ASDSO Dam Safety Toolbox

Static Deformation

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"Embankment deformations under static loading occur as a result of volumetric changes, lateral spreading, or shear displacements within the embankment and foundation materials. Volumetric changes are due to either an increase in the normal stresses on a soil element causing a decrease in void volume or dilation of soil elements undergoing shear. Lateral spreading and shear displacements are due to squeezing, distorting, and localized shear failures of material elements as the materials adjust to the stress conditions imposed by constructing the embankment and operating the reservoir. The rate at which these deformations occur depends on the dissipation rate of excess pore pressures and the rate at which steady-state seepage conditions develop."[1]

"The major effects of deformations are loss of freeboard, damage to appurtenant structures located within or upon the dam, loss of confidence in the dam due to swayback appearance, cracking of the embankment (most detrimental to the impervious core), development of localized zones susceptible to hydraulic fracturing, and failure of instrumentation. The effects of deformation can usually be mitigated by designing features based on experience gained from studying historical performance of existing dams without the need for performing any elaborate analyses. For most situations, simple “rules of thumb” and/or basic settlement calculations to determine the amount of over-build or camber to place on top of a dam and settlement estimates for appurtenant structures yields satisfactory results. Detailed attention to embankment zoning and foundation shaping can minimize differential settlements, thereby reducing the potential for cracking of the core or development of zones susceptible to hydraulic fracturing. For any large or hazardous dam, the designer should assume some cracking of the core is inevitable, and filters and drains must be incorporated into the design to control seepage and prevent movement of material. The determining factors for performing additional analyses lie in the potential for cost savings when the “rule of thumb” and/or simple settlement calculation approach suggests excessive design requirements."[1]

"The degree of analysis performed on an embankment is highly dependent on the design detail under consideration. For camber design, it is only necessary to estimate the amount of vertical settlement of the embankment crest. Often this estimate can be performed by applying simple guidelines that have been developed from observations of existing embankments. When cracking of the impervious core is of major concern or particularly compressible embankment or foundation materials are present, it is normal practice to perform some basic settlement calculations in order to decide whether a more complex analytical study needs to be performed or whether to simply incorporate more defensive design features. It is desirable to locate appurtenant structures or, for that matter, any structure off of the embankment. When possible, spillways and outlet works should be located through or over abutments or reservoir rim. If structures must be located on the embankment, settlement calculations are necessary."[1]

Best Practices Resources

Design Standards No. 13: Embankment Dams (Ch. 9: Static Deformation Analysis), USBR


Revision ID: 7077
Revision Date: 07/11/2023