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

在1995年的兵库县南部（科比）地震中，挤砂桩等挤密方法非常有效，因为尽管地震强度非常高，超过400 cm/s ^2，但在加固后的地基上没有发生液化破坏。然而，根据现行设计方法，地震后进行的反分析表明，密实地基在地震中应具有破坏性。笔者认为，实际现象与现行设计方法之间的矛盾源于以下两个原因：1。水平土压力增加，并产生高液化强度，由于压实。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土。然而，目前的设计方法并没有考虑这一影响，因此作者进行了三种试验来验证上述影响。在相同的试验条件下进行了室内标贯试验和循环扭剪试验，研究第一个问题。所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K 0对标准贯入试验N值与液化应力比的关系有影响。采用三个不同密度的特殊模型地基，对复合地基的断面进行了修正。试验结果表明，有挤密桩的地基不发生液化，而无挤密桩的地基不发生液化。

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.