VERY LOW GROUND SETTLEMENT IN TBM TUNNELLING IN
GRANULAR SOIL AT THE BUCHAREST METRO LINE 4
The second stage of the Bucharest Metro extension started with the metro
tunnel excavation presented in this paper, the M4 line. Tunnels in soft ground
usually imply drilling through sub-surface un-lithified deposits, namely
cohesive/non-cohesive/mixed soils that involve soft ground tunneling conditions
where the tunnel is driven using a closed shield as temporary ground support. This
must be followed by immediate involvement of tunnel lining due to considerable
potential of ground movement during excavation, realized using a high
mechanization level of the process. The Bucharest soil grain size distribution and
the rest of the soil geotechnical parameters was the main factor to deal with. The
water table also played a major role in determining the TBM type used, presenting
further influences on the stability of the excavation face, with the option of
dewatering the drilling site soil enhancing the TBM circumstances.
The chosen EPB type of TBM handled the geological variations that
occurred at the excavation face (e.g. voids) due to the non-compressive medium
which fills the working chamber at the TBM front and responds instantly and
efficiently at the combined activity of hydraulic jacks thrust pressure; advance
speed; cutter head torque and revolution speed; mucking screw speed which were
controlled via a complex software and proper manning to set a constant value to the
drilling head and TBM shield to achieve the smoothest and fastest advance of the
boring machine. Using a soil conditioner that presented an up-graded formula
rendered to the excavated soil considerably improved parameters. Moreover, the
high degree of mechanization and automation of the excavation activity with TBM
has decreased the high level of soil settlement and excavation face failure risk,
resulting a very low ground settlement value for the project.
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Keywords: tunnel, TBM (Tunnel Boring Machine), granular soil, settlement
Cuvinte cheie: tunel, TBM (mașina de frezat tuneluri), sol granular, tasare