Slope Failure of Embankment Dam under Extreme Flooding Conditions: Comparison of Limit Equilibrium and Continuum Models

During the spring of 2003, the downstream slope of an embankment dam began to move during an extreme flooding event. The current study was implemented to evaluate the slope stability of the embankment and as a case-study to compare the limit equilibrium and continuum modeling methods. Initially, stability analyses were performed using conservative shear strength of soils estimated from both boring log data and an infinite slope back analysis. The Mohr-Coulomb failure criterion was employed in the computer simulations. The computer program UTEXAS4 and Spencer method of solution was used to evaluate the limit equilibrium factors of safety and the critical failure surface. The computer program FLAC4.0 was used to evaluate the stress/strain behavior of the slope and determine factors of safety using the strength reduction method. Both these limit equilibrium and continuum models yielded slip surfaces that were close in geometry and location to the actual slip surface observed in the field. The factor of safety for the pre-failure embankment geometry was lower utilizing the limit equilibrium approach than that estimated by the continuum approach; however, the continuum approach appeared to yield more accurate results when using the best estimate of the shear strength. Back-analyses performed based on the observed failure surface in the field after flooding event provided estimates of shear strength of the embankment soil. The back calculated shear strength utilizing the continuum model fell within acceptable ranges as determined from the boring log data, with the continuum model yielding the more conservative (lower) back calculated shear strength. Back calculated shear strength using the limit equilibrium method was higher and its use may result in unconservative designs.