The 3.2km North Downs tunnel required construction of the largest cross-section tunnel ever to be built in the UK at 174m2.

The ‘Grey Rock’ approach considers the primary lining as enhanced quality ground surrounding the tunnel, leading to significant reductions in the long-term lining loads on the secondary lining. When combined with the accurate establishment of in situ stresses from hydrofracture testing this philosophy enabled: ​

• Reinforcement to be entirely removed from the tunnel secondary lining
• The tunnel to be constructed with a flat invert rather than the deep dished reference profile
• The lining thickness to be reduced from 600mm to 350mm, enabling substantial material savings along the 3200m length of tunnel.

This enabled this project to be completed five months ahead of schedule and saved £10 million.

The system was developed in partnership with Bemo Tunnelling.