| 摘要: | 在地下空間高度發展的都會區興建高樓建築時，地基開挖可能對鄰近既存隧道產生影響。開挖過程中土壓力解壓和土壤沉陷等因素可能威脅隧道結構的穩定性和安全性。因此，在施工前需要詳細評估和規劃，確定合適的施工方法和措施，以減少對隧道結構的影響。目前台灣的施工規範缺乏對既存隧道鄰近地區開挖工程的具體指引，因此多數案例會使用數值模擬分析開挖工程對隧道和周圍環境的影響，並應配合監測計畫，在施工過程中監控隧道的安全，調整施工方法或進行地盤改良等補強措施，以確保整體工程的安全性。總之，了解開挖工程對既存隧道的影響是未來都會區地下交通網路和高樓建築發展中不可或缺的課題之一。本研究使用有限元素軟體PLAXIS進行數值模型模擬，並配合離心模型試驗結果，對數值模型輸入參數進行驗證。針對相對密度為70%的乾砂地盤，評估既存矩形隧道在不同影響因素下受鄰近開挖工程影響的情況，影響因素包含隧道與擋土壁之距離、隧道覆土厚度以及擋土壁厚度等。
結果顯示，隨著隧道與擋土壁間距離以及隧道覆土厚度的增加，隧道的位移及旋轉角皆會隨之減小。表示當開挖工程距離隧道越遠或隧道的覆土厚度較深時，隧道受鄰近開挖工程影響較小；反之，開挖工程距離越近或隧道覆土厚度較薄時，則需要更多評估開挖工程可能造成的影響，或者採取補強工法保護隧道安全。根據模擬結果可通過從開挖工程擋土壁底部延伸的保守破壞面和Rankine主動破壞面，將擋土壁後區域分為低程度影響範圍、中等程度影響範圍和高程度影響範圍。當隧道位置位於保守破壞面外時，隧道的水平位移、垂直位移及旋轉角分別小於2 mm、1 mm及0.01度，在此為低程度影響範圍。在保守破壞面及Rankine主動破壞面之間為中等程度影響範圍；在Rankine主動破壞面內為高程度影響範圍。此外，以厚度為0.8 m及0.4 m的擋土壁為例，由於擋土壁之勁度降低至1/8，因此擋土壁本身產生更大的側向位移量，導致地表沉陷加深並增加隧道的位移和旋轉量。相較於厚度0.8 m之擋土壁，厚度0.4 m之擋土壁模型中擋土壁的最大側向位移增加了24%，隧道的水平位移、垂直位移及旋轉角則分別增加了29%、20%及27%，接近擋土壁側向位移增加量。;In the urban arear, the near-by basement excavation may be able to affect the existing tunnels during the construction of high-rise buildings. The stability and safety of the tunnel would be threatened by the soil pressure relief and soil settlement due to the near-by excavation. To minimize the effects on the existing tunnel by the near-by excavation, the detailed assessment and planning are required to design the appropriate methods before construction starts. Currently, Taiwan′s construction regulations lack specific guidelines for excavation projects near existing tunnels. Therefore, most cases would adopt numerical simulation to analyze the effects of excavation on the existing tunnels. To ensure the safety of tunnels, the construction methods should be adjusted or the reinforcement measures such as ground improvement should be applied if it is necessary according to the monitoring data from the tunnels. Understanding the effects of the near-by excavation on existing tunnels is essential for the development of underground transportation networks and high-rise buildings in future urban areas.
In this study, the finite element software PLAXIS was used for numerical modeling, and the input parameters of the numerical model were validated using centrifuge model test results.
Several factors such as the distance between the tunnel and retaining wall, the thickness of overburden soil on the tunnel, and the thickness of the retaining wall were selected to evaluate the effects of the near-by excavation on the existing tunnel in the case of dry sandy ground with relative density of 70%.
The results show that as the distance between the tunnel and the retaining wall increases, as well as the thickness of the overburden soil on the tunnel, the displacement and rotation angle of the tunnel decrease. This indicates that when the excavation is farther from the tunnel or when the overburden soil on the tunnel is thicker, the tunnel is less affected by the near-by excavation. Conversely, when the excavation is closer or the overburden soil on the tunnel is thinner, more evaluation is needed to assess the effects of the excavation on the tunnel, or reinforcement measures should be taken to protect the safety of tunnel.
Based on the results, the back area of the retaining wall can be divided into three area: low-level impact area, medium-level impact area, and high-level impact area. This can be achieved by considering the conservative failure surface extending from the bottom of the retaining wall and the Rankine active failure surface associated with the excavation engineering. When the tunnel is located outside the conservative failure surface, the horizontal displacement, vertical displacement, and rotation angle of the tunnel are less than 2 mm, 1 mm, and 0.01 degrees, respectively, indicating a low-level impact area. The area between the conservative failure surface and the Rankine active failure surface represents a medium-level impact area, while the area inside the Rankine active failure surface indicates a high-level impact area.
Furthermore, the retaining wall with thicknesses 0.4 m has larger lateral displacement than the wall with thicknesses 0.8 m due to the reduction of the stiffness of the retaining wall. This resulting in increased ground settlement and increased displacement and rotation of the tunnel. Compared to the retaining wall with a thickness of 0.8 m, the maximum lateral displacement of the retaining wall in the model with a thickness of 0.4 m increased by 24%. The horizontal displacement, vertical displacement, and rotation angle of the tunnel also increased by 29%, 20%, and 27%, respectively, approaching the increase in lateral displacement of the retaining wall. |
