| Summary: | 碩士 === 國立中央大學 === 土木工程研究所 === 87 === Shield tunneling has become more and more widely used to construct the subway to reduce interfering with the surface traffic in Taiwan. Tunneling is entirely a gravity problem, therefore, it is appropriate to solve the problems with the centrifuge model tests. In this study, a series of single and twin tunnel centrifuge models embedded in soft soils are used to investigate the distribution of earth pressures acting on tunnels and the change of lining stresses during the closure of tail voids. Impact on the existed tunnel caused by new tunneling nearby was discussed in detail as well. The test results were compared with the analytical results using "FLAC" under plane strain condition.
The clay beds were consolidated in a rectangular consolidometer. After shaping the clay beds, five pore water pressure transducers (PPTs) were instrumented at the selected positions and a row of eight linear variable differential transformers (LVDTs) were placed along the surface of model centerline. The clay bed was reconsolidated in an acceleration of 100 g. A horizontal hole of 6 cm in diameter was carefully cut and lined with a negligible stiffness rubber bag in the floor. A model liner of 5.5 cm in diameter was put inside the rubber bag. The model tested in a centrifuge gravity field of 100 g can model a prototype tunnel of 5.5 m in diameter with a tail void of 20 cm. The package was placed back onto the platform again and an air pressure line was then connected to the rubber bag. Centrifuge acceleration was increased to 100 g in a 5g increment. At each increment the air pressure in the rubber bag was cautiously regulated to serve as a support pressure in the tunnel so that no surface settlement occurred. The model was left rotating at 100 g for about 10 minutes. The tail voids closure test was performed by reducing the support pressure to zero at an increment of 10 kPa per 20 seconds. The surrounding soil gradually moved inward the tunnel and then pressed onto the liner. Two types of model liners were used. One can measure the liner bending strain caused by the earth pressures and the other one can directly measure the earth pressures acting on the liner. After finishing the tail voids closure test of a single tunnel, the test package was removed from the platform. A new tunnel 6 cm in diameter which was 9 cm away from the center of the existed tunnel (d/D=1.5) was driven. The second rubber bag and the model liner of 5 cm in diameter were then put into the new tunnel. The test package was ready to another rotation of 100 g. The same test procedures as the first closure test were repeated to investigate the interaction of the existed tunnel and the nearby new driven tunnel.
The obtained result of a single tunnel shows that the earth pressures acting on the tunnel lining decreased at beginning. Once the larger tail voids reach, however, non-uniform distribution of earth pressures along the liner may cause the larger bending moments on the tunnel liner. The moments and earth pressures on the liner decreased with increase of Young''s modulus and soil cohesion. For a deeper tunnel, the distribution of earth pressures is more uniform and the normalized earth pressures tend to be a constant. Arching effects due to the tail voids closures of the new tunneling nearby an existed tunnel may lead to increase the earth pressures acting on the liner of the existed tunnel. The increased earth pressures concentrate on the portion between the crown line and spring line of the existed tunnel. For a deeper tunnel, the liner subjects to the smaller increment of normalized earth pressure.
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