Presented by: Assist. Lect. Farah Hameed Abdul-Khudhair Earthen and sand dams are among the most widely used types of dams in the world, because they are built from locally available materials at a lower cost than concrete dams. However, they have little resistance to erosion. When the water level rises above the dam crest, the body of the dam is continuously eroded, then collapses and causes flooding within a short time. The causes of their failure are generally grouped into three categories: overtopping, seepage, and structural failure. Overtopping occurs when the spillway capacity is insufficient, when it becomes blocked with debris, or when the dam crest settles, and American statistics indicate that it accounts for about 34% of failures. Once erosion begins, stopping it is almost impossible. Internal erosion, known as piping, starts on the downstream face of the dam and progresses toward the reservoir until a water path forms through the body of the dam. The process begins with slow seepage, then the water speed increases and carries soil particles, and the channel gradually widens until failure occurs. Its danger lies in the fact that visible warning signs, such as springs at the toe of the dam or a whirlpool in the reservoir, appear only at a late stage. FERC data show that piping through the embankment accounts for about 32.5% of failure cases. Other causes include slope instability and foundation defects, such as settlement, which account for about 30% of failures. Earthquakes are another cause: saturated sandy soil loses its strength during shaking, a phenomenon called liquefaction, and parts of the dam slide. Maintenance also plays an important role, since tree roots and animal burrows create preferential seepage paths, and uprooted trees leave large voids in the dam body. Two case studies illustrate these mechanisms. The first is the failure of the Teton Dam in Idaho, USA, on 5 June 1976 during its first filling, which caused the deaths of 14 people and damage estimated at between 400 million and 1 billion dollars. The independent panel concluded that the cause was internal erosion in the core of the dam within the right foundation key trench, and a whirlpool was seen in the reservoir before a chunk of the dam fell into it and the whole dam collapsed within minutes. The second is the slide of the upstream slope of the Lower San Fernando Dam in California in 1971, about 40 seconds after the earthquake shaking stopped, caused by liquefaction of a zone of hydraulic sand fill near the base. There were no fatalities, but more than 80,000 people were evacuated until the reservoir level was lowered, and only about 5 feet of dam height remained above the water. To prevent such failures, the spillway should be designed to pass the maximum probable flood and be kept clear of debris, compaction quality should be ensured and filters designed to prevent the migration of particles, and monitoring instruments should be used to measure water pressure, seepage rates, and settlement, with regular inspection of the toe and slopes. Woody vegetation should also be removed, animal burrows filled in, the upstream slope protected with riprap, liquefaction susceptibility assessed in seismic regions, and an emergency plan and early warning system prepared for downstream areas. In conclusion, the causes of sand dam failure center on overtopping erosion and internal erosion, in addition to weak foundations and seismic liquefaction. Failure may occur suddenly, but its causes build up over years of poor design, construction, or maintenance, so regular monitoring, maintenance, and sound design remain the first line of defense. Source: Independent Panel to Review Cause of Teton Dam Failure, Report to U.S. Department of the Interior and State of Idaho on Failure of Teton Dam, 1976.