以離心模型試驗模擬沉箱式碼頭之受震行為

Centrifuge modeling on seismic responses of caisson type quay wall during earthquakes

蔡晨暉(Chen-Hui Tsai)

指導教授 ： 李崇正

國立中央大學/工學院/土木工程學系/碩士(2011年)

摘要

沉箱式碼頭於受震後，常因背填土壤液化以及自身慣性力，使沉箱朝海側方向產生側向位移，導致碼頭後方地盤的沉陷與滑動、港埠設施的損壞以及營運作業長時間的停擺，造成極大的經濟損失。本研究以離心模型試驗模擬沉箱式碼頭受震反應，探討沉箱的兩種振動模式，水平位移模式與旋轉模式，與周圍土壤之間的箱-土動態互制行為。
本研究以台中港3號沉箱碼頭原型尺寸的40%，設計八十分之一縮尺的沉箱碼頭模型，於80g離心力場下進行試驗。試體以石英矽砂霣降至本研究設計之固壁式蜂巢試驗箱而成，試體內部不同位置安裝加速度計、孔隙水壓計、土壓計，分別量測各項歷時。試體表面安裝線性差動變壓器(LVDT)，量測沉箱垂直、側向位移與後方背填土的地表沉陷歷時。另外，試體內部會擺設水平色砂層與土層變位計，觀察受振後的地盤變形。
研究結果顯示：(1)沉箱的水平位移模式主要影響沉箱後方淺層土壤的孔隙水壓變化，當沉箱愈朝海側移動則孔隙水壓激發量愈小；(2)沉箱的旋轉模式會影響沉箱下方基礎層土壤的孔隙水壓力變化，其正負交替的超額孔隙水壓力會使該區呈現抽吸的效應，造成大量背填土朝海側移動，引致沉箱後方顯著的沉陷量；(3)兩種振動模式的相位關係會影響旋轉中心的變化，兩者為反相關係時，沉箱朝陸側位移時會朝海側旋轉，並使旋轉中心朝陸側方向發展；(4)藉由Wolf(1988)所提出公式可預估沉箱運動模式的主頻值，其計算結果與試驗數據經由FFT之後的值相近。

關鍵字

沉箱式碼頭 ； 沉箱振動模式 ； 固壁式蜂巢試驗箱 ； 超額孔隙水壓 ； 主頻 ； 沉箱旋轉中心

並列摘要

The backfill liquefaction behind the caisson type quay wall and the inertial force of the wall may cause the horizontal seaward displacement during earthquakes. The lateral deformations and large surface settlements on the service area caused the severe damage of port facilities and led to large economical lost. A series of centrifuge model tests was conducted to simulate the dynamic response of caisson type quay wall, focusing on the investigation of the characteristic of vibration modes of quay wall and the dynamic soil-wall interaction. There are two types of vibration modes, i.e., the translation mode and the rotation mode.
In this research, a centrifugal scale-down model was specifically designed and tested at 80 g. The 40% dimension of No.3 quay wall in Taichung harbor was treated as the prototype. The test sand bed was prepared by pluviating quartz sand into the new designed rigid box. Several accelerometers, pore water pressure transducers, and earth pressure cells were instrumented in the quay wall model and backfill. LVDTs were also mounted at the surface of the backfill and on the quay wall, in order to record the displacement histories of the surface settlement and of the quay wall during shaking. Besides, the horizontal colored sand and ground displacement meter were put inside the test model to observe the ground deformation after shaking.
The test results draw the following conclusions: (a)The translation mode dominantly affects the variation of pore water pressure in the shallow layer of soil behind the quay wall, and the excess pore water pressure decreased as the increase of seaward movement of quay wall. (b)The rotation mode dominated the fluctuation of pore water pressure readings beneath the quay wall foundation, and caused drainage from backfill into sea region. Because of the positive and negative pore water pressures alternatively change results in the pumping and suction effects which makes the large surface settlement happened in backfill area.(c) The phase difference between those two vibration modes influences the location of rotation center of quay wall. Quay wall is moving toward land direction and rotating toward sea when it shows out of phase relation between translation mode and rotation mode. Meanwhile, the position of rotation center will be changed toward backfill.(d)The predominant frequency of two vibration modes can be predicted by the formula which was proposed by Wolf (1998), and the calculated values consist with the experimental results.

並列關鍵字

rotation center of quay wall ； excess pore pressure ； vibration modes of quay wall ； rigid box ； predominant frequency ； caisson type quay wall

參考文獻

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