現有水文地質調查結果顯示，台灣雲林地區在深度100公尺內之土層包含大量砂性土壤，而地下水位的變化將對砂性土壤產生有效應力的反覆改變造成體積壓縮及地表沉陷，本研究以渥太華砂為試驗材料，利用自動化三軸設備系統進行Ko反覆壓縮試驗，以模擬土壤元素側向束縛時因地下水位升降引致之壓縮行為；一般反覆壓縮試驗皆是以在垂直方向造成軸差應力的方式進行，但地下水位之變化實際上可以表現在試體孔隙水壓的升降上，因此本研究規畫了兩種試驗方法：(1)垂直有效應力控制法與(2)反水壓控制法，並進行數個階段反覆荷重試驗。試驗結果顯示，雖然兩種試驗方法之應力路徑不盡相同，但對顆粒性土壤所造成的壓縮量卻極相近，可論定此兩法皆可用於進行模擬地下水位升降造成的壓縮Ko反覆荷重試驗。此外因雲林地區細粒料含量約為20%，因此本研究另以雲母作為細粒料加入試體中，將試驗結果與乾淨渥太華砂比較發現雲母含量的增加會造成摩擦角減小及壓縮性增大，且在每一階段軸差的首次加壓時，應變量快速增加的幅度也越明顯，隨著應力振幅的增加壓縮量的差距也越大。

Hydrogeological surveys reveal that the soil stratum in Yunlin area are mainly consisted with sandy soils (sand and silty sand) within 100 m. Variations of ground water table could repeatedly change the effectives stress and cause ground subsidence. To evaluate the compressibility of granular soils subjected to repeating loads under Ko conditions, series of Ko triaxial compression tests were performed on Ottawa sand specimens. Two types of Ko triaxial compression test were conducted in this study. The first method is called vertical effective stress control method that varies the vertical effective stress in Ko conditions. The second one is back pressure control method that varies the back pressure in Ko conditions. The testing results show that the compressibility of Ottawa sand by the two methods are very close despite the different stress paths of the two methods. Additionally, 20% of mica was added in the Ottawa sand to study the effects of mica content on compressibility behaviors. The results reveal that the mica content reduces the friction angle but increases the compressibility. In addition, the mica content increases the plastic compressibility from the first cycle of loading and the increment increases as the loading level increases.
While the groundwater level change, it will changes in effective stress repeatedly to the sandy soil and cause subsidence. Therefore, we operate repeating load test in Ko conditions with reconstituted specimens of Ottawa sand. Repeat load tests are generally repeated in the vertical direction to caused axis deviation stress, but actually the change of groundwater level can be expressed in vary of pore water pressure. So we plan two test methods to simulate this phenomenon : (1)The vertical effective stress control method and (2)Back water pressure control method. Testing results reveal that although the stress path of this two methods are different, but the compression behaviors are extremely similar. So we can confirm that this two methods can be use for repeating load test in Ko conditions to simulate the compression result causing of groundwater level’s change. Then we use a specimen made of Ottawa sand with 20% mica content to do the repeating load test in K0 conditions and compare the results with purely Ottawa sand. The result show that with the mica content increases, the friction angle will decreases and the compressibility increases. And at each stage of the first axis deviation stress, the rapid increase in the amplitude of the strain is more obvious with the increase of axis deviation stress.

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