集集地震中因土壤液化所造成之地盤沉陷、側向流動處處可見，例如：霧峰鄉乾溪河岸太子城堡社區因地盤流動而嚴重滑移及傾斜。土壤液化後所產生地盤變位是地震災害中最常見也最具破壞性之地變型態，地盤液化後所產生的永久變位，受液化土層殘餘(臨界)強度所影響，因此，有必要瞭解土壤液化後產生流動時的殘餘強度，進而利用Newmark剛性塊滑體模式推估所產生的水平地盤永久變位。 本研究探討沉泥質砂與煤灰於壓密、受剪過程中的力學行為，並建立台灣地區液化層的SPT-N值與不排水臨界強度比之關係，期望能夠對地震所造成之液化行為更瞭解，進而利用於災害防治與工程設計上。土壤的不排水臨界強度為液化後穩定分析的主要參數，臨界強度可經由液化案例反算求得，但反算之臨界強度因假設液化後產生流動時可能發生排水的行為，所以強度較高。因此，使用實驗室不排水試驗推估臨界強度，在液化的穩定性分析上較為保守。 Chi-Chi earthquake was intensity led to severe damage due to soil liquefaction in Wufeng, an area beside the western foothills of the island. The post-liquefaction shear strength of sands, called the undrained critical strength. The stability of the original slope configuration at the instant of liquefaction is determined by the undrained critical strength of liquefied sand. The results from an experimental study on silty sands are presented and evaluated in view of the framework of critical-state or steady-state soil mechanics. Strain-controlled, consolidated-undrained triaxial compression tests were performed on silty sands and coal ashes on reconstituted soil samples. Silty sands are the most common type of soil involved in both static and earthquake-induced liquefaction. This conclusion is based upon an extensive review of the literature. For seismic stability analysis of an existing slope ,drainage of the slide mass during post-liquefaction flow cannot be assumed .Therefore, a value of undrained critical strength (Su) corresponding to constant volume or an undrained condition and the original slope geometry must be used in stability analysis. A technique for estimating undrained critical strength ratio using (N1)60-CS was sought from undrained laboratory test results. Newmark’s rigid block sliding model is used to estimate the permanent horizontal displacement with the steady state strength parameter obtained from laboratory.