2D and 3D Back analysis of Forest City (South Dakota) landslide

Landslides are a common geologic feature in the Missouri River trench and along the valleys of Missouri River tributaries. These landslides are commonly found to develop in the Pierre Shale formation in this region. Pierre Shale is a weakly cemented marine clay shale developed in Cretaceous age by sediments from Epeiric Sea. This clay shale is well known for imposing engineering challenges in the form of slope instability and foundation difficulties because of its non-homogeneity and high plasticity. It is known as heavily overconsolidated shale which can fail due to minor disturbances. Based on the development of dams and transportation infrastructure in this area understanding the behavior of Pierre shale is extremely important to assess the stability of those structures. As the shale material in this area is already fissured and has the history of numerous landslides, its residual strength is considered over peak strength to efficiently represent its strength. This thesis investigates a possible range of the residual friction angle for the Pierre Shale. The Forest City landslide which occurred in the Missouri River trench is selected as a case study for this purpose. The residual friction angle values are evaluated by performing a deterministic back analysis of the slope in two and three dimensions. The deterministic two dimension analysis is performed in limit equilibrium and finite element methods using SLOPE/W and SIGMA/W softwares from GEOSTUDIO 2007. A deterministic three dimensional analysis is performed by using CLARA/W software. The values obtained from these analyses are compared and a reasonable value of 4° to 6.64° is selected to represent the residual friction angle values for the Pierre Shale.