ABSTRACT: This paper deals with three case studies of famous dam failures, Teton dam (USA), St. Francis dam (USA) and Vaiont dam (Italy). The cases represent different dam constructions and failures, Teton dam was a rockfill dam, St. Francis and Vaiont dam was concrete gravity-arch dams. The reasons of failures are quite different, Teton dam failed because of seepage through the dam which expanded rapidly resulting in a collapse. Vaiont dam did not have a construction fault, instead a huge landslide pushed enormous amounts of water over the crest. St. Francis dam was built upon a weak layer of rock and had been built to high, this resulted in a possible overturn failure in combination with a landslide. These dam failures killed hundreds of people and caused devastating destruction to the surroundings. Dam construction is one of the most demanding areas of expertise because failure often leads to enormous casualties.
1 ST. FRANCIS DAM
1.1 Construction The dam was built between 1924 and 1926 and was located 64 km northwest of Los Angeles, California, and was designed as a reservoir to support Los Angeles with water for a year if the Los Angeles Aqueduct should be damaged or sabotaged. The chief engineer for the project was William Mulholland, a civil engineer that worked for Los Angeles Department of Water and Power.
The dam was planned to be 53 meters (175 feets) high and 185 meters (600 feets) long. When the construction began, see figure 1, in 1924 Mulholland decided to raise the height of the dam three meters (10 feets) to increase the capacity of the reservoir. Later when the dam was half finished in 1925 he made another decision to raise the dam 3 more meters. These decisions increased the capacity of the dam from 37 million cubic meters to more than 47 million cubic meters of water. The final height of the St. Francis dam where 59 meters. This change in construction was made without an increase of the base of the dam. The city of Los Angeles was growing rapidly and the size of the reservoir had to be this large to be able to support the city if needed.
Figure 1. Constuction of St. Francis dam. (1.)
1.2 The failure In 1926 to 1927 several cracks appeared in the construction, some of those were even leaking water. These cracks where inspected by Mulholland, who concluded that they were normal concrete cracks for a dam the size of the St.Francis due to natural settlements.
The damkeeper of St.Francis dam was a man named Tony Harnischfeger who lived in a cottage 400 meters downstream of the dam. He discovered new cracks and leaks when the reservoir was filled to its capacity for the first time on March 7th, 1928, see figure 2. He reported this to Mulholland who dismissed them as normal a second time. On March 12th Harnischfeger located new leaks and alerted Mulholland who once again pronounced the dam safe.
Figure 2. St. Francis dam filled with water. (1.)Figure 3. “The tombstone” and the landslide. (1.)
Less than twelve hours after the inspection the dam collapsed, this happened approximately at 23:57 on March 12, 1928. The dam broke into several large pieces, some of which was carried several hundred meters downstream, while the center section of the dam, called “The Tombstone”, remained standing as can be seen in figure 3. (1)
The collapse released 45 billion liters of water in a powerful wave that destroyed everything in its path. The damkeepers cabin was hit by a wave that’s been estimated to be 42 meter high. (4) The water continued to flow south though the San Francisquito Canyon and then turned left into the Santa Clara River bed, flooding the towns of Fillmore, Bardsdale and Castaic Junction. When the flood finally reached the Pacific Ocean it had travelled 87 km from the reservoir, figure 4. Bodies was found as far away as the Mexican border. (1)
Figure 4. The floods way to the Pacific Ocean, 87 km. (6.)
The exact number of victims is unknown but the official death toll is 495. No one knows how many undocumented farm laborers died that night. The city of Los Angeles paid several million dollars in recompense to families and landowners. (2)
1.3 The cause of failure One of the main factors for the collapse is the heightening of the dam without widening the base of the construction, according to Mulholland the factor of safety against overturning “was in excess of 4,” but the dams new height would have reduced the factor to as low as 0.77. The massive force from the water could have lifted the dam from its foundation causing a large tension crack in the upstream heal of the dam. (3)
In the 1920s the geological investigation showed no problems with the dam being built on the existing type of rock. This has later been proved to be an area that had been exposed to multiple landslides throughout history.
Post-failure studies of the surrounding environment shows that a massive landslide occurred along the dams left abutment. 500 000 cubic meters of schist could easily have torn the dams entire left abutment from the rest of the construction. (4)
Two of the worlds greatest geologists at the time had found no fault with the rock. Following the disaster Mr. Mulholland said “Don’t blame anyone else, you just fasten it on me. If there was an error in human judgment, I was the human, and I won't try to fasten it on anyone else.” He was cleared of all charges because no one could have known of the instable rock the dam was built upon. (1)
2 TETON DAM
2.1 Construction The construction of the Teton dam started in February 1972 in Idaho, USA. It took almost four years to finalize the dam and in October 1975 the dam began to store water in the reservoir. The contract for the 93 meter high dam was won by the company of Morrison-Knudsen-Kiewit in December of 1971, but the dam had been planned for many years ahead of the final decision to build it. The first site investigations took place as early as in 1932. Many other alternative sites along the Teton River where also investigated before the location of the dam was set. The intension of the dam was not only to provide the area with hydropower but also to provide the irrigation in the area (40.000 hectares) with water as well as flood control and recreation. (7)
Most of the dams in the world are embankment dams and so where the Teton dam. Because of the bad rock in the area a genuine foundation work had to be done. A cut of trench was done down to the rock and the rock under the abutments was excavated. Under the whole foundation a continuous grouting curtain was injected. The embankment dam consisted of four zones of different material and functions. Zone 1 was the core and this zone consisted of material with low permeability to prevent the water from escaping the reservoir. Zone 2 was the filter that prevented the material in the core to be filtrated out of the dam. Zone 3 was a layer that provided the dam with structural stability. Zone 5 was the rockfill, the most outer layer of a rockfill embankment dam. The Zones are shown in figure 5 below. (8)
Figure 1. Cross-section of the dam construction. (7)
2.2 The failure The first sign which indicated that something was wrong came on June 3 1976, when a small seepage of water showed on the right abutment near downstream of the spillway. The day after a spring showed closer to the embankment, the flow from the spring was about 80 liters per minute. (8) For the rest of the day the dam was thoroughly investigated and no signs of seepage water was found anywhere on the main dam construction. The flow in the spring remained constant until the morning of June 7 when it increased rapidly. At 9.30 that morning the first sign of leakage in the actual embankment started. At 10.30 the flow had gone from 2 cubic meters per second to 30 cubic meters per second and a whirlpool had begun to show in the basin on the upstream side of the dam. The police and media in the concerned areas had been notified when the first sign of leakage in the embankment came but it was not until now they began to understand the real danger of the situation. A few hours after the warnings had reached the people living in the risk area the dam failed and released the whole reservoir containing around 370 million cubic meters of water into the surrounding downstream area. (9)
Because of the basalt rock the dam was founded on the grout curtain was an essential part of the dam construction; and the geologists at the Bureau of Reclamation had specified that in the deeds. The curtain was injected in three rows of holes. It extended down to 100 meters under the foundation and 300 meters beyond the abutments. Even with such an extensive grouting program some people consider a failure in the grout curtain to be the reason for the dam failure. Commissioner Gilbert Stamm said that the water had either found its way around the grout curtain or went right through it.
In the catastrophe following the failure 11-14 (contrarious information) people died and some 2000 people where injured. The breach caused an area of more than 770 square kilometers to over flood. The flooding caused damages costing several hundred million dollars; among the things ruined were more than 7000 homes. (9)
The catastrophe got a lot of coverage in media because of the projects controversy. Before it got built the dam project met a lot of resistance from environmentalists and various experts, it was placed on a hit list for water constructions that were considered not cost effective or planed on to difficult sites. Maybe it was because of the bad publicity the whole project had endured that the US government did not put in more funds into dam inspection at this point. It took another 18 months and another dam failure in Georgia to get them to act. (9)
2.3 The cause of failure A panel of ten people where put together to investigate the incident and to find out what really caused the breach of the dam. They got to the conclusion that piping occurred in the base of the core in the keyway that was cut into the right abutment. They said the dam had been inadequately drained; that was the reason water had burst through the face of the downstream embankment. The piping phenomenon could have been due to the insufficient grouting. The committee completely freed the contractor from having anything to do with the dam failure. They had followed the deeds and done everything the Bureau of Reclamation had demanded; the responsibility according to the committee lay instead in the bureau’s hands. The final report in the matter said that the reason of the breach was poor embankment design and inadequate inspection of the Bureau of Reclamation. The poor embankment design was primary a bad filter layer that did not protect the core from eroding due to seepage water. The report also said that even if the conditions at the site of the dam where bad a safe dam could have been constructed. If the core had been better drained and protected against internal erosion their opinion was that it would still be standing there. (9)
2. http://www.scvhistory.com/scvhistory/sg031101.htm, 2007-12-14
3. http://web.umr.edu/~rogersda/st_francis_dam/lessons_learned_from_the_st_francis_dam_failure(geostrata_mar-apr_2006).pdf) , 2007-12-14
4. http://web.umr.edu/~rogersda/st_francis_dam/reassessment_of_st_francis_dam_failure.pdf, 2007-12-14
5. http://web.umr.edu/~rogersda/st_francis_dam/Impacts%20of%20St.%20Francis%20Dam%20Failure-compressed.pdf, 2007-12-14
6. Google earth, 2007-12-14
7. http://www.eng.uab.edu/cee/faculty/ndelatte/case_studies_project/Teton%20Dam.htm 8. http://www.geol.ucsb.edu/faculty/sylvester/Teton%20Dam/narrative.html Books:
9. Ross, Steven S. 1984. Construction Disasters. New York, USA. McGraw Hill Book Company.