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  Water ingress and groundwater control in double shield TBMs at the Follo Line Pro ject - Norway

  Tunnel projects in Norway run the risk of high water ingress during construction phase, resulting in pore pressure reduction in both rock and soil above the tunnel. This is due to risk of settlement in sensitive clay sediments and draining wetlands. Thus it is crucial to identify areas sensitive to pore pressure reduction above the tunnel alignment during the design phase. As a result of the isostatic uplift since the last ice age, settlement sensitive marine clays are often located in rock surface depressions. Infrastructure and buildings constructed on top of these sediments are therefore exposed to pore pressure reduction during tunnel excavation. In addition, avoiding drought damage to vulnerable vegetation above the tunnel alignment is important. The Follo Line Project has established a customised concept for probe drilling and on demand pre-grouting from the TBMs, in combination with infiltration wells, in order to maintain the pore pressure in sensitive areas whilst passing with the TBMs.

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