Embankment Dam Slope Stability 101
URS Corp., November, 2013
Embankment Dam Slope Stability 101” provides an overview of embankment dam slope stability with an emphasis on practical evaluation methods for small to medium dams, particularly those with limited subsurface data. Slope stability is defined as the ability of an embankment to resist downslope movement caused by gravity, changes in loading, seepage, or material loss. Common factors that can adversely affect stability include rapid changes in reservoir level, elevated internal pore pressures, erosion, and excessive loading on the crest or slopes.
The article highlights the importance of regular visual inspections as the primary tool for monitoring embankment performance, especially for low-hazard dams where formal stability analyses may be waived. Visual indicators of developing instability include longitudinal cracking, wet areas or seepage on the downstream slope or toe, slope slumps or failures, erosion and over-steepening, displaced features such as riprap or fences, toe bulging, sinkholes, and changes in the upstream waterline. Repeated inspections using benchmarking photographs are emphasized as an effective means of identifying subtle changes over time.
Triggers for conducting more detailed, quantitative slope stability analyses include new dam design, dam raises, berm construction, hazard reclassification, post-flood condition changes, deterioration of embankment slopes, or field observations suggesting potential instability. Stability analyses evaluate a factor of safety, defined as the ratio of resisting forces to driving forces, with minimum acceptable values varying by loading condition. Typical minimum factors of safety include approximately 1.5 for steady-state conditions, 1.3 for end-of-construction, and 1.2 for rapid drawdown or post-seismic conditions.
The article outlines the basic components of a slope stability analysis, including selection of the critical cross section, estimation of the phreatic surface, and assignment of material properties for embankment and foundation soils. A key consideration is whether soils behave in a drained or undrained manner under a given loading condition, which depends on permeability, loading rate, and drainage path length. Guidance is provided for evaluating drainage behavior using consolidation parameters or hydraulic conductivity when detailed data are unavailable. Typical loading conditions addressed include steady-state seepage, end-of-construction, rapid drawdown, and seismic loading. Each condition requires appropriate assumptions regarding pore pressures and shear strength parameters. The article stresses that slope stability analyses are comparative tools used to evaluate relative performance and risk, rather than absolute guarantees of safety. When factors of safety are low or uncertain, additional field investigations and laboratory testing are recommended.
This article was published in Vol. 1, Iss. 3 (November 2013) of Western Dam Engineering News and provides a general overview of embankment dam slope stability evaluation, with particular emphasis on visual inspection, screening-level assessments, and common loading conditions. Western Dam Engineering News is a collaborative effort between the States of Colorado, Montana, New Mexico, and Wyoming and is funded by the FEMA National Dam Safety Act Assistance to States grant program. Special thanks to URS Corporation in Denver, Colorado for authoring the article.
Revision ID: 8326
Revision Date: 02/02/2026