interaction effects can be visualized as an added mass, an added
damping, and an added force. Such added hydrodynamic terms can significantly
affect dynamic response of concrete dams to earthquake loading.
The added hydrodynamic mass tends to lengthen resonant periods of
the dam. The added force presents additional seismic inertia load
acting on the dam. The added damping term arises from the transmission
(absorption or loss) of energy at the reservoir boundaries and from
propagation of pressure waves in the upstream direction. The energy
loss or transmission at the reservoir boundaries is usually approximated
by a wave reflection coefficient known as "alpha". An
alpha=1 corresponds to a total reflection (i.e. no transmission)
and alpha=0 represents total transmission into the boundary materials.
If the reservoir boundary materials are relatively soft, an important
fraction of the reservoir water energy can be transmitted, leading
to a reduction in hydrodynamic pressures and thus dynamic response
of the dam.
Structures has developed and applied three different geophysical
techniques to measure in-situ values of the reservoir-bottom reflection
coefficient (alpha). These include seismic reflection, seismic refraction,
and acoustic reverberation techniques that have been employed at
two dams in China and seven dams in the United States.
Army Corps of Engineers and US Bureau of Reclamation Sponsored Projects
- Dongjiang Dam, China
- Longyangxia Dam, China
- Monticello Dam, California
- Pine Flat Dam, California
- Hoover Dam, Nevada-Arizona
- Glen Canyon Dam, Nevada
- Morrow Point Dam, Colorado
- Crystal Dam, Colorado
- Folsom Dam, California
and refraction at a plane interface