SLOPES OF EMBANKMENTS AND EARTH DAMS
Embankments are constructed by placing and compacting successive layers of a fill material onto a foundation soil. Construction causes the total stress in the embankment layers themselves, and also in the foundation soil, to increase. The initial pore water pressure (uo) depends primarily on the placement water content of the fill.
At the end of construction of an embankment, the Factor of Safety is lower than in the long term. This is because water dissipates after the end of construction, with the pore water pressure decreasing to the final, long term value. This assumes the permeability of the compacted fill layers is low, so not much dissipation takes place during construction. The construction period is usually quite short.
Stability may also depend on the shear strength of the foundation soil.
Embankment (Earth) Dams
(see also Andrew Graham's site on Dam Design)
These are used where concrete dams are unsuitable due to abutment or foundation conditions. Suitable materials for the fill need to be accessible and close at hand.
A thorough Ground Investigation is vital before planning to construct an earth dam. Importance must be placed on examing the "cut and fill" areas - we need to know the quality and quantitty of the material available.
The structure of these dams is not homogeneous, but "zoned", having a low permeability soil core, with shoulders of other material on each side. There is usually a layer of "rip-rap" (rockfill), on the upstream slope surface to prevent wave damage. Usually an internal drainage system is included in the dam.
Failure of the dam could arise from instability of either the upstream or downstream slope, so it is important that careful design of the slope angles is undertaken. Over-conservative design is undesirable as it results in excess material being used, thus raising expense.
Failure may also arise from internal erosion or erosion of the crest and the downstream slope from overtopping.
The Factors of Safety to be designed for are:-
(1) The most critical stage for the upstream slope; at the end of construction and during rapid drawdown of the level in the reservoir.
(2) The most critical stage for the downstream slope; at the end of construction and during steady seepage when the resevoir is full.
Often there will be a Piezometer system installed for the purpose of measuring the actual pore water pressures and comparing them with those used in design. These values are influential in the Factors of Safety of the slopes.
For reasons discussed earlier (relating to porewater pressures), the majority of slope failures in embankment dams take place either during, or just at the end of, construction.
An "effective stress analysis" method is most suitable here. A "total stress" method would result in over-conservative design, because the construction period although short, will probably allow some dissipation of water.