大型振動台飽和麥寮砂受振行為之研究

Abstract

麥寮砂為台灣西部海岸沖填工業區之典型砂土，不但含有相當高的細料含量，而且具有高壓縮性，因此工程性質與受振後的行為與純淨石英砂有所不同。國家地震工程研究中心以麥寮砂來進行大型振動台試驗，並與2000年到2005年間所進行之純淨越南砂振動台試驗結果比較。 麥寮砂試體於準備完成後進行壓力波波速量測，經量測結果發現試體內部傳遞波速與水中傳遞波速相近，所以試體飽和度佳。另外由薄管取樣所得乾密度與細料含量分佈圖得知，整體而言乾密度值差異亦不大，且除了表面薄層細料含量較高以外，其餘底下差異並不大。故利用分階濕砂水中沉降法準備大型試體有不錯的飽和度與均勻性。 經由試體內部埋設水壓計之反應得知，麥寮砂振動後其消散時間比純淨越南砂還要長大約15倍左右。從滲透試驗得知兩者的滲透性也同樣相差15倍左右。由此得知滲透性對振動後孔隙水壓的消散確實有明顯的影響。同時本研究也整理歷次麥寮砂大型振動台試驗之體積應變，結果顯示麥寮砂土在受振液化後，體積應變只與相對密度有關而與振動頻率、振動加速度無關，並隨著相對密度的增加而減少。由此結果可觀察麥寮砂與越南砂試驗液化後之體積應變與相對密度關係，發現麥寮砂在相對密度45 ~ 70 %時體積應變比越南砂高，而在相對密度高於70 %時兩者之間則差異不大。

Mailiao silty sand is a typical soil of the reclaimed lands in the western coastal area in Taiwan. Shaking table tests with various acceleration amplitudes, frequencies, durations, and shaking directions were performed on the saturated Mailiao silty sand inside the large laminar shear box at National Center for Research on Earthquake Engineering (NCREE). The test results were compared with those obtained in the previous shaking table tests on Vietnam clean sand. After sample preparation of Mailiao sand, the results of P-wave velocity measurement confirmed the saturation of the sand specimen. The dry density and fines content obtained by thin-walled tube sampling showed no significant difference within the sand specimen, except a thin layer of high-fines-content sand on the top of the sand specimen. According to the measurements by the mini-piezometers, the pore water pressure dissipation of Mailiao silty sand is about 15 times longer than that of Vietnam clean sand. The permeability test results also showed that the ratio of permeability of Vietnam sand to that of Mailiao silty sand is also approximately 15. It appears that the permeability have a significant effect on pore water pressure dissipation after shaking. The results of shaking table test of Mailiao silty sand, showed that if liquefaction occurred, the volumetric strain is only related to relative density regardless of the acceleration of vibration, frequency of vibration or direction of vibration. Volumetric strain decreases with increase of relative density of the specimen. The test results were compared with Vietnam clean sand, it showed that the relative density of the volumetric strain of Mailiao silty sand were higher than that of Vietnam clean sand in Dr = 45 ~ 70 %, but there is no significant difference between Mailiao silty sand and Vietnam sand after Dr = 70 %.