CS12B - Concurrent Session 12B: Evaluating Slope Stability of a Reclamation Dam using Three-Dimensional Limit Equilibrium Analysis

Three-dimensional (3D) effects can have significant impacts on the stability of earthen embankment dams. These effects can be dramatic for dams in narrow valleys, dams where geometry or material properties vary over short distances, embankment-abutment contacts areas, and sites with very weak layers. 3D effects are generally neglected in the traditional two-dimensional (2D) slope stability analyses. Two-dimensional analysis assumes a plane-strain condition that is not representative of most embankments. However, most assessments are still done using 2D methods, with 3D effects generally only considered qualitatively. While the conservative estimates of factor of safety (FS) produced by 2D analyses are generally considered acceptable in practice, overestimating the potential for slope instability can lead to overdesign and increased costs when designing new dams or modifications to existing dams. For this reason, assessing 3D effects is important when estimating the risk of dam instability and the need for dam modifications. To demonstrate the influence of 3D effects on stability, a case study is presented in which slope stability of a Reclamation Dam was evaluated using both 2D and 3D methods. Materials in both the foundation and dam embankment have been found to be susceptible to significant strength loss during earthquakes. As a result, post-seismic stability of the dam is a concern. 2D post-seismic stability analyses performed using limit equilibrium methods (LEM) and finite difference methods (FDM) indicated significant potential for post-seismic instability at the dam (FS < 1). Because the dam is located in a narrow canyon, and it was considered worthwhile to assess potential 3D effects on stability of the dam. To this end, 3D LEM analyses were also conducted. Results were compared to results from the 2D LEM and FDM analyses to evaluate 3D effects on slope stability of the dam. The FS from 3D LEM were significantly larger than those from 2D analysis, with increases as high as 100 percent compared to FS at the critical 2D section. In contrast to the 2D analysis where FS for some cases were much lower than 1, FS from 3D LE analysis were all greater than 1, indicating post-seismic stability would likely be acceptable. As a result, while 2D analyses suggest potential risk of post-seismic stability at the dam, incorporating 3D analyses into evaluation of this dam could lead to smaller estimates of post-seismic risk. This analysis demonstrates the importance of considering 3D effects when evaluating slope stability of earthen embankments.