Empirical prediction of horizontal movements induced by tunnelling in fine-grained soils

For tunnelling in fine-grained soils, available evidence indicates that the focal depth of displacement vectors along the ground surface decreases with the transverse distance from the tunnel centre-line. Based on field and centrifuge observations, this paper presents a new empirical method in which the focal depth is a function of the transverse offset and, possibly, the tunnel volume loss. The comparison with experimental data from centrifuge experiments and case histories confirms the reliability of the proposed method, in terms of both surface horizontal displacements and strains. In particular, the predicted horizontal displacement profiles, both in terms of maximum value and extent, are in better agreement with the available experimental evidence than those obtained using the empirical and analytical methods currently adopted in design. Finally, an operational way to obtain the complete horizontal displacement field at the surface, near-surface, and at depths down to the tunnel crown is suggested and the importance of carrying out field monitoring for surface and subsurface horizontal displacements is highlighted.