Circumferential forces and deformations of shield tunnels due to lateral excavation

To investigate its transverse forces and deformations subjected to lateral excavation, a theoretical approach for estimating the transverse forces of a shield tunnel considering the influences of deflections of retaining wall proposed. The calculated radial additional loads on the tunnel is compared with the results of three-dimensional finite element analysis of a case history and centrifuge modeling of excavation effects on a nearby existing tunnel in dry sand, which verifies the reliability of this approach. Based on the measured pile deflections, ground settlements, tunnel deformations and structural strains from this case history, the interaction mechanisms of the transverse forces, deformations and internal forces of the tunnel are analyzed. The results show that: (1) The initial radial loads on the tunnel are symmetrically distributed in a "gourd shape". The lateral excavation results in the decrease in the loads on the tunnel crown and right springline near excavation, while the loads on the tunnel invert and left springline away from excavation increase. This phenomenon is related to the relative values of the free-field ground displacements and measured tunnel displacements caused by excavation. The horizontal and vertical unbalanced loads are balanced by the shear force between neighboring rings caused by longitudinal differential deformations of the tunnel. (2) The elliptical deformations, clockwise rotations and bending moments of the tunnel all increase as the excavation proceeds. (3) The distribution of the circumferential bending moment of the tunnel is closely related to the relative position of the bolts. There are bolts near the right tunnel springline closer to the excavation at the investigated cross section, which bear more circumferential tension stress. In comparison, there are no bolts adjacent to the left tunnel away from the excavation, and thus the circumferential tension stress is mainly undertaken by the segments. Hence, the maximum circumferential bending moment of the tunnel at the investigated cross section occurs at the left springline.