- 學位論文
土壤二次壓縮受應力歷史影響之室內試驗研究
A laboratory study on the effects of stress history on secondary compression of soils
摘要
軟弱土壤的含水量大、壓縮性高、強度低、透水性差，如果未對其施作土壤改良，在建築物的長期荷載下會產生危害建物安全之沉陷。超載工法常被用來改良軟弱黏土層，可避免建物完成後可能所產生的不均勻沉陷。過去的研究多以純超載解壓為主，然而現地的情況可能包括反覆的加載與解載動作。 為探討不同的應力歷史對軟弱土壤之二次壓縮影響，本研究利用六種土壤試樣，加以重模後使用中型單向度壓密儀製作出均質且有一致性的重製土壤試體，選定超載比為0.125及0.25以傳統單向度壓密試驗儀對六種土壤進行三種不同的超載壓密試驗，分別為純超載解壓試驗、超載後解壓至27.4kPa的低壓再回壓試驗、正常加載後解壓至27.4kPa的低壓再回壓試驗，針對其試驗結果如壓縮曲線、沉陷量(LVDT –logt讀數)分析其變型速率、二次壓縮曲線斜率、主壓密與十倍主壓密之孔隙比變化量、二次壓縮沉陷量等，並與傳統單向度壓密試驗之結果相互比較。 根據六種土壤之分析結果顯示純超載解壓試驗二次壓縮曲線斜率落在0.001~0.121mm，主壓密與十倍主壓密之孔隙比變化量為0.001~0.023為三種超載應力歷史中最低的，表示對軟弱黏土的二次壓縮沉陷量改良效果較佳。至於另兩種解壓至低壓再回壓之試驗，有超載者其二次壓縮曲線斜率落在0.017~0.168mm，無超載者則落在0.028~0.223mm，主壓密與十倍主壓密之孔隙比變化量為0.002~0.035，無超載者則為0.002~0.046，表示儘管皆曾經解壓至低應力再回壓，有超載者其改善二次壓縮沉陷量效果仍較無超載者佳。而正常加載試驗之二次壓縮曲線斜率範圍落在0.029~0.283mm，主壓密與十倍主壓密之孔隙比改變量為0.006~0.114，表示三種不同應力歷史之超載方法皆能降低土壤二次壓縮沉陷。
並列摘要
Soft soil has high water content, high compressibility, low strength, and poor permeability, if soft soil is not improvement, building load will produce a subsidence hazard. Surcharging methods are often used to improve the soft clay layer, and it can avoid uneven settlement. Previous research is mainly about pure unloading. However, in reality it include repeating loading and unloading. To explore the effects of different stress history on secondary compression of soft soil, in this study, six soil samples were re-mixed and poured into a medium-sized one-dimensional consolidation cell to produce homogeneous soil specimens. Three different surcharging operations with surcharge ratios of 0.125 and 0.25 where designed. The three surcharging operation included removing surcharge only, removing surcharge to 27.4kPa and then reloading back to final pressure, and unloading without surcharge 27.4kPa and then reloading back to final pressure. The slope of secondary compression curve, strain rate, change of void ratio are analyzed. According the test results, the slope of secondary compression curve from the test of removing surcharge only is about 0.001~ 0.121mm and the void ratio change amount of tp-10tp is about 0.001~0.023, this is the lowest among these three operations. The results of the other two operations with unloading to 27.4kPa, the slope of secondary compression curve with surcharge is about 0.017~0.168mm and the void ratio change amount of tp-10tp is about 0.001~0.023, and the slope of secondary compression curve without surcharge is about 0.028~0.223mm and the void ratio change amount of tp-10tp is about 0.002~0.046. The slope of the secondary compression curve of traditional one-dimensional consolidation test is about 0.029~0.283mm and is greater than those from the other three operations, means that surcharging with different loading history can effectively reduce the magnitude of secondary compression settlement amount.