Strains in sprayed concrete tunnel linings at Heathrow Terminal 4 and back-calculation of stress

This paper describes the installation and interpretation of strain gauges embedded in the sprayed concrete lining of the concourse tunnel at Heathrow Terminal 4 station, with readings taken over nearly 20 years. The rate of flow method was used to back-calculate stresses from the measured strains, which were then compared with the stresses obtained from pressure cells at the same locations. Interpretation of the pressure cell data and the pressure cell results have been published in earlier papers. The resulting 20-year history of stress and strain gives precious insights into the short- and long-term behaviour of a sprayed concrete lining. An increase in compressive strain up to 3 years followed by a slight decrease over the subsequent 15–16 years has been revealed. It is hypothesised that this is due to radial groundwater flow through the primary lining followed by the gradual application of hydrostatic water pressure to the waterproof membrane and secondary lining. The sprayed concrete primary lining was not designed for permanent loads but nevertheless appears to be supporting all of the effective stress, whereas the secondary lining is not supporting any ground loads but may be loaded by groundwater pressure.