ABSTRACT
The stresses and strains in a sprayed concrete primary lining at Heathrow Terminal 4 were monitored for nearly 20 years. Vibrating wire pressure cells and strain gauges were installed on and in the sprayed concrete and provide a unique history of stress and strain during construction and many years into operation of the tunnel. Many lessons were learnt about the survivability and reliability of these kinds of instruments in the long-term. They were generally very reliable and losses were more likely to be due to cable damage than to any malfunction of the instrument. This kind of monitoring can provide information that is very useful to designers, about how loads in tunnel linings vary over time, and how loads may be shared between primary and secondary linings. The pressure cells showed no increase in loads in the long-term, except around the invert of the tunnel. The strain gauges, on the other hand, showed a small decrease in load in the long-term. Since pressure cells measure total stresses, an explanation could be that groundwater has gradually seeped through the primary lining. Pressure has then built up on the outside of the sheet waterproof membrane causing an unloading of the primary lining that is only measured by the strain gauges.
