Double-O-Tube shield tunneling for Taoyuan International Airport Access MRT

From 1989 to 2010, 20 tunneling projects have been carried out with the Double-O-Tube (DOT) shield tunneling method in the world. In this paper, the DOT shield tunneling for the construction of Taoyuan International Airport Access (TIAA) Mass Rapid Transit (MRT) system is introduced. A 6.42. m-diameter, 11.62. m-wide DOT shield machine was used to build the first DOT tunnel in Taiwan. Field data indicated that, throughout the tunneling operation, the rolling angle of the DOT shield varied between +0.23° and -0.39°, which was within the limiting design values of +0.6° and -0.6° proposed by both TIAA MRT and Shanghai Metro. For the six surface settlement troughs collected from Tokyo, Shanghai and Taipei, the ground loss due to DOT shield tunneling ranged from 0.23% to 1.30%, and the average ground loss was 0.78%. As compared with the ground loss due to single-circular Earth-Pressure-Balance (EPB) shield tunneling in cohesive soils, the range of ground loss due to DOT shield tunneling was relatively narrow, and the peak ground loss value was significantly less. Underground excavation with the DOT tunneling method would increase the tunneling duration for about 32%. The cost per meter of tunnel constructed with a DOT shield was about 1.5 times that constructed with single-circular shields. The cost of shield machine and segment lining were 23% and 53% of the total tunneling costs respectively. The expensive DOT shield machine and the complicated manufacturing and assembly of DOT lining segments are the main reasons for higher cost of tunneling. However, it would cost a lot more budget and it would be much more risky to excavate three cross-passages between the single-circular tunnels under the river.