The TBM excavation disturbed zone

Some basic differences between TBM tunnels and drill-and-blasted tunnels are illustrated in Figure 4 . These simple sketches from Wanner (1980) suggest a fundamentally different disturbed zone around such tunnels. In moderate to good rock conditions the zone with measurable change of properties (permeability, seismic velocity, deformation modulus) may be only some 10s of centimeters thick around a small TBM tunnel, but many times this value around a drill-and- blasted tunnel, especially under the invert. Overbreak of unstable rock also tends to occur during the blasting and does not need supporting to maintain the profile as it may in a TBM tunnel. Gripper problems that are unique to the TBM tunnel may be experienced in the zones prone to instability. These problems may occur only hours after excavation, or days or many weeks or even months later if the cutterhead gets blocked and major void formation occurs.