The theory of plate tectonics explains that the Earth's crust is broken into many plates. The plates are in continuous contact with each other and move as a whole. They move slowly over the Earth's surface and are propelled by heat generated inside of the Earth. Plates meet along fault zones. Earthquakes are caused by the sudden release of energy from the movement of the Earth's crust mostly along fault zones. Areas along the fault zones may become stuck or locked in place storing a tremendous amount of energy. When the area moves, the energy is released in the form of seismic vibrations causing ground shaking. The sudden release of energy can cause enormous amounts of damage to structures including earth embankment dams. Damage to a dam can cause the dam to fail releasing large amounts of water into a river valley. The result can be catastrophic including loss of life and property.
Most of the early dams were built with no consideration of earthquakes. For instance, dams built by the U.S. Army Corps of Engineers in the Louisville District before 1950 were not designed for earthquake forces because designers did not consider earthquakes probable threats (USACE 1990). As more information of earthquakes was collected, the need to built dams that could withstand earthquakes was recognized. Earth embankment dams may be damaged by earthquakes in several ways including dam movement, liquefaction of fill in a dam, water waves caused by an earthquake over topping a dam, and direct damage caused by a dam being located on a fault.
The dam or parts of the dam may move vertically, horizontally or a combination of both resulting in vertical cracks or settlement of the dam. The West Yellowstone Earthquake of 1959 damaged the Hegben Dam in Montana. The Hegben Dam is an earth embankment dam with a concrete core and the earthquake had a magnitude of 7.5. The earth embankment settled more then a meter and the concrete core cracked in four places. The reservoir floor was tilted but the dam embankment did not fail (Bonilla 1991).
In 2001, the Howard Hansen Dam located northwest of Enumclaw was damaged by the Washington Earthquake. The earthquake had a magnitude of 6.8. The damage consisted of cracks in the embankment and movement of instrument housing buildings on the dam (USGS 2001).
Seismic studies of the areas considered for dam construction increase the knowledge of potential problems that may be caused by earthquakes. For dams that are in place or planned to be built, modifications can be made to the structure to increase the structural stability including crack stopping zones, grouting, adding berms, and flattening of slopes (Babbitt and Verigin 1996).
Liquefaction of fill in the dam may occur. Liquefaction is the large drop in stiffness and strength of soil due to seismic movements (Byrne and Seid-Karbasi 2003). As a result, part of a dam may slump and slide off the structure. During the Santa Barbara Earthquake of 1925, the Sheffield Dam in California failed due to liquefaction apparently in the area just below the dam embankment containing silty sand. Little damage and no loss of life resulted. The earthquake had a magnitude of 6.3 and the dam was constructed in 1917 (Bonilla 1991).
The Lower San Fernando Dam located northwest of Los Angeles, California was damaged by the San Fernando Earthquake of 1971. The earthquake had a magnitude of 6.7. Liquefaction in the upstream side led to a major slide involving the core, the crest, and the upstream slope along nearly half the length of the dam. The dam did not fail because a cracked rim on the downstream side of the dam was higher then the water level in the lake. No loss of life resulted but 80,000 people were evacuated downstream of the dam. The dam was constructed between 1912 and 1915 with hydraulic fill methods and was enlarged in 1930. The older part of the dam included a clay core with outer zones of silty sand (Bonilla 1991). Hydraulic fill methods involve the mixing of fill soil with water, transporting the fill to the dam site by pipelines, depositing the fill and water on the embankment, and allowing the water to drain away. The fill is loose and is susceptible to liquefaction (VTU 2000). Because of the damage to the Lower San Fernando Dam, the California Department of Water Resources, Division of Safety of Dams analyzed over 100 dams. Of these, 60 dams have been physically modified, 19 have permanent storage restrictions, 36 have preliminary restrictions pending mitigation of deficiencies, and 4 have been removed (Babbitt and Verigin 1996).
The Los Angeles Dam in California was built 3,000 feet upstream of the Lower San Fernando Dam in 1976. The information from the damage to the Lower San Fernando Dam was used to design the Los Angeles Dam. In 1994, the Los Angeles Dam was damaged by the Northridge Earthquake. It had a magnitude of 6.6 that is comparable to the San Fernando Earthquake of 1971. The Los Angeles Dam only suffered minor deformation and superficial cracking (USGS 1995).
The recognization of materials susceptible to liquefaction and slumping would allow their removal before the construction of a dam would occur. During the design analysis of Patoka Dam in Indiana, loose fine sand in the foundation was found to be susceptible to liquefaction during shaking from a moderate earthquake. The revised design included removal of the valley fill and placement of the entire embankment on bedrock. The dam was built between 1972 and 1978 (USACE 1990).
Water waves caused by an earthquake may over top a dam. Water going over the top of a dam would result in erosion to the surface and could lead to failure of a dam. Water waves over topped the Hegben Dam in 1959 (Bonilla 1991).
All dams should be constructed with ample freeboard to withstand waves caused by earthquakes (USACE 1990). For dams that are already constructed, freeboard could be increased by adding to the embankment or by lowering the spillway (Babbitt and Verigin 1996).
The dam may be located on a fault and have direct damage caused by the movement of the plates. The Great San Francisco Earthquake of 1906 caused damage to the San Andreas Dam. The dam only incurred minor damage but the tunnel spillway that ran through the San Andreas Fault was ruined. The fault offset approximately 2 m in the locality of the dam during the earthquake. The dam was built from 1868 to 1870, before the fault was recognized (Bonilla 1991).
Increased knowledge of where faults are located now affect where and how dams are built. Building of dams across faults should be avoided. If the construction across a fault cannot be avoided, special measures need to be taken to ensure the structural stability of the dam. The Coyote Dam southeast of San Francisco was constructed across the Calaveras Fault in 1936. The dam was designed to accommodate 6.1 m of horizontal and 1 m of vertical fault displacement. The Palmdale Dam was originally built across the San Andreas Fault in 1891. The dam was completely reconstructed in 1969 because of the possibility of faulting. The new dam was designed to accommodate 6.1 m of horizontal and 1 m of vertical fault displacement. The Cedar Springs Dam in California was built over active faults. Because of the faults, the original design of the dam was modified including a reduction in height, shifting of the axis, and changes in construction material zonation (Bonilla 1991).
As more is learned about earthquakes and earth embankment dams over time, damage to earth embankment structures is being reduced. Older dams constructed on poor foundations, with poor fill, or with poor construction techniques are more likely to be damaged or fail. Dams constructed on firm foundations, with appropriate fill, and using current construction techniques are less likely to be damaged or fail. Knowledge from past failures, increased technology, increased knowledge of earthquakes, and improved construction techniques are the primary reasons for better structural stability in earth embankment dams.
Babbitt, D. H. and Verigin, S. W., 1996. General Approach to Seismic Stability Analysis of Earth Embankment Dams. California Department of Water Resources, Division of Safety of Dams.
Bonilla, M. G., 1991. Faulting and seismic activity, in Kiersch, G. A., ed., The heritage of engineering geology; The first hundred years. Boulder, Colorado, Geological Society of America, Centennial Special Volume 3.
Byrne, M. P. and Seid-Karbasi, M., 2003. Seismic stability of impoundments. 17th Annual Symposium, Vancouver Geotechnical Society.
U.S. Geological Survey, National Strong-Motion Program, 2001. Howard Hansen Dam. World Wide Web homepage URL: http://nsmp.wr.usgs.gov/data_sets/20010228_1/20010228_hhd_pics.html.
U.S. Geological Survey, 1995. Fact Sheet-096-95, The Los Angeles Dam Story. World Wide Web homepage URL: http://quake.wr.usgs.gov/prepare/factsheets/LADamStory/.
U.S. Army Corps of Engineers, Louisville District, Dam Safety Section. 1990. Earthquake Fact Sheet. Louisville, Kentucky.
Virginia Tech University, 2000. Geotechnical Engineering Photo Album. World Wide Web homepage URL: http://cgpr.ce.vt.edu/photo_album_for_geotech/.