Environmental Engineering Reference
In-Depth Information
12
Design of embankment dams to withstand earthquakes
12.1
EFFECT OF EARTHQUAKE ON EMBANKMENT DAMS
Earthquakes impose additional loads on to embankment dams over those experienced
under static conditions. The earthquake loading is of short duration, cyclic and involves
motion in the horizontal and vertical directions. Earthquakes can affect embankment
dams by causing any of the following:
- Settlement and cracking of the embankment, particularly near the crest of the dam;
- Instability of the upstream and downstream slopes of the dam;
-Reduction of freeboard due to settlement or instability which may, in the worst case,
result in overtopping of the dam;
- Differential movement between the embankment, abutments and spillway structures
leading to cracks;
- Internal erosion and piping which may develop in cracks;
- Liquefaction or loss of shear strength due to increase in pore pressures induced by the
earthquake in the embankment and its foundations;
- Differential movements on faults passing through the dam foundation;
- Overtopping of the dam in the event of large tectonic movement in the reservoir basin, by
seiches induced upstream;
- Overtopping of the dam by waves due to earthquake induced landslides into the reser-
voir from the valley sides;
- Damage to outlet works passing through the embankment leading to leakage and
potential piping erosion of the embankment.
The potential for such problems depend on:
- The seismicity of the area in which the dam is sited and the assessed design earthquake;
- Foundation materials and topographic conditions at the damsite;
- The type and detailed construction of the dam;
- The water level in the dam at the time of the earthquake.
The amount of site investigation, design, and additional construction measures (over
those needed for static conditions) will depend on these factors, the consequences of fail-
ure, and whether the dam is existing or new.
There are four main issues to consider:
- The general (or “defensive”) design of the dam, particularly the provision of filters, to
prevent or control internal erosion of the dam and the foundation, and provision of
zones with good drainage capacity (e.g. free draining rockfill);
- The stability of the embankment during and immediately after the earthquake;
- Deformations induced by the earthquake (settlement, cracking) and dam freeboard;
- The potential for liquefaction of saturated sandy and silty soils and some gravels with
a sand and silt matrix in the foundation, and possibly in the embankment, and how this
affects stability and deformations during and immediately after the earthquake.
