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.

Peter Gilbert is Chief Engineer for Jacobs UK Tunneling & Ground Engineering, and is the Ground Engineering Lead for Align-D on HS2 Contract C1. He has led the geotechnical engineering on numerous major Design & Build projects, and takes a lead role on preparation of earthworks standards in the UK and Europe. He has specialised in the ground engineering of highspeed rail projects since 2012.

Antonios Paraskeva has 30 years professional experience which has been gained from a broad spectrum of experience within the civil engineering and geotechnical engineering fields in the UK, Europe and the Middle East. His experience covers project management and leading design teams on large geotechnical and multi-disciplinary heavy civil engineering schemes and Expert Witness work. Antonios has experience of construction supervision and monitoring for major embankment dams, highway and rail infrastructure schemes. Antonios has also led the scoping and interpretation of complex ground investigations for major heavy civil engineering and large infrastructure schemes.

Martin Klein is a design engineer with 10+ years of experience in continental Europe, the UK as well as the United States. His experience includes viaduct design for high speed rail projects in France, as well as design of underground structures on major transportation schemes such as the Grand Paris Express. Martin’s experience also includes leading an on-site resident engineering team on a tunnel retrofit project in the Paris area.

Diogo Ferreira is a Geotechnical Engineer with four years’ experience on major engineering schemes such as High Speed 2 and Thames Tideway Tunnel. His experience includes detailed design and advanced numerical analysis for temporary and permanent works for shallow and deep foundations, retaining structures and earthworks under static and dynamic loading.