Stresses in sprayed concrete tunnel linings at Heathrow Terminal 4

A wide variety of instrumentation was deployed during construction of the sprayed concrete lined tunnels at Heathrow Express Terminal 4 Station in the mid-1990s, some of which continues to function and be accessible for the taking of readings today. This paper presents a nearly 20-year history of stress in the primary lining of the Concourse Tunnel measured using radial and tangential pressure cells on and in the sprayed concrete. Data from tangential pressure cells require careful interpretation and the new and complete methodology for achieving reliable results described in Jones & Clayton (2021) was used to provide the stress history from construction into the long-term. This is a unique case study, in terms of both the detail of the measurements and interpretation and the time period over which measurements have been taken. The results show that pressure cells are very sensitive and respond to changes in stress due to nearby construction activities, and that after construction has ceased, stresses stabilise at a value well below full overburden pressure (the vertical total stress at tunnel axis level).