Analysis of Earth dam failure during Tohoku Earthquake Magnitude 9.0 on march 11, 2011

Fujinuma Dam (Source:wiki)
The 2011 Tohoku earthquake, or the so–called Great East Japan Earthquake, was a magnitude 9.0 (Mw) undersea megathrust earthquake off the Pacific coast of Japan, which occurred on 11 March 2011. The epicenter was about 70 km east of the Oshika Peninsula, in Miyagi prefecture, at a depth of approximately 32 km (JMA). It was one of the most powerful earthquakes in the world since the establishment of the modern recording system. You can watch video of earthquake shaking and tsunami by clicking here.
The ground motion records obtained from Station FKS017, Sukagawa, Fukushima prefecture, Japan, was provided by the Kyoshin network that operated by the National Research Institute for Earth Science and Disaster Prevention. These data were observed about 15 km away from the dam site. Furthermore, Hata et al. 2011 also investigated the simulated ground motion at the dam site. They estimated the ground motion for the dam site by microtremor observation by using the site effect substitution method. These motions were used in the dynamic analysis of the dam. The peak ground acceleration amax was 4.198 and 4.25 m/s2 for the observed motion and simulated motion, respectively. The predominant period of all motions was 2.71 and 2.96 Hz, respectively. In addition, to verify the effects of the phase of ground motion, the opposite wave forms of observed motion and simulated motions were also used as additional input motions for this study.
In general, earth dams are the most common type of dam found because of its cost effectiveness. A number of earth dams exist throughout the country of Japan for the purposes of irrigation water supply, flood mitigation and hydropower. Since the country of Japan lies in one of the most seismically active areas of the world, one of the main issues in the dam management and construction is the seismic safety. Therefore, to assure dam’s safety, a proper safety evaluation of such dams is crucial and necessary. This research deems to get insight and explain the dynamic behavior during earthquake and the possible failure of the Fujinuma dam. The study included the site investigation, laboratory experiment and numerical simulation.

Location of Main and Auxiliary dam (Source: Google Earth)

Failed main dam
Failed auxiliary dam 

The Fujinuma dam was an earth–fill embankment dam in Sukagawa city, Fukushima prefecture, Japan. It was established on the Ebana river, a tributary of the Abukuma river, 16 km west of the city office of Sukagawa city (37°18′07″N, 140°11′41″E)(Wikipedia). The dam was constructed in 1937 and it was completed in 1949 after construction was halted due to World War II. The dam served the primary purpose of water supply for irrigation. The dam was 18.5 m in height and 133 m in long embankment–type with a structural volume of 99,000 m3 and a crest width of 6 m. Also, there is an auxiliary dam with a height of about 6 m and length of approximately 60 m. The dam sat at the head of 8.8 km2 drainage area and its reservoir had a capacity of 1,504,000 m3.
In the study of the dam failure, the site’s investigation showed that the dam was not susceptible to liquefaction. The microtremor observation show that the natural frequency of the remaining of the main dam is in a good agreement with the modal analysis of the remaining dam. So, the material properties are proper for modeling the original dam model and also for conducting the dynamic analysis of the dam. The dynamic analyses of the dam were conducted using numerical simulation. The results obtained using the Tohoku recorded ground motion showed that the dam was subjected to the high amplitude and long duration earthquake which also characterized by a wide band that possibly induced resonance in the dam. In addition, with a long duration of shaking, the build–up pore pressure was then developed within the dam’s body which might cause the reduction of the shear strength within dam body. Accordingly, large shear strain can be observed on the top portion of the dam and on the downstream slope. The dam was therefore settled and the plastic deformation was then accumulated and then the loss of freeboard was occurred that later cause the breaching of the dam. This failure mechanism exhibit good agreement with the facts gathered from field observations.

Numerical Results

In summary, through this study it can be seen that there is always a risk for those people who lived near the dam site especially at downstream side. In fact, this study presented only one case, there are still numerous of existing fill dams were needed to be evaluate their seismic safety to ensure the safety of people and their properties. Therefore, the seismic safety evaluation of existing dams is crucial and indeed urgent. Otherwise, when future quake strikes it might bring about a disaster to those who live downstream. Thus, this study demonstrates that it is very important not only to design a dam that is capable of withstanding future quakes, but the investigation, maintenance and mitigation of existing dams are vital for seismic safety of dam.

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