TBM and spoil treatment selection process - case history Crossrail C310 Thames Tunnel - Slurry TBM versus EPB TBM

Slurry TBM versus EPB TBM
The contract C310 comprises the construction of the Plumstead and North Woolwich Portals and the twin tube Thames Tunnel, which has a length of approximately 2.6 km between the two portals. The two TBMs used for the construction of the Thames Tunnel will drive through varying ground conditions (Thanet Sand, River Terrace Deposit (gravel), and chalk) below the water table. During the drive under the River Thames, the tunnels will only have an overburden of approximately 12 m. The effect of pressure variation due to the tidal River Thames has to be accounted for in the control of the tunnelling. The tunnel will pass underneath several grade II listed buildings, utilities, adjacent to operational railway tracks and close to existing subway tunnels. Previous experience of the handling and disposal of excavated chalk has been gained on several tunnelling projects in chalk, most notably the Dartford Road Tunnels, the Channel Tunnel, the Brighton Stormwater Tunnel, the Lille Metro Tunnel, the Socatop Road Tunnel near Paris and Channel Tunnel Rail Link CTRL 320. The Contract allowed for both Mixshield and EPB TBM Technology. The advantages and disadvantages of a TBM-S with Earth Pressure Balanced face support (EPB-TBM) and a TBM-S with Slurry Face support (Mix-Shield TBM) for the C310 Thames Tunnels were discussed after contract award and a comparative risk assessment was developed. A Mixshield TBM is more expensive but outperformed the EPB TBM in the overall scoring of risk assessment and therefore it has been decided to use this type at C310.