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

点击翻译按钮获取中文摘要

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.