An Investigation Into the Effects of an Annular Gap on Surface Settlement

Ground displacement is an inevitable problem in the excavation of tunnels and underground spaces near the surface. The importance of surface settlement increases with a reduction of the depth of tunnelling. In urban areas, when settlements exceed the allowed value, they can damage surface and subsurface structures. Therefore, the identification of factors that affect settlements is critical. One of the essential factors is the grouting behind the segmental lining of a tunnel. An annular gap behind the segmental ring usually causes more problems such as surface settlement and displacement and damage to the precast concrete and unsuitable sealing. Void thickness has a large effect on the amount of maximum surface settlement and the shape of the surface settlement curve. This study focuses mainly on the effects of the aforementioned parameter on surface settlements. Both numerical and empirical methods were used in this study. In addition, the field data were presented in order to support the predicted surface settlements by 3D numerical modelling. The results obtained from the 3D numerical modelling were in agreement with the field measurements. The obtained results revealed that the amount of maximum surface settlement and shape of the surface settlement curve is strongly related to the annual gap thickness and the grouting operation.