TBM operation and surface settlement

We all know that the amount of surface settlement that a TBM generates depends on the ground and groundwater properties. In addition, it also depends on how well the TBM is able to balance the in-situ ground pressure. The ground surface settlement profile is usually idealised as an inversed bell curve, calculated from an assumed volume loss.

The volume loss adopted at the design stage is usually an assumed value based on elastic theory or experience. The estimation can be significantly different from what happens in reality. In reality, the total volume loss, or surface settlement, depends on the following factors related to the TBM operation (in addition to the ground properties):

1. Face pressure. This is the primary pressure-balancing mechanism of a TBM. See this blog for more details. Ground movement happens ahead of the tunnelling face; the ground has a tendency to flow towards the tunnel face, which leads to volume loss and surface settlement. Adequate face pressure can help reduce or prevent this movement

2. The annulus gap behind the shield. The TBM shield is tapered with an over-cut outside of the tunnel segments. There are many reasons for the overcut, including steering for alignment and friction reduction. Once the TBM excavates the profile, the ground has the tendency to close in around the shield. This radially inwards movement adds to the volume loss generated at (1). The maximum movement the ground can realise would be the full annulus gap, but this leads to much higher friction and can result in TBM getting stuck. Modern TBMs can inject bentonite slurry into this gap to reduce or prevent the ground from closing in onto the shield, which can reduce both surface settlement and the friction onto the shield to ease advancement.

3. How the annulus gap behind the segments is filled. The Annulus gap is usually permanently sealed with fast-setting grout, injected through a grout line embedded in the tail shield. The grout is pressure-applied and enclosed by tail brush and spring plate mounted at the end of the tail shield. Once set, the annulus grout ensures contact between the tunnel segments and the ground and also serve as a layer of water resisting barrier. See this blog for more details.

4. Whether the TBM is advancing or stationary. An advancing TBM usually leads to less volume loss, as the ground movement only happens gradually over time. If you keep advancing with adequate pressure and fill in the gaps before the ground has a chance to realise its full movement, there will be less volume loss.