In situ behaviour of an instrumented ring subjected to incipient TBM steering around a curve

Abstract

During the TBM tunnelling of curved tunnel sections with passively articulated shield tails, the concrete segmental rings are susceptible to ring deformation and structural damage. In this paper, the in situ loading condition and response of the CAM4 instrumented ring in the Crossrail’s Thames tunnel (CTT) during tail seal passage at incipient TBM steering around a left curve are examined in detail. The field data comprises selected TBM data, the distributed strain measurements of a Brillouin Optical Time Domain Reflectometer (BOTDR) and the output quantities of vibrating wire strain gauges (VWSGs) and biaxial micro-electro-mechanical system (MEMS) tiltmeters. The field data was completed with relevant analytical models (Gil Lorenzo, 2019a, 2021c) and compared with the outputs of the finite element modelling conducted alongside (Gil Lorenzo, 2021a, 2021b). It was found that the eccentric tail seal passage was the main cause of the highest lining pressures and pressure gradients detected at the early stages of tunnelling. The machine turned around the thrust symmetry axis so that the shield posture changed from vertical and upwards to left and inwards. The transverse jack forces near the CAM4 ring switched orientation from downwards to rightwards. The locking mechanism of opposing TBM transverse actions during the eccentric tail seal passage triggered the ring deformations in the unsupported tunnel, which became essentially irrecoverable. The radial rotations of the outer springline segment were largely affected by the interfacial quality between ram shoe and segment first and between segments once the next ring was erected. The moderate sealing pressure gradients and the effective ring coupling ensured by the high thrust loads translated into a very limited ring distortion. The top half of the unbedded ring behaved as an arch-like structure with the crown segment being supported by the neighbouring segments. The in situ hoop curvatures were the result of ring ovalisation developed under the sealing pressure gradients and, in the first TBM cycle, the segment interaction with the hydraulic jacks. A temporary intrados front longitudinal crack near the mid length of the outer springline segment was detected at the time of incipient machine turning. The crack was caused by the combination of the spalling stresses between ram shoes with the front hoop tensile stresses typical of segmental rings loaded at the rear. Initial in-plane angularities of longitudinal joints with contact at the segment front can be corrected by the sequential loading of the ring from rear to front. There was good agreement between the in situ and FE-computed deformational quantities.