以動態離心模型試驗模擬沉埋隧道上浮機制 ; Dynamic centrifuge modeling on uplift mechanism of immersed tunnel

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Title:	以動態離心模型試驗模擬沉埋隧道上浮機制;Dynamic centrifuge modeling on uplift mechanism of immersed tunnel
Authors:	莊汶雅;Wen-Ya Chuang
Contributors:	土木工程研究所
Keywords:	上浮;沉埋隧道;液化;振動台;離心模型試驗;centrifuge modeling test;shaking table;liquefaction;immersed tunnel;uplift
Date:	2010-12-23
Issue Date:	2011-06-04 14:54:27 (UTC+8)
Abstract:	隧道建構於高液化潛能之砂土層時，其周圍土壤受到強震作用會發生液化現象，隧道周圍土壤受到超額孔隙水壓力激發之影響，而喪失對隧道的束縛效果，進而使得隧道在強震作用下而上浮。台北-板橋鐵路地下化路段，因鐵路地下化隧道採明挖覆蓋的施工方式進行，且隧道處於有液化潛能之土層中，上方覆土較少，當隧道周圍土壤遭受強震而液化時，可能產生上浮破壞的現象。本研究利用地工離心振動台試驗，於80g離心重力場情況下，模擬飽和砂土受振液化與沉埋模型隧道於液化土層之受振行為。試驗過程量測沉埋隧道上浮量及隧道周圍土壤的超額孔隙水壓力與加速度歷時反應，藉此深入探討沉埋隧道上浮機制。根據研究結果顯示，當土體受振時激發超額孔隙水壓，使砂土顆粒間以及砂土隧道間摩擦力降低，產生隧道上浮。此外根據水力梯度分析結果顯示，土體振動時所激發之超額孔隙水壓，會因有效覆土應力不同而有所差異，故所激發之超額孔隙水壓有所差異，使產生水頭差，造成隧道下方土層會有往隧道底部垂直向上之滲流力，以及隧道側方土層會有往隧道底部之水平滲流力，將液化或接近液化浮動之砂土往隧道下方流動並擠壓，造成隧道上浮。由試驗得到之超額孔隙水壓力，可代入日本道路協會所提出之抗上浮安全係數(FS)評估方程式，經計算結果顯示，當安全係數小於0.98時隧道開始產生上浮，FS於0.74~0.98間時隧道會持續上浮。 Saturated loose sand may liquefy during strong earthquakes. Floating of embedded tunnel due to the lighter unit weight of the tunnel during the surrounding soil liquefaction may cause severe damage of tunnel. A series of dynamic centrifuge model tests was conducted in order to investigate uplift behavior of tunnel in the liquefiable sand during 1-D shaking. The model tunnels used in the study have different unit weights and are embedded in two different embedment depths and in the different sand beds saturated with viscous fluid and water, respectively. Four accelerometers are instrumented in the model tunnel to measure the seismic response of tunnel. A dense array of accelerometers and pore water pressure transducers are also installed to measure the seismic response and the generation of pore water pressure in the surrounding soil during shaking. In addition, several LVDTs are used to measure the uplift of model tunnel, the surface settlement, and the lateral displacements on the wall of laminar box. Several colored sand layers are placed at various depths for observing the ground deformation after the tests. According to the analysis of model test results, the following conclusions are made: (1) The magnitude of tunnel uplift is significantly influenced by the viscosity of pore fluid and the embedded depth of tunnel. The tunnel will experience the less uplift if the tunnel is embedded in the deeper depth and in the less viscous pore fluid. (2) Once the tunnel begins floating the liquefied sand will squeeze into the tunnel bottom due to high seepage forces form the outside of tunnel and below the tunnel toward the bottom of tunnel. (3) Once the Safety factor against uplift (FS) calculated with the proposed method is less than 1 the tunnel will start floating during shaking.
Appears in Collections:	[Graduate Institute of Civil Engineering] Electronic Thesis & Dissertation

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