Date of Graduation
Statler College of Engineering and Mineral Resources
Civil and Environmental Engineering
Hema J Siriwardane
H Ilkin Bilgesu
Raj K Gondle
Udaya B Halabe
John D Quaranta
Modern computer capabilities enable complex slope stability problems to be analyzed using the finite element method (FEM), including three-dimensional slopes. This research presents the results of analyzing two- and three-dimensional, unreinforced and soil-nailed, reinforced slopes. Previously performed two-dimensional slopes were modeled as three-dimensional slopes in an effort to validate the modeling technique. The Shear Strength Reduction (SSR) Method was used throughout this study to determine the Factor of Safety (FOS). Both two- and three-dimensional FEM models compared well with conventional, two-dimensional Limit-Equilibrium (L-E) results. Overall, results show that the FEM is an extremely diverse and robust alternative to conventional, L-E slope stability analyses, especially when complex site geometries or conditions exist.;When modeling two-dimensional, unreinforced soil slopes using FEM, the most efficient and accurate element type that provides an acceptable failure mechanism is the CPE4 (4-noded bilinear quadrilateral) element in conjunction with either the Mohr-Coulomb or Drucker-Prager soil failure yielding criteria. When modeling three-dimensional, unreinforced soil slopes using FEM, the most efficient and accurate element type that provides an acceptable failure mechanism is the C3D8 (8-noded linear brick) element in conjunction with the Mohr-Coulomb soil failure yielding criteria. Although the Drucker-Prager soil yielding criteria assumptions seem to offer more potential for three-dimensional applications, this study found no significant benefit from its use.;For unreinforced slopes, three-dimensional, unit-width FEM models provide identical results to FEM slope models with depth, when end conditions are not considered. For soil-nailed reinforced slopes, three-dimensional, unit-width FEM models yield FOS values marginally higher than two-dimensional FEM and L-E models for all slope angles. Unit-width FEM models provide designers with a valuable tool for performing parametric studies and preliminary design. Three-dimensional FEM models of soil-nailed reinforced slopes can be used to effectively determine the soil nail orientation that yields the highest FOS, also known as the optimum soil nail orientation. For slopes with a level backfill, the optimum soil nail orientation (in degrees measured downward from horizontal) can be first approximated using the equation 58°- 0.6beta, where beta is the slope angle in degrees. Three-dimensional FEM models can also be used to effectively determine the most efficient soil nail length. For slope heights of about 10 meters, the most efficient soil nail length can be first approximated using a soil nail length to slope height ratio equal to 1.0. A maximum vertical soil nail spacing of 2.4 meters and 1.9 meters is recommended for soil-nailed slopes with slope angles less than or equal to 60° and greater than 60°, respectively.;A maximum horizontal soil nail spacing less than 1.9 meters is recommended for soil-nailed slopes for all slope angles. When performing three-dimensional FEM modeling of reinforced slopes, the FOS value is relatively insensitive to a soil's Elastic Modulus, Unit Weight, and Poisson's Ratio value. The most significant soil parameters are the Angle of Friction and the Cohesion. Further, the results of this study indicate that regardless of the soil type, there is no significant increase in the FOS of soil-nailed reinforced slopes with slope angles less than 80°.;Three-dimensional FEM models can be used effectively to evaluate unreinforced and soil-nailed reinforced slopes that have a surcharge load and can be used to estimate slope deformation, even prior to failure. In addition, pre-tensioned soil nails can be modeled using an FEM approach. Fully modeled, three-dimensional FEM slope stability analyses yield FOS values that are slightly lower than either traditional two-dimensional L-E models or three-dimensional FEM unit-width models that use a "worst case" section technique. Overall, three-dimensional FEM slope stability modeling is superior to other methods due to its capabilities and versatility.
Stauffer, Kerry D., "Three-Dimensional Stability Analyses of Soil-Nailed Slopes by Finite Element Method" (2015). Graduate Theses, Dissertations, and Problem Reports. 6719.