Estimation of Damage on Uplift of buried manholes and piles due to Liquefaction of Replaced and Surrounding Soils

在2004年的新潟县中越地震和2003年的德立树地震中，由于回填材料的土壤液化，造成了许多地下结构，特别是人孔的隆起。最大提升量超过1米半，总提升人孔频繁。1995年阪神大地震时，井口隆起量小于20 cm。这两种破坏类型的不同是由周边地面的沉积条件决定的。从损伤观察来看，可液化地层中人孔的上拔量小于非液化地层。为了阐明充填砂液化引起地下结构抬升的机理，进行了充填模拟模型试验和振动台试验。从试验研究中，我们假设了井口抬升的附加机制。首先，当回填土发生液化时，取决于液化程度的向上力会作用于井底，成为井底隆起的触发因素。井口微微上移时，井底周围产生空化和吸力。在沙质地基中，不仅回填土会发生液化，而且周围的地面也会发生液化。考虑到液化破坏，我们通常集中在沙质地面。但是，软土地基中的地下结构也必须引起重视。

In the 2004 Niigata-ken Chuetsu Earthquake and 2003 Tokachi-oki Earthquake, many underground structures, especially manhole, were uplifted caused by soil liquefaction of the backfill materials. The amount of maximum uplift were over one meter half and total uplift manholes were frequent. On the contrary, the amount of uplift of manhole was less than 20 cm in 1995 Great Hanshin Earthquake. The difference between the two types of damage derives from depositional conditions of peripheral ground. From the observation of damage, the amount of uplift of the manhole buried in liquefiable ground was less than in un-liquefiable ground. The authors conducted model tests for back filling simulation and shaking table tests in order to clarify a mechanism of uplift of underground structure due to liquefaction of backfill sand. From the experimental study, we suppose the additional mechanism on the uplift of manhole. At first, when liquefaction occurred in the back fill soil, upward force depended on the degree of liquefaction will act to the bottom of manhole as trigger of uplift. While manhole slightly move upward, cavitations and suctions was generated around the bottom of manhole. In the sandy ground, liquefaction will occurred not only in backfilled soil but also in the surrounding ground. Considering liquefaction damage, we often concentrate to the sandy ground. However, we also have to pay attention to the under ground structures in the soft ground.