Abstract: A complete 3D numerical simulation, by using FLAC3D, is developed to investigate the influence of longitudinal displacement profile (LDP) relaxation on ground deformation and contact force acted on the shield in squeezing ground excavated by a double shield tunneling boring machine (TBM). The shapes of LDP curves using the stress relaxation are well controlled and the defect of control method by time step is represented in comparison with the stress method. The model is based on the real geometry and considers the nonuniform gap between the shield and ground. The characteristics of the LDP curves, the contact force and the plastic zone on the cross-section of tunnel are studied. Some useful conclusions are drawn as follows: 1) The sensitivity of LDP curves to the stress relaxation is not uniform for various rock mass parameters. 2) The contact pressure and frictional resistance acted on the shield increase gradually with the increasing stress release rate, and the corresponding behavior of LDP curve of ground and the contact area on the shield are also different. 3) The shape of plastic zone outside the shield is closely related to the stress release rate and the non-uniform gap between shield and ground. The plastic zone will gradually reduce from top to bottom when the stress release rate reaches a relatively large value.

Key words: the relaxation of LDP curves, double shield TBM, non-uniform gap, ground deformation, contact force acted on the shield