Seismic Design Code for Tailings Dams in Japan Revised After the 2011 Great East Japan Earthquake

Abstract eng:
The 2011 Great East Japan Earthquake, with a magnitude of MW=9.0, caused severe damage to many structures not only in the Tohoku district of northeastern Japan, but also in the Kanto district, surrounding Tokyo. Seismic intensity in the affected area, as measured by Japanese Meteorological Agency (JMA) scale, was 5 to 7, which corresponds to an intensity of 7 to 11 according to the Modified Mercalli (MM) scale. The earthquake caused the failure of three abandoned tailings dams that had been examined about 30 years previously and had been judged to be safe against earthquakes. Detailed study conducted after the earthquake demonstrated that the main reason for these failures was the intense shaking generated by the earthquake, which exceeded the shaking level applied in the examinations. Following this earthquake, the Japanese Ministry of Economy, Trade and Industry (METI) revised the application of its seismic design code for tailings dams to insure safety against Level 2 earthquake motion, , which is extremely strong but very unlikely to strike a structure during its lifetime. Then a new design concept based not on the occurrence of sliding but on the likely degree of damage to structures from sliding was introduced. The METI lists 388 major tailings dams in Japan. About one-third of these dams were constructed by inner filling method. It was considered necessary to inspect following types of tailings dams deemed susceptible to damage by very strong Level 2 earthquake motion: i) constructed by inner filling method, ii) with a slope steeper than 15 degrees, and iii) with a ground water table shallower than GL-10 m. These tailings dams have been inspected since 2012. In the inspection, detailed soil investigations, including borings, standard penetration tests, PS loggings, measurement of the ground water table, undisturbed samplings, triaxial tests, and cyclic triaxial tests, were conducted. Then, the stability of the slope of the dam under Level 2 earthquake motion was analyzed in the following four steps: i) Step 1; the static stress distribution in the cross section was analyzed by static finite element method, ii) Step 2; the dynamic stress distribution in the cross section was analyzed by seismic response analysis, iii) Step 3; the distribution of the safety factor in the section was calculated based on the static and dynamic stresses and on soil strength, and iv) Step 4; the slip surfaces were assumed and the sliding displacements along the slip surfaces were estimated by Newmark’s method with consideration of an increase in excess pore water pressure. Then an inspected result for a tailings dam was introduced.

• Export as: BibTeX | MARC | MARCXML | DC | EndNote | NLM | RefWorks
• Add to your basket