Uncoupled Limit Equilibrium Analysis of Embedded Walls

Embedded foundation elements such as diaphragm walls, drilled shaft walls, and heavy gauge sheet pile walls are often used to stabilize soil slopes and to create retaining walls for grade separation. These walls are typically analyzed for global stability using limit equilibrium analysis methods where the slip surfaces pass through the wall below grade. This type of analysis assumes that the slip surface is continuous and shears through the wall. However, the failure mode for these heavy foundation elements is not shearing through the wall but instead is a deformation failure as the foundation elements are pushed into the soil mass downslope in a passive failure mode. The author presents a modified limit equilibrium analysis approach where the slip surface is uncoupled at the wall instead of being modeled as a continuous slip surface through the wall. This method includes a search for the critical slip surface beginning upslope and terminating along the vertical wall below grade. The slip surface downslope of the wall is replaced with a line load acting upslope. The line load represents the resultant of an allowable passive pressure, less than the full passive pressure to limit deformation to an acceptable magnitude. A comparative example of an embedded wall for grade separation is presented. The uncoupled analysis is more realistic and often results in a lower calculated global factor of safety than a limit equilibrium analysis assuming a continuous slip surface through the wall.