Design of Chiltern Tunnel South Portal

The tunnel portal structures to each end of the Chiltern Tunnel are the first example of high-speed rail porous portal structures in the UK. Porous portals exist in other countries (e.g. Japan and France) but not to the scale of those required for the Chiltern Tunnels. The primary purpose of the porous portals is to prevent excessive noise pollution by controlling aerodynamic pressure waves due to high-speed trains entering, traversing, and exiting the tunnel. However, the portal structure has many other related aspects, including TBM launch requirements, visual appearance, and interface between open route and tunnel environment. This paper illustrates how the Align project team considered portal structural forms (noting the limited precedents for such a structure), and how a co-ordinated design and construction approach was used to deliver good value and confidence in the performance for this major structure.

This paper will discuss the portal structural form, comprising twin in-situ concrete exposed portal structures on a single ground-bearing slab with no movement joints with a total length in excess of 250m, and will review how this proposal met the strict HS2 requirements around durability, aerodynamic pressures, and fire resistance. In addition, it will discuss the integration of the TBM launch operation and the incorporation of the associated temporary work requirements into the permanent works, which was crucial in enabling construction efficiencies.