東京における地震時の地盤工学上の課題

摘要

東京および周辺に首都直下地簾が襲つた場合に懸念されるニつの地盤災害を取り上げ，筆者達 が行ってきた研究を紹介し，今後の課題に関して述べた.ーつは地盤の液状化である.1923年 関東地震では沖積低地の各地で液状化が発生したが，2011年東北地方太平洋沖地震ではほとん ど液状化しなかった.その原因として経年効果により液状化強度が増加したことが挙げられる が,定量的に明らかになっていない.将来の首都直下地震時の予測等に対し研究を進める必要が ある.もうーつは谷底低地の地震応答である.関東地震時の水道管や家屋の被害は東京の谷底低 地に被害が集中していた.そこで谷底低地で詳細な地盤調査を行い，それをもとに地震応答解析 を行ったところ.谷底低地に堆積している腐植土が大きな地震応答を生じさせたと考えられた. 将来の首都直下地震に対して局所的に堆積している腐植土を考慮して被害想定などを行つておく 必要がある.%Our recent research on two geotechnical problems in Tokyo and its surrounding area to predict the damage by an epicentral earthquake is introduced, and future tasks are described. One of these problems is soil liquefaction in lowland areas where loose alluvial sand is deposited. During the 1923 Kanto Earthquake, liquefaction occurred in various places in the alluvial lowland, but almost no liquefaction occurred during the 2011 Great East Japan Earthquake. A possible reason for this is that liquefaction strength had increased due to aging, but this is still not proved quantitatively. It is necessary to advance research on the aging effect to predict liquefiable sites by the future earthquakes. The other problem is that the response of the deposits under the valley floors to the earthquake. Damage to houses and buried pipes during the 1923 Kanto Earthquake was concentrated in valley floors. Detailed soil investigations and seismic response analyses in a valley floor revealed that soft humic soil filling the valley floor enhanced the ground response to the seismic motion. To predict the damage that might be caused by an epicentral earthquake, it is necessary to consider the distribution pattern of soft humic soil in the valley floors.