The shield tunnel method was introduced to Taiwan 157 years after its invention. Because of the newborn ground,plenty of groundwater, complex geological conditions, and the cultural and construction environments of the island, this method can be considered a new technology.
This article reviews each stage of sprout, growth, robustness, and creation and describes the concerns related to the design and construction of the shield tunnel engineering applications at these stages. In addition, we briefly demonstrate the design concept evolution and a special case of construction. Finally, we propose a shield tunnel technology and focus on a deeper understanding of technologies that are more complex. In addition, we recommend improved directions for ring segment lining design rationalization, sustaining mechanisms of the stability of excavation faces, the maintenance and management of complete life cycles, and the application of new methods.
This paper reports the development and application of large cross-section tunnels constructed using shield machines with diameters greater than 10 m. This paper discusses the application of large-diameter shields in the construction of road, railroad, and water drainage tunnels; the checklists for planning and design; lining segments; selection of a shield machine; and considerations for the construction jobsite. Of 128 cases of large cross-section shield tunnels collected from Asia and Europe, 49% of them were used for road tunnels, and 38% were used for railroad and metro tunnels. Thus, the large cross-section tunnels were mainly constructed for road and railroad tunnels. Of the 73 large cross-section tunnels excavated in Japan, 59 of them (81%) were lined with reinforced concrete (RC) segments. The lining segments that were used most often were composed of RC. The relationship between the thickness of the lining segment and the outer diameter of the tunnel was defined as the thickness–diameter ratio. According to previous construction cases, the thickness–diameter ratios of the large cross-section tunnels ranged from 3.5% to 4.0%. In Japan, from 1985 to 2005, 74% of the large-diameter tunnels were constructed using slurry shields. However, since 2005, most large-diameter tunnels have been constructed using earth-pressure-balance shields.
For removing underground obstacles for tunnel boring machines (TBMs) operating on tunnel routes, many methods depend on the time, site environment, construction schedule, cost, and risk of treatment. This paper presents the methods and factors that must be considered for three obstacles: anchors, hard rocks, and PC piles of dikes. The case in which a TBM struck a PC pile group while tunneling underneath the north dike of the Keelung River at the Wen-Hu Line of the Taipei MRT Project was the most sophisticated. To ensure the safety of the dike and enable a smooth procession of the tunnel work, the dike was broken step by step to remove the PC pile group according to the principle that, to prevent flooding, an existing dike cannot be dismantled until a temporary dike is built. After removing the PC piles, which were 80 cm in diameter and 39 m in length, 100-cm-diameter and 16-m-long cast-in-situ piles were constructed using ground treatment within the piling zone to increase their bearing capacity so that the TBM could smoothly pass and the settlement of the dike could be controlled within an allowable value. Finally, the original structure of the dike body was completely restored. The results showed that structural analysis should be performed for each construction step during the breaking of dikes and PC piles to ensure the safety of dikes.
In previous years, a prestress anchor method was used in an earth support system for deep excavation in Taiwan. Because the method was limited to removing whole anchors after construction, the follow-up construction may encounter such anchors. This paper presents a case study of a shield tunnel in the Taipei MRT. During the construction of a diaphragm wall in a driving shaft, anchors for excavating the adjacent building were found. These anchors on the path of the shield tunnel had to be removed before shield driving. This problem was solved by changing the alignment, and a diaphragm wall method was used to remove the remaining anchors. Because some anchors located within the intersection were difficult to remove, we strengthened the cutter bit of the shield machine and considered a repair plan for the supposed location. This paper proposes other special measures.
Shield tunnel construction requires a working shaft to continue. Once a shield machine stops, difficulties occur. However, the uncertainty of buried driftwood regularly affects shield tunneling. In addition to understanding the driftwood distribution in the Taipei Basin, it is crucial to study the effects of buried driftwood on construction that includes shield tunneling, launch/arrival tunneling works, and excavation cross passage. This paper provides a reference for future designs and construction of shield tunnels.
Shield tunneling that encounters obstructions, such as existing retaining structures and piles, has been a troublesome concern for geotechnical engineers. With the continual development of the MRT network in the Taipei metropolis, removal obstruction experiences for shield tunneling works have increasingly accumulated. This paper presents the design lot DG166 of the Songshan Line of the Taipei MRT network as an example in which design considerations and strategies against obstructions such as retaining structures and pile foundations for viaducts, basements, and bridges were applied. Because of site conditions and restraints, the adjustment of the alignment to prevent conflicts with most obstructions was the first step at the design stage. For obstructions which could not be avoided, certain removal strategies and mitigation measures were applied, and an automatic monitoring system was introduced to sure smooth construction of shield tunnels and to maintain the normal operations and functions of the existing structure.
A shield tunnel method was used for constructing a tunnel between an underground station and a ground station in the Taipei MRT project. The shield machine type and ground improvement method were selected according to pipeline investigations, drilling data, and whether the obstacles should be removed in the design phase or at the early construction stage. However, when the construction was underway, shield tunnel boring encountered unexpected underground obstacles. At this point, evaluating all possible solutions in the available resources and existing conditions was essential for expediting construction progress.
This paper presents the case of the Taipei MRT Luzhou Line CL804 lot that encountered sheet piles beneath the sewer to illustrate the processes of estimation, review, and elimination during shield tunnel construction in which unexpected underground obstacles were encountered.
Mining an MRT underground tunnel by using a shield machine is one of the most secure and economic methods for urban tunnel construction. Increasing the number of underground pipelines, viaduct foundations, underpasses, and other underground structures makes urban shield tunnel construction more challenging. Shield tunnel construction may be more difficult and time consuming if the underground structures and obstacles are not designed properly or if investigations are conducted before construction. Therefore, mitigating underground obstacles is crucial for shield tunnel construction.
The construction cases of the Songshan Line tail rail shield tunnel indicated that shield tunneling encountered underground obstacles and structures. This case used an all-casing pile oscillator for cutting and removing underground obstacles. Initial cutting and removing were performed using the grab and hammer, respectively, and the latter involved using an all-casing pile oscillator for cutting and removing obstacles directly to reduce vibration and noise.
In recent years, an increase in the living standards and rapid urban development has led to an increase in populations and building density. In the planning phase of major transportation projects, the obstacles that existing structures present should be considered. After the completion of the first and second phases of the Taipei MRT project in the near future, the additional following routes will overlap the operating routes or stations to connect with the existing MRT network. Therefore, the means of reducing the construction risk must be considered in the design and analysis phases. This paper presents a circular line case of a shield tunnel that passes underneath an operating MRT station. To reduce the risk of shield machine launching, the vertical and horizontal ground improved grouting method used in front of diaphragm wall of station and beneath station was considered in the planning and design stage. Moreover, in the detail design, twoand three-dimensional analytical software was used. According to the construction process, the influence of a shield passing underneath the existing station structure was simulated on the basis of the actual boundary conditions and reasonable assumptions. New construction technologies were applied during construction. For example, a horizontal drilling locator was used to confirm the construction accuracy, and a high-pressure water jet and diamond chainsaw were used to cut both the existing and new diaphragm walls of the station. The analysis results were considered for the subsequent construction risk control. The process of the shield tunnel passing underneath the existing MRT station was managed by the launching control and automatic monitoring system. This successful case can be referred to for similar projects in the future.