- 學位論文
含單粉土層之砂土地盤於強震作用下之受震反應
SEISMIC RESPONSE OF A SANDY STRATUM WITH A SILT LAYER UNDER STRONG GROUND MOTIONS
摘要
具液化潛能的砂土層中有可能存在一層滲透係數低的粉土層，於地震作用下在粉土層底部會產生具有高孔隙水壓的水膜。從大地工程的觀點而言，此一水膜會造成地層，尤其是傾斜地層，的滑動。本研究的目的在利用數值模擬的方式闡明P波和S波抵達時間差，具不同特性之地震動以及粉土層在某一時間發生開裂等對含有單一粉土層且具液化潛能的砂土層之受震反應的影響。 本研究採用三維有效應力有限元素程式。首先以離心機試驗的結果進行驗證該程式的正確性，然後建立了14組分析模式，其中八組用來了解P波和S波抵達時間差和具不同特性之地震動的影響，剩下的六組則是用來探討粉土層在某一時間發生開裂時對含有單一粉土層且具液化潛能的砂土層之受震反應的影響。本研究使用了三個真實地震的歷時記錄和一個在離心機試驗箱底部量測到的諧和震動歷時。分析的結果以地表的水平位移與沉陷以及孔隙水壓呈現。 對於三維土層模型受到單一方向的地震作用時，諧和地震歷時會造成比實際地震較大的反應，顯示以諧和地震歷時來預測反應時會比較保守。粉土層的開裂會使該土層底部的高超額孔隙水壓以較快的速率消散而導致較大的沉陷。粉土層的開裂也會造成上面土壤的超額孔水壓突然增加，而下面土壤的超額孔水壓突然降低，如此使得粉土層上面的土壤的超額孔隙水壓會較沒開裂者為大，也因此有可能使原本粉土層沒開裂時不會液化的上層土壤液化，至於粉土層下面的土壤則呈現相反的趨勢。此外，與不開裂的情形比較，粉土層的開裂會使得粉土層底部的土壤的超額孔隙水壓散速率增快，但此速率隨深度而遞減。
並列摘要
The presence of silt layer with small permeability may exist in the liquefiable sandy ground and can produce the water film beneath silt layer with high pore water pressure under earthquakes. From the geotechnical point of view, the water film can cause instability of ground especially for slope ground. The objectives of this study is to clarify the effects of interval of P-wave and S-wave arrival, input motions with different of characteristics and crack inside the silt layer at certain time on the seismic responses of ground of liquefiable sand stratum with a silt layer through numerical simulations. A nonlinear 3D effective stress finite element program was used in this study. Its validity was first validated by comparing with centrifuge results. Then, a total of 14 models were constructed; eight of the models were used to gain a better understanding the effect of interval of P-wave and S-wave arrival and input motions with different characteristics and the remaining six models were used to investigate the effect of possible crack inside silt layer at certain time on the seismic responses of ground of liquefiable soil sand stratum. Three real earthquakes with different characteristics and one harmonic loading measured in a centrifuge test were used in this study. Horizontal displacement and settlement on the surface and excess pore water pressure were presented for all models. For 3D model with 1D input motion, in general, the response behavior of liquefiable soil stratum by using Harmonic input is much larger than that by using the real earthquakes, meaning that the prediction by using Harmonic input is very conservative. The crack in the silt layer can lead to the larger settlement due to the faster dissipation of EPWP beneath the silt layer and the breakage of silt layer can lead to the sudden decrease in EPWP in the soil beneath the silt layer and sudden increase in EPWP in the soil above the silt layer; such a phenomenon may cause the soil above the silt layer to have the larger EPWP and that below the silt layer to have smaller EPWP. Sometimes the upward movement of pore water may cause the soil to liquefy, which will not occur without the breakage of silt layer. The crack in the silt layer leads to the faster dissipation of EPWP below the silt layer; such faster dissipation progresses from the location beneath the silt layer to the bottom of the soil stratum.
參考文獻
延伸閱讀
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