砂土細料含量對動力三軸試驗超額孔隙水壓量測之影響

Effects of the fines contents on pore water pressure measurements in dynamic triaxial tests

吳紹華(Shau-Hua Wu)

指導教授 ： 翁作新

國立臺灣大學/工學院/土木工程學研究所/碩士(2006年)

https://doi.org/10.6342/NTU.2006.00517

摘要

摘要

一般動力三軸試驗水壓量測系統，於土壤試體頂部與底部以硬質Teflon管線連接至外部水壓計來進行水壓量測。不排水動力三軸試驗均假設土壤試體、儀器設備及水壓量測系統沒有體積變化；而實際上體積變化行為是存在於整個動態試驗系統中，如壓力變化將使連接管線及量測設備產生體積變化。因此，當試體某一點孔隙水壓改變時，不只是在試體中水壓之傳遞作用，也會使端部外接水壓量測系統產生體積變化，體積變化的空間必須要有水流產生來遞補，端部水壓計才能感受到試體內孔隙水壓的變化。細料含量較少的試體，其滲透性高，水流流動可迅速補水壓量測系統體積變化之空間，水壓計可即時反應孔隙水壓的變化；細料含量高之試體，滲透性低，水流流動遞補水壓量測系統體積變化之時間較長，使試體端部外接水壓計，不能即時反應土壤內部孔隙水壓變化之情形。

本研究為探討砂土細料含量對動力三軸試驗，超額孔隙水壓量測之影響，採用改良型動力三軸試驗儀，於試體內部不同高程位置裝設水壓計，並於動態試驗過程中量測試體端部、1/2及3/4高程處之孔隙水壓值。試驗結果發現，於細料含量較少之土壤試體，無論試體任何部位所量得之水壓值幾乎一致；細料含量較多之土壤試體，其試體3/4高程處水壓計水壓量測值會先達有效初始圍壓，1/2高程處水壓值次之，端部水壓值最慢。故於高細料含量試體，採用端部水壓計量測水壓值來判定土壤試體之抗液化強度，可能會有高估之情形。本研究採用不同位置水壓計量測值作為判定試體液化之依據，求得不同滲透係數試體，其內部不同位置水壓量測值判定試體所能承受的反覆作用次數，並與此試體端部水壓量測值判定試體所能承受的反覆作用次數相比。依不同試體滲透係數，用以修正一般動力三軸試驗，採用端部外接水壓計量測水壓值判定液化之反覆作用次數。

關鍵字

細料含量 ； 液化 ； 動力三軸試驗 ； 體積變化 ； 孔隙水壓 ； 量測系統

並列摘要

Abstract

In the ordinary cyclic triaxial tests, the researchers often use small and stiff Teflon tubes to connect the cap and bottom of the specimen to the water pressure transducer. They assume that there is no change in the volume of specimen and measurement system in undrained tests. In fact, the volume change exists in the dynamic testing. When the pore water pressure changes in a certain location of the specimen, not only the water pressure transmission within the specimen, but the pressure would probably lead to the volume change of the water pressure measuring system. There must be water flow to fill the volume change, that the ends pore water pressure transducer can experience the pore water pressure changes. Water flow can occur quickly and the measurements reflect the pore water pressure change immediately for low fines contents specimen with higher permeability. Howerer, for the high fines contents specimen with lower permeability, the measurements cannot reflect the pore water pressure changes inside the specimen immediately.
This study evaluates the effects of the fines contents on pore water pressure measurements. The pore water pressures at different locations (both ends, mid-height, 3/4-height) in the test specimen were measured in the dynamic triaxial tests. It is found that in an ordinary triaxial test, for specimen of lower fines contents, the pore water pressure sensors can obtain the same pore water pressure value. For specimen with higher fines contents, different pore water pressure values were obtained at different locations. The pore water pressure at 3/4-heigh of the specimen reach the initial effective pressure first, followed by that at mid-height of the specimen, and then that at the ends. Therefore, there is probable overestimate of the liquefaction resistance of the specimen with high fines contents in the ordinary dynamic triaxial test with pore water pressure measurements at the ends. This study obtain the correction ratios to adjust the liquefaction resistances obtained in the ordinary dynamic triaxial tests according to the permeability of the specimens.

並列關鍵字

fines contents ； liquefaction ； dynamic triaxial test ； volume change ； pore water pressure ； compliance ； measurements

參考文獻

[6] Prakash, S. and Sandoval, J. A. (1992), “Liquefaction of low plasticity silts, ” Soil Dynamics and Earthquake Engineering, Vol. 11, pp. 373-379.

[10] 葉俊麟，(2005)，「高細料土壤動力三軸試驗試體孔隙水壓激發之探討」，國立台灣大學土木工程學研究所，碩士論文。

[11] Kramer, Steven L. (1996), Geotechnical Earthquake Engineering , Prentice Hall, New Jersey.

[13] Head, K. H. (1986), Manual of Soil Laboratory Testing, Pentech Press, London, Vol. 3.

[15] Seed, H. B. (1979), “Soil liquefaction and cyclic mobility evaluation for level ground during earthquakes”, Journal of the Geotechnical Engineering Division, ASCE, Vol. 105, No. GT2, pp. 201-255.