飽和砂土孔隙水壓變化對側向靜止土壓力之影響

Abstract

本研究以不同滲流情況考慮孔隙水壓的變化，使用滲流儀施加向上滲流，藉以造成孔隙水壓的變化，並搭配水壓計、土壓計、流量計與資料擷取系統，量測孔隙水壓力變化時側向土壓力的變化。然後計算不同水壓力變化情況下的靜止土壓力係數。 實驗結果顯示，孔隙水壓比ru值在0到0.4之間，側向有效應應力的變化並不大，推測是受到顆粒互鎖(interlocking)的影響所導致。但ru值在0.4以上時，側向有效應應力下降幅度明顯，表示顆粒互鎖的消失。 由於側向有效應力有顆粒互鎖的影響，孔隙水壓力增加，垂直有效應力降低，而側向有效應力的變化不大。因此所計算出來的Ko值會增大。現地土層因孔隙水壓增加，Ko值可達1.0以上。 Jaky公式中所預測的Ko值與量測值接近，但是在水壓力變化下，Ko會隨著水壓力變化而變化，因此Jaky公式可能不適用於水壓力變化的情況。以Mayne and Kulhawy所提出針對過壓密土壤Ko值的修正式計算不同過壓密比(OCR)的Ko值，與量測結果接近，適用於過壓密土壤。

In this study, changes of pore water pressure were induced by applying upward water flow with a permeameter. Water pressure transducers, soil pressure transducers, flow meter and data acquisition system were used to measure the changes of lateral earth pressure. The coefficient of earth pressure at rest in different seepage conditions can then be calculated. Experimental results showed that lateral effective stress did not change much for pore pressure ratio ru from 0 to 0.4. The particles may be subjected to interlocking during pore pressure changes. However, for the value of ru over 0.4, effective lateral stress declined clearly. It indicated that the particle interlocking might disappear under a higher excess pore pressure. The vertical effective stress decrease with increasing pore water pressure. But the lateral effective stress changes little due to the effect of the particle interlocking on the lateral effective stress. As a result, the Ko value increases when pore pressure increases. In the field, the Ko value can reach over 1.0 due to increasing of pore water pressure.

The Ko value predicted by Jaky formula is close to the measured value in this study. The value of Ko changes with changing pore water pressure, so Jaky formula does not suitable in the condition when pore water pressure changes. For over-consolidated soil, the measuring value is close to the predictive value according to the relation proposed by Mayne and Kulhawy for various values of OCR.