Research on displacement modes and development of high-precision geodetic survey technique for tunnels in operation
Date Issued
2014
Date
2014
Author(s)
Chiu, Ya-Chu
Abstract
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.
Subjects
隧道
隧道維護
隧道監測
隧道變形
營運中隧道
Type
thesis
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