| 摘要: | 摘 要 國內都會區捷運系統及下水道工程興建時，為減少對地面交通的衝擊，常採用隧道工法施工。然而在都會區進行隧道施工，周圍地盤往往有許多鄰近結構及地下維生管線。因此如何預測隧道開挖引致的地盤沉陷，評估其對鄰近結構物及地下維生管線之影響，就成為工程師關心的主要課題。另外，由於都市路權取得困難，上下行隧道通常併行而且相當靠近；因此兩隧道之互制行為及雙隧道開挖對周圍地盤之影響，值得進一步研究。 本研究在平面應變條件下分別以離心模型試驗和有限差分程式FLAC進行隧道開挖解壓變形之模擬，在隧道內施加支撐氣壓以維持隧道初始的穩定，然後逐步解除支撐氣壓來模擬開挖的過程。分別針對單隧道及左右併行雙隧道進行模型試驗及數值分析，藉由改變隧道的埋置深度和兩隧道的中心距離，探討單隧道及雙隧道的破壞機制，以及隧道開挖對周圍地盤變位、土壤應力及孔隙水壓之影響。另外，以15組國內外潛盾施工監測資料進行案例分析，驗證模型試驗和數值分析之預測成效。 根據試驗結果，考慮隧道埋置深度，建立土壤漏失量和地表沉陷之關係；若已知單隧道施工的土壤漏失量，便可由所建議的方法預測單隧道和雙隧道開挖所引致的地表沉陷。至於地下沉陷槽，考慮隧道所在深度，提出地下沉陷槽寬度參數隨著所在深度而遞減的關係式。統計經驗法之應用方面，以常態機率曲線迴歸單隧道地表沉陷槽求得沉陷槽寬度參數，將此沉陷槽寬度參數乘上0.85配合單隧道最大地表沉陷量，以兩個單隧道沉陷槽曲線疊加，可合理預測距徑比為1.5至2之雙隧道地表沉陷槽；對於距徑比大於2之雙隧道，建議直接以單隧道沉陷槽寬度參數配合最大沉陷量疊加。此外，以彈性理論分析隧道周圍地盤之應力變化，由塑性力學之上限理論推導單隧道和雙隧道之地盤破壞機制，和試驗量得之地盤位移方向相符。經過15組國內外潛盾施工監測資料之案例分析，由試驗結果所預測之地表沉陷和監測資料頗為接近，驗證了本研究所提出建議公式之預測成效。 數值分析部份，以FLAC程式建立和離心模型相同邊界條件與材料參數的數值模型，分析單隧道和雙隧道開挖引致的地盤沉陷及土壤應力變化。在隧道破壞前，數值分析所得橫斷面地表沉陷槽分佈型態和試驗量測結果大致接近。根據試驗量測之孔隙水壓反應和數值分析所得之地盤應力變化，繪製地拱效應的作用範圍，並以此解釋隧道周圍地盤應力的轉移機制及兩隧道的互制行為；若是在此作用範圍內有地下結構物，則必須考慮隧道開挖對地下結構物應力增量之影響。由試驗量測、數值分析及塑性理論求得超載係數之下限值與上限值，可用來評估隧道之穩定性。綜合離心模型試驗與數值分析成果，經過與現場監測資料相互驗證之後，提出預測單隧道和雙隧道施工引致地盤沉陷之方法，提供工程應用之參考。 Abstract In order to reduce the interference with the surface traffic, tunnelling methods have been widely used in the construction of metro and sewage systems in Taipei. However, these tunnels were driven through the ground where structures and lifelines already existed. Therefore, it is very important for engineers to evaluate the impacts of tunnelling on the environment, i.e. the effect of surface settlement on adjacent buildings and the influence of subsurface ground movements on underground pipelines. Besides, because the tunnels in cities are usually driven in limited areas, the down track tunnel and up track tunnel are parallel and very close to each other. Hence, it is necessary to evaluate the settlements induced by parallel tunnelling and the interaction between two tunnels. In this study, both centrifuge model tests and a finite difference program (FLAC) have been taken in plane strain conditions to investigate the tunnel stability and soil movements for both the single tunnel and parallel tunnels. A series of model tests was performed in centrifugal acceleration of 100g, including the single tunnel with various cover-to-diameter ratio (C/D) and the parallel tunnels with various C/D and distance-to-diameter ratio (d/D). The supporting air pressure was applied inside the tunnels to keep the tunnels in equilibrium during the accelerating stage, and then the supporting pressure was reduced gradually to simulate the process of excavation. From centrifuge modelling and numerical modelling, the ground movements and the changes of soil stresses and pore water pressures could be studied. Besides, 15 field cases of measured surface settlements due to shield tunnelling in Taipei were used to verify the predictions from centrifuge modelling and numerical modelling. From test results, the relationships of both the volume of ground loss versus the maximum settlement and the burial depth of tunnels versus the width parameter of the settlement trough have been proposed. Since the volume of ground loss could be evaluated prior to tunnel construction and the burial depth was determined in the preliminary design stage, the settlement troughs induced by single tunnelling and parallel tunnelling could be predicted. Using empirical methods, the settlement troughs can be reasonably fit by a Normal Distribution Curve suggested by Peck (1969). By superimposing two curves with a factor of 0.85 times the width parameter fit from the single tunnel, the settlement trough due to parallel tunnelling with d/D less than 2 may be estimated. For the parallel tunnels with d/D greater than 2, the simple superposition of two settlement curves from single tunnel is suggested. Besides, the collapse mechanisms of both single tunnel and parallel tunnels derived from upper bound theorem compared well with the observed ground movements. From 15 case studies in field, the predicted settlements show reasonable agreement with the field data. Hence, the prediction from proposed formula is verified. In addition, the program FLAC was carried out to solve the whole problem in the same material and boundary conditions as those in centrifuge tests. The surface settlement troughs computed from FLAC show reasonable agreement with results measured from model tests. According the pore water pressures measured from model tests and the soil stresses computed from numerical analysis, the extent of soil arching for single tunnel and parallel tunnels could be defined. It may be adopted to explain the transferring mechanism of soil stresses around tunnels and the interaction between parallel tunnels. If the underground facilities existed within the extent of soil arching, the influence of tunnelling to those structures should be taken into account. Based on test results, numerical modelling and theoretical solutions, the lower bound and upper bound of overload factors are determined for evaluation of tunnel stability. From test results verified by numerical modelling and field measurement, this study may provide reliable predictions of ground movements induced by single tunnelling and parallel tunnelling. |
