Pregrouting combined with double shield TBMs

Of the 36km of 9.9m o.d. segmentally lined double-shield rock TBM excavation required on the twin tube Follo Line railway between Oslo and Ski in Norway, nearly 8,000m or 22% required systematic pre-excavation grouting to lower the pore pressure and ground water levels in the host geology and protect against the surface settlement.

The contract established and defined grouting requirements and provided information on grouting concepts, with the contractor required to further develop these concepts and the chosen methods of construction.

Optimisation of the process, modifications of the equipment and revision of trigger levels markedly improved pregrounting cycle times while meeting the water control criteria and maintaining project programme by the four double-shield hard-rock TBMs.

A two-hole probe drilling concept with the litres of water per minute from each triggering the need for pre-excavation water control grouting