The CTRL Project involves a total of 40 km of running tunnels with an internal diameter of 7.15 m. The segmental construction of the tunnel linings comprises nine pre-cast concrete segments plus a ‘key’, each 350 mm thick.

Halcrow is a member of Rail Link Engineering (RLE), a consortium of Arup, Bechtel, Halcrow and Systra. RLE is the designer and project manager of the CTRL.

Prior to the contract award, fire and loading test programmes were carried out in order to provide essential design information.

Following a severe hydrocarbon fire (1350°C for 2 hours) on concrete containing monofilament polypropylene fibres, it was found that over half the lining had a residual compressive strength of > 80% of the concrete cube strength, and a further third had > 66%. For a severe hydrocarbon fire (1200°C for 45 minutes) on plain concrete, the predicted reduction in capacity would be 49% of its “normal” 20°C capacity.

The main conclusion of the fire testing programme was that monofilament polypropylene fibres should be added to the concrete mix to reduce spalling in the event of a fire.

Calculations of the residual lining capacity after a fire demonstrated that fire-damaged lining would still support the ground loading and allow repairs to be carried out.