營運中隧道變位模態解析與高精度監測技術之研究

Abstract

隧道的分析理論肇始於對彈性介質中存在孔洞時周圍應力應變組成關係之探討，由古典力學封閉解開始，演變至閉合收斂曲線概念後，隧道開挖的理論已逐漸臻成熟。這些既有理論大部分係歐美國家所提出，其地質條件優良，隧道損壞之報導多為開挖階段所發生，建成之後少有異狀出現。故一般皆認為隧道完工後便趨於穩定，對隧道力學行為的了解僅完善至完工為止，關於隧道營運之後出現的異狀，現仍無嚴謹的分析理論提出。 在如今國際間廣泛推動工程結構永續經營的氛圍下，隧道工程勢必將朝永續發展前進，因此掌握隧道全生命週期之力學行為變得極為關鍵，而其中隧道營運後長期安全性和穩定性的評估，是亟需補足的一環。受板塊運動頻繁、岩石膠結相對較差等因素影響，針對營運中隧道行為之研究目前以我國和日本較多。相關研究多係假設特定造成隧道安全性下降的因子，如岩體參數弱化、隧道結構劣化，或者外力變化，以模型試驗或數值模擬探討因子對隧道的影響；另有一類乃是蒐集大量隧道案例，將隧道異狀定性分類，透過監測資料確定案例異狀肇因後，由統計理論或經驗推斷歸納隧道異狀與肇因的關聯性。然既有方法與評估營運中隧道結構安全與力學行為之需求間仍存在落差，如何有效定義隧道結構狀態，係目前隧道永續發展之門檻。 本研究由隧道變位著手，開發一隧道變位分析方法，提供特殊的變位模態，用以解析隧道營運後複雜的變位。營運中隧道多變而繁複的異狀與變位一向為了解其行為的重要指標，也是困擾隧道工程師與研究者已久的問題。將隧道二維斷面變位視為一個向量，假定該向量可以拆解為每個監測點上產生單位變位的隧道變位之和，以有限元素軟體為工具獲取監測點上產生單位變位的隧道變位後，通過矩陣和向量推導，本研究提出物理意義明確、彼此間相互獨立正交的變位模態，稱之為特徵模態。模態中包含斷面整體平移、旋轉，以及各種隧道純變形行為如純剪、三角變形、方形變形、五角變形等，當監測點數增加，解析能力亦隨之提升。對於圓形斷面、橢圓形斷面和馬蹄形斷面之分析結果顯示不同斷面形狀有其相應之特徵模態，橢圓形斷面和圓形斷面隧道的特徵模態形態相近，但馬蹄形隧道則在左右側壁下部與仰拱之間有較特殊的形態出現。岩體彈性模數的上升則使得特徵模態徑向方向分量變小。 有鑑於現今隧道變位監測技術囿於精度不足、斷面監測點不夠、無法獲取監測點絕對坐標以致未能有效取得隧道完整變位，本研究與台北科技大學王泰典教授及銢欣公司合作研發高精度量測技術－隧道襯砌微變監測技術，針對營運中隧道之需求，整合衛星定位控制測量、導線控制測量、路線測量暨回歸計算與襯砌三維絕對坐標全斷面測量等多種項目成一完整技術。為了解特徵模態法及微變監測技術的可行性與應用性，實際於台灣某山岳隧道施測，選取其中為期近三年的6次監測成果進行討論，期間導線閉合精度介於1/90,518至1/29,915之間，已達基本控制測量二等導線之精度，高程導線精度為-3.88至5.00 mm(K)^0.5，斷面上單點測量精度在±3-5 mm之間，遠高於其他隧道變位量測技術。獲得之監測資料經由本研究建議的流程處理，並採用特徵模態法分析後，根據其變形特性將隧道分為五個區段，可解釋案例隧道變位之物理含義，提供隧道變位肇因診斷的依據，以至於隧道結構穩定分區或分段的參考。

Analytical approaches of tunnel engineering begins with pursuing the stress-strain relationships around a hole in an elastic media. Start from close-form solution, the development of underground excavation support design method is nearly mature when it comes to convergence-confinement method. The existing methods were mostly proposed by European and American countries, where geological conditions are fairly good, and tunnel damages occur during excavation rather than after completion. Thus, it is generally recognized that tunnels are stable after construction. Understanding of tunnel mechanical behavior reaches only to the end of excavation, no rigorous theories aims to analyze the anomalies of tunnels in operation. Amid the international fever of advancing sustainable engineering structure, knowing the mechanical behavior throughout the whole service life is the key to future tunnel engineering. However, one the crucial part, the methods to evaluate the long-term safety and stability for tunnels in operation, is still absent. Influenced by frequent plate tectonics and lose rock consolidation, a major researches concerning these topics come from Japan and Taiwan. Some of the researches assume a specific factor that reduce tunnel safety, e.g. weakening of rock mass properties, tunnel structures, or change of external forces, and figure out the consequences by experiments or numerical modeling. The others collect a large amount of tunnel cases, and classify lining anomalies according to the feature. After confirm the cause to lining anomalies by surveillance data, it is possible to generalize the relationships between lining anomalies and its cause by statistics or experiences. However, there is a drop between existing methods and the needs to evaluate the safety and mechanical behavior of tunnels in operation. How to efficiently define tunnel structure conditions is still the threshold to sustainable tunnel engineering. This study focuses on tunnel displacement. An approach is proposed to analyze the complicated tunnel displacements by providing particular displacement modes. Displacements of tunnels in operation, with varied and intricate feature, is always an important index to interpret tunnel behavior, and a difficult problem to engineers or researchers. Regarding the displacements of a two-dimensional tunnel section as a vector, and assume that vector can be decomposed as the sum when a unit displacement occurs on every monitoring points. A finite element software is chosen to be the tool to generate the tunnel displacements. Followed by matrix and vector deduction, this study suggests displacement modes that has definite physical meaning, and independent with each other, the characteristic modes. Characteristic modes includes overall motions like translation and rotation, and also deformations including pure shear, triangular deformation, square deformation, pentagonal deformation,…etc. The ability of characteristic modes to describe tunnel displacements rises as the monitoring points increases. According to the results of circular, elliptic and horseshoe tunnel, the characteristic modes vary with shape of tunnel section. The form of characteristic modes are similar for elliptic and circular tunnels, but horseshoe tunnel possesses exceptional details on the lower part of the sidewalls and on invert. The increase in Young’s modulus of rock mass reduce the radial component of each characteristic mode. In respect that present survey technique cannot obtain the complete tunnel displacements owing to deficient precision, insufficient monitoring points and that the results are not in an absolute coordinate, this study cooperate with Pr. Tai-Tien Wang and Yong-Hsin Ltd. to develop a high precision geodetic survey technique named micro-displacement monitoring technology. This technique incorporate global positioning system, traverse survey, route survey and regression calculation, and three-dimensional global coordinate full section lining survey to be as one technique. To understand the feasibility and applicability of characteristic mode method and micro-displacement monitoring technology, a mountain tunnel located in southeast Taiwan is selected to survey and analyze. The traverse survey precision of six monitoring results within three years is between 1/90,518 to 1/29,915, higher than the required value of second-order traverse. The height survey precision is -3.88 to 5.00 mm(K)^0.5, while the single point precision is ±3-5 mm, higher than other current technique. The monitored data after processed with the procedure suggested by this research indicate that the case tunnel can be divided into five zones according to displacement properties. The zoning explains some of the physical meaning of tunnel displacements, provides a basis for determining the cause to lining anomalies, and may serve as a reference to decide the zoning of structural safety and stability.