The Shear Strain Analysis of Large Sand Specimen in Shaking Table Test

本研究共分兩部份進行，一為動力三軸試驗，一為振動台大型剪力盒試驗。在動力三軸試驗方面，試體準備方式採用多重篩霣降法，以兩種不同相對密度51 %及82 %進行頻率0.1 Hz的反覆荷重試驗，讓試體在達到初始液化後，持續以反覆荷重作用，觀察初始液化後剪應變、孔隙水壓及再壓密的體積變化情形。在大型振動台剪力盒試驗方面，試驗盒採用多層框架堆疊而成，使土壤試體能如現地水平土層隨地震波作用而變形，模擬實際地震剪力波在水平土層中傳遞的情形。以濕沉降法準備試體，振動台進行單向及多向振動試驗，其振動振幅0.03 g至0.1 g，振動頻率包括1、2、4、8 Hz，振動延時5、10、20及30秒，並利用位移計、水壓計及加速度計等，觀察試體受振時不同深度剪應變的變化情形及其與量測數據間之關係。 在動力三軸試驗結果中，發現緊密砂土在液化後繼續承受反覆荷重作用下，試體的再壓密體積應變與試體上部直徑縮小區段的(necking)長度有關，疏鬆砂土液化時常是整個試體高度都是直徑縮小區段，故其直徑縮小區段沒有意義。在振動台大型剪力盒試驗中，發現液化層內液化後剪應變振幅在振動過程中會逐漸增加，使砂土試體軟化現象更加嚴重。也發現液化後的剪應變之峰谷值隨振動時間增加而逐漸減少，是由於試體受振過程中有排水，有效應力增加，土壤勁度回復所造成，且這種現象皆發生在液化層與非液化層交界附近。振動台大型剪力盒試驗振動過程中，液化深度會有逐漸減少的現象。在振動台大型剪力盒雙向振動試驗方面，發現液化後剪應變的向量和隨振動時間增加而逐漸減少的過程中，同高程超額孔隙水壓下降的速度較單向振動緩慢，可能是由於剪動方向影響。求出剪力盒各次試驗最大剪應變與再壓密體積應變，發現最大剪應變與再壓密體積應變無明顯關係。There are two types of tests carried out in this study. One is the cyclic traixial test, another is the large scale shear box test on the shaking table. In the traixial test, we prepared the sand specimen with relative density of 51% and 82% by dry pluviation. We continued the cyclic loading on the satured specimen after initial liquefaction until the given axial strain limit to observe the shear strain, the pore water pressure and the reconsolidation volumetric strain. In the shaking table test, the soil sample deformed according to the shear waves induced from the shaking table motions. We prepared the sand specimen by the wet sendimention method. We input acceleration amplitudes from 0.03 g to 0.15 g, frequence of 1 Hz, 2 Hz, 4 Hz and 8 Hz and duration of 5 sec, 10 sec, 20 sec and 30 sec. In the cyclic traixial test, we find the reconsolidation volumetric strain is related to the necking length on the upper part of the specimen after initial liquefaction of denser specimens. But for the looser specimens, because the reduction of diameter occured in the whole specimen length, the necking length of loose specimen becomes unmeaningful. In the shaking table test, the increase of shear strain amplitude with time after initial liquefaction implies the sand specimen softened further. At the interface between liqufied and non-liquefied soil layers, we find that the shear strain decrease with time after initial liquefaction resulting from the drainge which leads to the soil stiffness recovery. We also find the interface between the liqufied and non-liquefied soil layer will gradually shift upward. In the multi-directional shaking table test, when the combined shear strain decreases with time, the pore water pressure reduces slower than that in the single directional shaking table test. We also find the maximum shear strain after initial liquefaction during vibration have no obvious relationship with the re-consolidation volumetric strain.誌謝....................................................Ⅰ Abstract................................................Ⅱ 摘要....................................................Ⅲ 目錄....................................................Ⅳ 表目錄..................................................Ⅶ 圖目錄..................................................Ⅷ 第一章 前言.............................................1 1-1 研究動機與目的...................................1 1-2 研究內容.........................................1 1-3 各章節涵蓋內容...................................2 第二章 前人研究 2-1 土壤受振後的沉陷行為.............................3 2-2 土壤受振剪應變之影響.............................4 第三章 試驗設備與操作流程...............................25 3-1 試驗土樣與基本物理性質..........................25 3-2 動力三軸試驗....................................25 3-2-1 試體準備......................................26 3-2-2 試體飽和......................................27 3-2-3 試體壓密......................................28 3-2-4 液化強度試驗..................................28 3-3 大型剪力盒試驗..................................28 3-3-1 試驗設備......................................28 3-3-1-1 雙軸向剪力試驗盒............................28 3-3-1-2 大型砂土霣落箱..............................29 3-3-1-3 量測儀器....................................30 3-3-2 量測儀器安裝與試體準備........................30 3-3-3 試體飽和測定..................................31 3-3-4 振動台振動試驗................................32 第四章 試驗資料之整理...................................42 4-1 動力三軸試驗.....................................42 4-1-1 液化後的動力三軸試驗..........................42 4-1-2 試體上部直徑縮小的區段之量測..................42 4-2 振動台大型剪力盒試驗.............................45 4-2-1 試體內部水壓計之量測..........................46 4-2-2 試體內部與框架加速度計之量測..................46 4-2-3 土層液化深度之判定............................47 4-2-4 框架上位移計的量測與剪應變計算方式............47 第五章 試驗結果分析與討論...............................71 5-1 動力三軸試驗結果.................................71 5-1-1 液化曲線......................................71 5-1-2 液化後的體積變化情形..........................71 5-2 振動台大型剪力盒試驗結果..........................72 5-2-1 不同的液化後剪應變變化對超額孔隙水壓激發之影響72 5-2-1-1 維持等振幅變化的剪應變.....................72 5-2-1-2 隨時間增加而逐漸增加的剪應變...............73 5-2-1-3 隨時間增加而逐漸減少的剪應變...............74 5-2-1-4 非液化土層中的剪應變行為...................75 5-2-2 不同深度的剪應變對超額孔隙水壓之影響..........75 5-2-3 雙向振動剪應變行為............................77 5-2-4 液化後剪應變對再壓密體積應變的影響............78 第六章 結論與建議......................................118 6-1 結論............................................118 6-1-1 動力三軸試驗.................................118 6-1-2 振動台大型剪力盒試驗.........................118 6-2 建議............................................119 參考文獻...............................................12