— June 2017
Feasibility study and conceptual design.
Holborn is a LU station in central London. It is served by the Central and Piccadilly lines. Demand for both lines at Holborn is high. The principal design challenges are the close proximity of the proposed tunnels to existing operational tunnels, complex tunnel geometry and alignments, limited clearance to existing buildings, restricted work sites and strict settlement criteria. The lining system is a double shell lining with both linings considered part of the permanent load bearing structure.
The alignment of the tunnels mainly crossing limestone formations with laminated black shales of varying thickness and locally glacial till.