不飽和土壤孔隙壓力參數之研究

Abstract

古典土壤力學的研究領域通常專注在飽和土壤的行為上。如此發展的趨勢造成在古典土壤力學中缺乏對不飽和土壤行為機制的瞭解及關於氣候變動因素影響地下水文狀態邊界模式評估等兩個主要弱點。然而，許多工程師在進行一般基礎建設中即會不斷遭遇關於地下水位以上的不飽和土壤相關工程問題。本研究之目的在於探討不飽和土壤孔隙壓力參數之變化，許多研究指出該參數為介於0與1之間的數，飽和為1、完全乾燥為0，但在這範圍間的變化仍相當模糊，如此會大大地影響到實際工程應用上對不飽和土壤有效應力的預估，更甚者，剪力強度參數及體積變化模數也不可得知，為解決這層困境，本研究乃利用改良式單向度壓密滲透儀來進行不飽和土壤體積變形試驗，藉由改變土壤結構張力的方式觀察體積變化以推求孔隙壓力參數;最後並利用土壤—水保持曲線試驗結果來建立孔隙壓力參數隨結構張力變化之線性方程式。實驗結果顯示：孔隙壓力參數會隨飽和度下降而成非線性減小，最後並趨近於零，而分析中也發現孔隙壓力參數的變化與正規化土壤—水保持曲線有非常近似的關係，藉由這層關係推導出孔隙壓力參數隨對數化結構張力變化之線性方程式。

Classical soil mechanics has focused on the behavior of saturated soils. The major weakness associated with classical soil mechanics is the lack of understanding of the behavior of unsaturated soils and missing of a model for the moisture flux boundary condition imposed by climate conditions. However, most of the geotechnical problems which engineers encounter involve unsaturated soils above the groundwater table.The objective of the research is to study the pore pressure factor of unsaturated soils. The result of many of previous researches indicate that the value (pore pressure factor) is only between one and zero. When the soil is saturated, the value is one. For completely dry soil, the value will be zero. However, the variation of the pore pressure factor is difficult to determine within the range between one and zero. As a result, it is difficult to estimate effective stress of unsaturated soils when encountering practical engineering problems. Unless the pore pressure factor can be evaluated, the effective shear strength parameters cannot be found for an unsaturated soil; nor can the compressibility or expansibility be measured in terms of effective stress. In order to solve these problems, the main task of this research is to perform volume change test of unsaturated soils in use of specially designed consolidometor. The volume change of soils during every stage of matrix suction is used to determine the pore pressure factor in this research. Finally, by formulating simple linear equation for pore pressure factor and matrix suction in logarithmic scale, the pore pressure factor can be estimate easily.The experimental results indicate that pore pressure factor will decrease nonlinearly with saturation descent, and will approach to zero consequently. Moreover, the results show that pore pressure factor is closely related to soil-water characteristic curve and can be compute by linear equation.