| The basic seismic design of dams in Japan has been conducted using traditional pseudo-static analysis in compliance with the present technical standards. Fortunately, dams in Japan designed in accordance with the standards have not suffered any earthquake damage so severe as to affect people’s lives and property in the lower reaches. In the “Guidelines for Seismic Safety Evaluation of Dams (Draft)” issued by the River Bureau of the Ministry of Land, Infrastructure, Transport and Tourism in March (MLIT), 2005, seismic performance of embankment dams is essentially evaluated based on sliding deformation by dynamic analysis. This evaluation method stands on technical and empirical judgments that earthquake-induced settlement without accompanying sliding formation due to a large earthquake motion would be about equal to future consolidation settlement and smaller than the settlement by sliding deformation. Nevertheless, post surveys of embankment dams that had been damaged in some recent earthquakes in Japan revealed that relatively large-scale settlements without sliding deformation is expected to be during large earthquakes. In addition, immediately after the Tohoku Earthquake, special safety inspections were carried out at over 300 dams in the affected area. As a result of special safety inspections, more than 10% of all inspected dams reported some damages. This ratio rose to 18% for embankment dams. Damage to embankment dams included relatively wide and/or long cracks mainly on the crest of earthfill dams. However, none of the dams under the jurisdiction of the MLIT sustained damage severe enough to threaten the safety of the dam, although an old earthfill dam for an irrigation pond located outside the area regulated by the River Law was breached due to the earthquake. To improve seismic performance evaluation of embankment dams, we have conducted research on evaluating cracks and settlement on the crest of embankment dams during earthquakes using dynamic analysis. To evaluate the cracking phenomena of core material, the characteristic of core material under tensile action is very important. For the input data of dynamic analysis to reproduce cracks on the crest of embankment dams, we conducted splitting and direct tensile tests for core material of an existing rockfill dam with a center core. We find that tensile strength of direct tensile tests is larger than that of splitting tensile tests. We also find that tensile strength of compaction degree 100% is larger than that of 95%. |
