Evaluation of Seismic Damage to the Mound of Takamatsuzuka Tumulus Based on Dynamic Finite Element Analysis

Innumerable cracks and fissures caused by huge earthquakes were found on the excavated walls inside the mound of Takamatsuzuka Tumulus during archaeological investigation. The largescale excavation prior to the dismantling of the stone chamber in 2006 gave additional information about the three-dimensional distribution of those cracks and fissures. Serious ground motion due to huge subduction earthquakes hitting Japan every 100 to 140 years as well as nearfault earthquakes possibly caused the damage to the tumulus mound. Thus, seismic stability is of importance for the conservation of tumulus mounds made of soil without artificial reinforcement. In this study, a series of three-dimensional dynamic finite element analyses was conducted for the mound of Takamatsuzuka Tumulus to investigate the possibility of the occurrence of cracks and fissures induced by the outer forces of earthquakes. Based on archeological investigation, realistic modeling for the tumulus structure was conducted. Calculated performance shows that the natural period of Takamatsuzuka Tumulus that can generally be measured for near field earthquakes is 0.15 s. This means that such a small mound as Takamatsuzuka Tumulus is more fragile to near field earthquakes than subduction earthquakes. Serious amplification of acceleration can be seen in the tumulus mound. The response acceleration derived on the mound becomes much larger than the input acceleration monitored in the original ground. Calculated results also exhibit that serious stress concentration is found at the foot of the mound and at the boundary of the mound and the chamber stones, possiblycausing the occurrence of cracks in the mound of Takamatsuzuka Tumulus.