Thermally-induced longitudinal decompression of simultaneously backfill grouted tunnels

Abstract

The longitudinal compression of concrete segmental linings (CSLs) can enhance the durability of the tunnel structure. The compressive force reduces the risk of water ingress through ring joints and the differential displacements between adjacent rings. The hydraulic jacks of tunnel boring machines (TBMs) compress longitudinally the tunnel tube during construction, but subsequent longitudinal movements in the lining can cause the migration of the prestressing forces to the ground. One significant, yet disregarded, contributor to the longitudinal decompression of CSLs is the time-varying lining temperatures induced by tunnelling, particularly in CSLs that are simultaneously backfilled with cementitious grouts. This paper proposes analytical solutions for the prediction of longitudinal mechanical strains thermally-induced in CSLs during and after tunnelling. The solutions are compared with field data collected from the Crossrail’s Thames tunnel. It is proved that time-varying lining temperatures in simultaneously backfill grouted CSLs are responsible for the substantial loss of longitudinal compression exhibited in the months following construction. Tunnels subjected to moderate overburden pressures may experience a total loss of compressive forces at thermal equilibrium. The magnitude of the thermal decompression depends mainly on the temperature profile developed behind the TBM, which is in turn governed by the grout exothermic properties and ambient temperatures. A method for the assessment of longitudinal forces over the CSL design life is proposed for asset owners of future tunnels.