一維與二維振動對砂土液化行為之影響

Abstract

本研究為探討一維振動與二維振動對土壤液化行為的作用，將翁作新等人在國家地震工程研究中心進行一系列之振動台試驗的資料進行整理與分析。振動試驗所採用的砂為越南石英砂以及麥寮砂，越南砂為自越南峴港進口的白色次角狀石英砂，而麥寮砂是台灣西海岸典型的灰黑色粉土質砂。振動台試驗包含9組越南砂試驗以及2組麥寮砂試驗，將振動台試驗，依振動模式分為一維振動試驗與二維振動試驗兩大類。二維振動試驗若X向與Y向有相同振動模式而且與一維振動試驗之單一X向或Y向振動模式相同，則將此兩次試驗配為一組。以作一維與二維振動時砂土反應的比較。 本研究利用埋設試體內部水壓計之量測資料，比較一維與二維振動對孔隙水壓激發的影響。越南砂和麥寮砂未液化區二維振動下引致的孔隙水壓總是大於一維振動下的孔隙水壓。還發現各水壓計埋設處，砂土受二維振動與一維振動時激發孔隙水壓的比值相近。水壓計深度會隨試驗而變動，因此引入孔隙水壓比（ru=∆u/σv'）作為比較參數。比較一維與二維振動下所激發之最大孔隙水壓比，越南砂受二維振動下引致之孔隙水壓比大約為一維振動下引致孔隙水壓比的5.76倍，麥寮砂受二維振動下引致之孔隙水壓比大約為一維振動下引致孔隙水壓比的3.2倍。 根據水壓計及加速度計之量測數據判定砂試體液化深度，然後進行不同維度振動下液化深度之比較，由分析結果可看出振動頻率、以及延時對液化深度的影響並不顯著，但隨著振動振幅(最大加速度)增加液化深度有些微增加。此外發生液化時一維與二維振動作用引致之液化深度約略相等，但少數二維振動作用可能發生稍大的液化深度。

In order to study the effect of one- and two-dimensional shakings on soil liquefaction, the test data obtained from shaking table tests at the National Center for Research on Earthquake Engineering were processed and analyzed. Test specimens include two kinds of sand. One is clean Vietnam silica sand and the other is Mailiao sand with fines. The Vietnam sand is white and sub-angular quartz sand from Da Nang, Vietnam. The Mailiao sand is a typical silty sand in the western coastal area of Taiwan. The test data contain nine series of Vietnam sand and two series of Mailiao sand. This study used the data from piezometers within the specimen to investigate the effect of two-dimensional shaking on pore water pressure generation in sand. Within the non-liquefied zone of Vietnam sand and Mailiao sand specimens, excess pore water pressures generated under two-dimensional shakings were always greater than those under one-dimensional shakings. It was also found that the ratios between the maximum excess pore water pressure generated under one- and two-dimensional shakings were similar at all locations of the piezometers within the specimen. This study uses the pore water pressure ratio (ru = Δu / σv ') as the parameter to compare the test results under one-dimensional shaking and two-dimensional shaking. In the case of the Vietnam sand specimen, the ratio of maximum ru under 2D shaking to that under 1D shaking is about 5.76, and the ratio of maximum ru under 2D shakings to that under 1D shaking is about 3.2 in the case of the Mailiao sand specimen. The depth of liquefaction was determined based on the measured pore water pressures in the sand specimen and accelerations on the frames of the shear box. It was found that the influences of shaking frequency and duration on the depth of liquefaction are not remarkable. The liquefaction depth increases slightly with increasing amplitude of shaking (maximum acceleration). With the same testing conditions, the liquefaction depth under two-dimensional shakings is approximately equal to that under one-dimensional shakings. Only a small number of two-dimensional shaking tests caused larger depth of liquefaction than one-dimensional shakings.