| 摘要: | 日本神戶港二座人造島 (港島與六甲島) 於阪神大地震時，因大規模的土壤液化而導致沉箱式碼頭的側移破壞情形，與台灣921大地震時，台中港1至4A號碼頭的破壞型式頗為相似。因此，藉由數值模擬分別針對日本和台灣兩案例進行研究，幫助了解港區沉箱式碼頭於地震作用下的破壞機制、穩定性和土壤液化範圍，及地表變形等現象，將有助於今後港灣結構之耐震設計和抗液化處理。 本研究所使用之數值模擬程式為有限差分連續體分析之套裝軟體FLAC，以有效應力法進行動態分析。研究範疇包含日本－神戶港和台灣－台中港兩案例之案例研究。首先進行兩案例之資料收集包括：a) 設計斷面圖；b) 現地及室內試驗；c) 地下水位面和地震記錄。依上述所收集之資料，分別進行兩案例之數值模擬。數值模擬共分十大步驟：1) 建立網格；2) 給予材料強度參數；3) 設定邊界條件；4) 加入界面元素並重力平衡；5) 施加海水之側向力；6) 指定地下水位面；7) 力學平衡；8) 使用Finn模式；9) 給予阻尼參數和動態邊界條件；10) 施加地震力。最後將兩案例之數值分析結果，與現地觀察量測值、前人之物理模型試驗和前人之數值模擬結果一一作比較，以分別探討其破壞機制。 由數值分析結果顯示：港島及六甲島和台中港的沉箱式碼頭的主要破壞機制皆為碼頭背填區土壤液化，累積大量超額孔隙水壓，產生水平推力使碼頭有往海側移、沉陷或傾斜的破壞。而孔隙水壓和有效應力部分：a) 遠離沉箱的背填砂區所激發的超額孔隙水壓，較沉箱底部土壤所激發的超額孔隙水壓高；b) 鄰近沉箱的背填砂區有效應力不為零，而是離沉箱較遠處的背填砂區，有效應力值幾乎為零，已達液化狀態。 During the Kobe Earthquake of magnitude 7.2 occurring in 1995, intense liquefaction resulted into the lateral spreading of caisson type quay walls in two artificial islands (Port & Rokko Islands). This type of failure is quite similar to that of Piers #1 to #4A in Taichung Harbor during the 921 Chi-Chi Earthquake in 1999. Accordingly, numerical study of these two failure cases in Kobe and Taichung is very beneficial to help identify the failure mechanism, stability, liquefied zone and ground deformation of backfill behind caisson type quay walls during severe earthquake. The FLAC 3.2 is the main analysis tool in this study, including a dynamic analysis module. Prior to numerical analysis, the basic data of the above two cases (composed of quay wall cross-section design diagram, in-situ and laboratory test results, groundwater level, earthquake records, and damage document) are compiled. The general procedures of numerical modeling for each case include generating geometric mesh for the port site, assigning material parameters, setting up boundary conditions, adding interface elements and turning on gravity, applying lateral water pressure, leveling groundwater table, checking mechanical equilibrium, using Finn mode, setting dynamic damping and dynamic boundary conditions, exerting earthquake loading, and monitoring the variation in displacement and pore water pressure. The analysis results of both two cases are compared with field observations, and those of shaking table tests and numerical analyses performed by other researchers. The numerical simulation results of this study show that the failure mechanism of both two cases in Kobe and Taichung is due to liquefaction of backfill (a hydraulic sand fill) during earthquake, the same as that found in literature. The increasing excess pore water pressure in the backfill produces large lateral pulse acting on the caisson, leading to its lateral spreading, rotation and settlement. The excess pore water pressure stimulated in the backfill is higher than that beneath the caisson. The effective stress of soil just behind the caisson does not reach zero during shaking, but the further inside portion of the backfill is liquefied. Learning from this study can provide an insight to understand the interaction between quay wall and backfill during strong ground motion, as well as a future guideline to design a caisson quay wall and soil system sensitive to liquefaction damage. |
