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

在2004年新泻县中越地震和2003年十胜冲地震中，许多地下建筑物，特别是检查井，由于回填材料的土壤液化而被抬升。最大上举量超过1.5米，总上举人孔频繁。与此相反，1995年神户大地震时，检查井的隆起量小于20 cm。这两种破坏类型的差异主要取决于周围地层的沉积条件。从破坏情况来看，埋在可液化地基中的检查井的上举力小于埋在不可液化地基中的检查井。为了阐明回填砂土液化引起地下结构上拔的机理，进行了回填模拟模型试验和振动台试验。通过试验研究，推测了沙井上拔的附加机理。首先，当回填土发生液化时，取决于液化程度的向上力将作为上拔的触发器作用于检查井底部。当人孔略微向上移动时，在人孔底部周围产生空化和吸力。在桑迪地基中，不仅回填土会发生液化，而且周围地基也会发生液化。考虑液化破坏时，我们往往集中在桑迪地基上。然而，我们也必须注意软土地基中的地下结构。

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.