Structural response of concrete segmental linings in transverse interaction with the TBM. Part 2: Non-axisymmetric conditions

Abstract

In the second part of this paper, the behaviour of the last assembled ring when loaded simultaneously by the tail seal pressures and the jack forces is investigated in detail for eccentric scenarios of tail seal passage such as those encountered in the Crossrail’s Thames tunnel (CTT). In these non-axisymmetric scenarios, the sealing pressures grow linearly with depth and the ram pad restraints against the tunnel uplift are unevenly distributed over the ring circumference. The parametric study targeted the effect of uneven pad boundaries, tunnel unsupported length, Lu, and lining features on the numerical outputs of selected ten-ring finite element (FE) models. The numerical outputs of interest were ring deformations, internal forces, segment interactions and stress concentrations within the segment bodies. It was found that ring distortions can develop within Lu. The distortion mode is determined by the ram pad boundary. The ring deformations increase with the advance of the sealing pressures and decay with distance from the TBM pads. For a given pad boundary, Lu governs the magnitude of distortion, which, in the cases studied, raised up to 0.30% the tunnel diameter D by the end of the first advance. The tangential component of the jacks’ downward reaction can induce hoop tension at the ring front. The radial reaction forces result in significant hoop and torsional moments, differential radial rotations at transitional longitudinal joints and longitudinal moments in constrained segments. The total angularities of longitudinal joints can exceed recommended tolerances. The lower joints are at a higher risk of developing permanent angularities. Gasket offsets in joints undergoing differential radial rotations are sensitive to Lu. Temporary spear bolts can trim extreme offsets, e.g. 12 mm, down to moderate values. The transverse action of the pads increases the magnitude of peak tensile stresses in constrained segments: hoop spalling stresses at pad interspaces plus radial bursting stresses and ‘longitudinal’ spalling stresses near the rear corner of the longitudinal joints. Torsional shear stresses combined with hoop tension can cause excessive extrados surface tension near the front of constrained segments. Shear failure was identified in convex-convex transitional joints with considerable joint imperfections.