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Although stiff London clay is 'ideal' for tunnelling, extreme care is needed in this project, according to Darcy. En route to the Piccadilly line platforms one of the new pedestrian tunnels comes within 6m, or a tunnel's diameter, of the platforms in King's Cross mainline station. 'One tunnel diameter is about the minimum you can have. We are at the limit of what's safe, ' explains Darcy.