Tunnelling on the Kaohsiung MRT is a combination of cut and cover and twin bored construction.
Bored work began in March 2003 and is expected to finish in March 2006. A total of 45.5km of tunnelling is needed to form the twin tunnels (double the total route length). At the peak, 28 earth pressure balance TBMs were employed on the project.
Work is being carried out on 67 drives averaging 680m long, although the longest is 1,396m.
Of these, 64 are in soft ground (43.7km) and three, totalling more than 1.7km, are in rock.
Ever-changing ground conditions require strict control of earth pressure balance face pressure, calculated on the basis of interpreted soil profile. This uses a lower bound of active earth pressure at 0.2bar and an upper bound of earth pressure at rest of 0.2bar, with driving typically carried out at an active earth pressure 0.4bar (+/-0.2bar).
All finite element analysis predictions of settlement were based on Taipei MRT experience, using presumed ground loss of 0.8% to 1.2% with a settlement trough factor of 0.35 to 0.45. Results of the initial drives and variable face pressure control indicated ground loss of 1.2% to 1.3%, with the loss on curves at 3%.
However, when polymer was used as a soil conditioning agent through the cutterhead in tandem with good face pressure control, this was reduced to 0.5%.
Some excessive ground losses were associated mainly with face pressure loss, through over-excavation at station interfaces and when tunnelling through grouted zones of cross passage locations, as well as the failure to grout tail voids.