地震所引致反覆剪力，會使飽和砂土激發超額孔隙水壓，造成土體剪力強度改變。為深入瞭解砂土受剪後之力學行為，本研究利用放置於三軸室試體上、下蓋內的一對彎曲元件(bender elements)，藉元件壓電特性，於緩慢剪動飽和砂試體同時，在擾動極小的情況下量測傳遞經試體之剪力波波速(Vs)。掌握剪力波速的變化，即可由彈性波傳理論計算土體隨應力狀態改變而演化的重要動態參數最大剪力模數(Gmax)。
　　本研究首先將彎曲元件作適當的防水及絕緣措施，以利進行飽和砂的三軸壓密不排水試驗，並降低接收波形雜訊及電磁干擾。為瞭解彎曲元件的性能，先於等向壓密階段，測試不同激發波形及頻率對接收訊號之影響，以選擇最適用的激發波，並建立波傳時間判定準則。爾後以此準則計算三軸試驗各階段所量測之砂土剪力波速。
　　研究結果顯示，本研究配置下最適激發波為頻率12kHz至15kHz之單一週期正弦波；試體受到等向圍壓時，剪力波速與圍壓在雙對數座標下呈現斜直線分布，等向圍壓越高砂土剪力波速越高；試體剪動階段，藉超額孔隙水壓激發程度與剪力波速之關係，可看出波速不同於等向壓密階段只受等向圍壓的影響，試體受剪時，軸向壓縮會使剪力波速增高、軸向伸張則使剪力波速降低。另外，剪力波速與有效平均主應力的改變呈正相關，故利用回歸分析可得到以孔隙比與有效平均主應力為參數之剪力波速預測公式，預測與實測值接近且趨勢相似，僅在軸向伸張至主應力比小於0.5時實測值出現明顯低於預測值之情形。

The cyclic shear stress induced by earthquakes would increase the excess pore water pressure and decrease the soil strength in a saturated sand deposit. For investigating the mechanism of sand during shearing, a pair of bender elements was mounted in both the cap and pedestal of the triaxial apparatus. Using the bender elements can measure the shear wave velocity while the saturated sand specimen was subjected to slow shearing. Therefore, the maximum shear modulus,Gmax, one of the key parameters for predicting dynamic behavior of soil, can be evaluated by the measured shear wave velocity (Vs) during the consolidated-undrained triaxial test at different stress paths.
Before the test, the bender elements were waterproofed, shielded, and grounded in order to work in the saturated sand specimens and to reduce electromagnetic interference of received signals. Besides, for knowing the properties of bender elements and determining the criterion of arrival time of the measured wave, three kinds of exciting waveforms and different ranges of exciting frequencies were examined at the consolidation stage in advance.
The test results showed that the most suitable transmitting signal was a single sine pulse with frequencies between 12 kHz to 15 kHz. The increase in the isotropic confining pressure increased the shear wave velocity. Nevertheless, during shearing, the shear wave velocity was affected by not only the effective confining pressure but also the stress state on the soil specimen. By the regression analysis, an empirical equation was proposed to predict the shear wave velocity based on the void ratio and the effective mean stress of soil specimen. The predicted values were closed to the measured ones when the stress ratio was greater than 0.5 and less than 3.

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