The maximum risk for shield tunneling construction is shield machine approached ventilation shaft and left that. The variation of ground water table and nonuniform settlement of Taipower 345 kV pylon by super well point (SWP) dewatering will be discussed. And further, the lateral displacement of diaphragm wall might cause seepage at mirrorface during the excavation stage, therefore that will be also discussed.
The decomposition of shield machine include several works, that is dewatering by SWP, and install standpipe piezometer to measure the variation of ground water table at 3rd SongShan layer(EL.92.42m) and 5th SongShan layer(EL. 72.42m), respectively. In addition, nonuniform settlement of nearest building such as Taipower 345kV pylon is monitored. The JSG grouting was conducted for ground soils outside the diaphragm wall several weeks before shield machine approached and then the leakage test for JSG grouting soilcrete is conducted. The backfill grouting is executed immediately when shield machine approached. Finally, there are two stages to decompose shield machine, that remove cutter face, cutter motor and screw conveyor, respectively. Therefore, the mirrorface can be broke safely and through the shield tunnel.