| Summary: | 碩士 === 國立成功大學 === 土木工程學系碩博士班 === 101 === 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.
|
|---|
