A study on the effect of countermeasures against liquefaction under strong shaking

在1995年神户地震中，夯实砂桩等方法是有效的，因为加固后的地基虽然地震烈度超过400 cm/S^2，但没有发生液化破坏。然而，地震后根据现有设计方法进行的反分析表明，地震时加密层应该有液化。笔者认为，实际现象与现行设计方法的矛盾源于以下两个原因：1.水平土压力增大，压实产生较高的液化强度。然而，目前的设计方法没有考虑压力的影响。挤密桩上的密实地基是一种复合地基。然而，在目前的设计方法中没有考虑这种影响。然后，作者进行了三种试验来验证上述效应。为了研究第一个问题，在相同的试验条件下进行了室内标准贯入试验和循环扭转剪切试验。使用的土壤是干净的沙子和粉砂。进行了几种密度和水平围压的试验。试验结果表明，静止土压力系数K0对SPT N值与液化应力比的关系有一定的影响。将特殊的模型地基分成三个不同的密实区，对复合地基的横截面进行修正。试验结果表明，虽然没有挤密桩的地基液化，但有挤密桩的地基并未液化。

Compaction methods, such as sand compaction piles were effective during the 1995 Hyogoken-nambu (Kobe) earthquake because no damage due to liquefaction occur on the improved grounds though the seismic intensity was very high more than 400 cm/s^2. However, back analyses, conducted after the earthquake based on the current design methods showed that densified ground should have liquefy during the earthquake. The author thought that the contradiction between the actual phenomena and current design methods is derived from the following two reason :1. Horizontal earth pressure increases and produces high liquefaction strength due to compaction. However, effect of the pressure is not considered in the current design methods.2. A densified ground on compaction piles is a composite soil. However this effect is not considered within current design methods.Then the author conducted three kinds of tests to demonstrate the above mentioned effects. Laboratory SPT and cyclic torsional shear tests were carried out under the same test conditions to study the first problem. The soil used was a clean sand and a silty sand. Tests were conducted for the several density and horizontal confining pressures. Test results showed that the relationship between SPT N-value and liquefaction stress ratio is affected by the coefficient of earth pressure at rest K_0.Shaking table tests were carried out to study the second problem. Special model grounds, separated into three different density zones, were used to modify the cross section of the composite ground. Test results showed that the ground with compaction piles did not liquefied though the ground without the piles liquefied.