The Chiltern Tunnel South Portal Headwall has been designed to provide a subvertical face for the launch of the TBMs, having an approximate retained height of 20 m. The face was designed to have an angle of 70 degrees above the tunnel crown, through 6 m of structureless chalk and drift deposits, and 80 degrees below the tunnel crown a further height of 14 m. The subvertical face is essential to enable the beginning of the bore.

The South Portal structure is part of the wider South Cutting works, and interfaces with the Chiltern Tunnel at the headwall location. The tunnel portal extends South East for approximately 230 m towards the Colne Valley Viaduct.

The route traverses down the side of a dry valley. These dry valleys often have a significant influence on the depth and type of drift deposits and weathering across the chalk hills.

This presentation will describe the various options considered for the headwall, the reasons for the selection of the soil nail solution, how detailed design and geotechnical risk management issues were resolved, and performance of the structure during construction.