Effects of Material Damage and Structural Characteristics on Tunnel Shield During Loading and Unloading

To study the effects of material damage and structural characteristics in a tunnel shield during loading and unloading, we conducted an investigation using a plastic-damage constitutive model for the concrete material, nonlinear contact theory, Python's secondary development, and a unidirectional compression spring around the shield circle, which represented the ground reaction force. A Two-dimensional (2D) discontinuous segment-joint model along with a three-dimensional (3D) precision discontinuous segment-joint model were built to simulate the tunnel shield's structural distortion and internal forces when exposed to varying material damage conditions during both loading and unloading. These conditions were explored using three different soil-condition scenarios. The influence of bolt abnormality on the structure is discussed, with graphs showing the corresponding soil radial displacement, the joint stretching value, material stress, and the damage factor for both the loading and unloading condition. Graphs for the joint stretching value and bolt stress levels are presented for two kinds of abnormalities:pre-tightening force loss and corrosion depth. The results show that the deformation index for the macro-structure given three kinds of soil conditions are 510, 340, and 170 kPa, which are considered safe values for loading and unloading. Regarding material stress level to avoid extensive damage, the safe values for loading and unloading are 340, 170, and 170 kPa. The influence of bolt preload loss on the structure is primarily controlled by the opening of the dome joint. The influence of bolt corrosion on the structure is primarily controlled by the stress level of the bolt. The safety limit for the corrosion depth is 6 mm.