of the most important materials involved in the landslides is the McNairy I formation, which consists of interbedded layers of clay, silt, and sand. The McNairy I is inherently anisotropic, and strength variations with inclination of the failure surface were incorporated in the back analyses.