Case Study of Mixed Face Tunneling with Shield Machines
5. 隧道中心線深度Z越大，地表沉陷槽越寬。若以O’Reilly and New（1982）建議之經驗公式(i=kZ)評估所收集到之沉陷槽寬度監測值可知，k介於0.2至0.7間。|
ABSTRACT This paper investigates the problems encountered during the construction of shield tunnels through gravelly soils. Case histories reported in the literature are studied and recommendations are made regarding how to solve the tunneling difficulties. The tunneling experience obtained for the construction of lot CN259C, CH222, CH223 and CH224 of the Taipei Rapid Transit Systems (TRTS) have been collected, and the following conclusions are drawn. 1. Open shields could be used to tunnel through gravelly soils. It would be easier to excavate large particles at the face, and to transport the particles to the ground surface. However, auxiliary methods, such as dewatering, compressed air, grouting, and ground freezing methods, are commonly employed to stabilize the face, and to resist the seepage of ground water into the tunnel. 2. When tunneling through gravelly soils with a closed shield, it would be necessary to break large particles at the face with cutter bits and roller bits mounted on the cutter disc. The smaller particles could then be transported through the screw conveyor or slurry discharge pipe. 3. The shield machine for lot CN 259C of TRTS had to tunnel through soils with different stiffness, such as clayey, sandy, gravelly, and rock layers. To effectively control the direction of excavation, the soft clay encountered at the upper part of the face was modified with JSG method. So that its stiffness becomes consistent with the gravelly soil encountered at the lower part of the face. 4. At the same depth/diameter ratio Z/D, the maximum surface settlement Smax due to an open shield is obviously greater than that due to a closed shield. 5. The deeper the tunnel centerline would result in a wider surface settlement trough. By comparing the monitored data with the empirical equation(i=kZ) suggested by O’Reilly and New (1982), it is found that the k value varies from 0.2 to 0.7 for tunneling through gravelly soils with shield machines.
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