低塑性細粒料土壤抗液化強度之研究

Abstract

影響土壤液化之因素包括土壤密度、有效圍壓、過壓密比、塑性、礦物組成、顆粒形狀以及試體準備方法等，本研究係以CKC動力三軸儀，採完全通過兩百號篩之石英粉及鐵立庫地區之細粒料為土樣，以濕搗法重模試體，將試體控制在相同圍壓下，以乾密度、塑性指數以及動態試驗之加載速率為控制變因，進行動力三軸試驗，探討各變數對於低塑性粉土液化之影響。 根據本研究試驗結果顯示，當石英粉試體於相同圍壓下，以乾密度為控制變因，以相同之加載速率進行動力三軸試驗，可發現土壤之抗液化強度隨乾密度上升而增加，即土壤愈緊密，其抗液化強度亦愈高。 根據不同塑性指數之鐵立庫細粒料土壤之動力三軸試驗結果，可得在相同圍壓下，以相同之加載速率進行試驗，土壤之抗液化強度呈隨塑性指數增加而有下降之趨勢。此外，土壤之塑性指數不同，其液化之行為亦不同。當土壤之塑性指數較低時，於反覆應力作用下，試體會先達有效應力為零，即發生初始液化，爾後才達到5% 雙振福軸向應變。當土壤之塑性指數較高時，於反覆應力作用下，試體會先達到5% 雙振幅軸向應變，爾後有效應力才下降至零，甚至不會發生初始液化。 本研究亦針對加載速率對於土壤抗液化強度之影響進行探討，由試驗結果可得，當試體為透水性較高之石英粉時，於相同之反覆應力比下，不論加載速率之快慢，土壤之抗液化強度皆相近，且孔隙水壓激發情形亦相近，即加載速率對透水性較高之石英粉之抗液化強度較無影響。當試體為透水性低之低塑性粉土時，於相同之反覆應力比下，土壤之抗液化強度會隨加載速率下降而降低，且孔隙水壓激發情形亦不同，即加載速率會影響低透水性之低塑性粉土之抗液化強度。

There are many factors affecting soil liquefaction including soil density, confining pressure, over-consolidation ratio, plasticity, mineralogy, and particle shape, etc. This research examined liquefaction behavior of non-plastic and low-plasticity silts by a series of cyclic triaxial tests. Materials used in this study were quartz powder and Tieliku silt, which both pass #200 sieve (0.074mm), Specimens were prepared by moist tamping. All tests were carried out under the same initial confining pressure with different dry densities, plasticity indexes and loading rates. From the test results, it can be found that the liquefaction resistance of the quartz powder increased as its dry density increased. The results of Tieliku silts show that the liquefaction resistance decreased as the plasticity index increased under the same initial confining pressure. In addition, silts with different plasticity indexes lead to different liquefaction behaviors. For the specimens with lower plasticity index, specimens first reached the state of zero effective stress, indicating the occurring of the initial liquefaction. It was then followed by the state of 5% double amplitude axial strain during undrained cyclic loading. On the other hand, for the specimens with higher plasticity index, specimens reached 5% double amplitude axial strain first, and then reached the state of zero effective stress or not reached the state at all. This study also investigated the effect of loading rate on liquefaction resistance. According to the test results, for quartz powder with higher permeability, whether the loading rate was fast or slow, its resistance was almost the same and the pore water pressure generation within the specimen was in a similar pattern. It indicates that the liquefaction resistance of quartz powder with higher permeability will not be affected by loading rate. However, for low plasticity silts with lower permeability, the liquefaction resistance decreased as the loading rate decreased and the pore water pressure generation in the specimen followed different patterns. It suggests that the liquefaction resistance of low plasticity silts with lower permeability will be affected by loading rate.