Numerical analyses of twin stacked mechanized tunnels in soft grounds – Influence of their position and construction procedure

Twin stacked tunnels become more and more common due to the limited surface area and underground space. Most researchers have already paid attention separately to the influence of the twin stacked tunnels’ position and construction sequence. However, the influence of the lagging distance and pillar depth of twin stacked tunnels has not yet been fully investigated when considering the tunnels construction sequence. The present study aims at analysing the effect of the construction sequence, pillar depth between tunnels, and lagging distance between stacked tunnels’ faces on the structural forces induced in the tunnel linings and on the surface settlements. For this purpose a three-dimensional (3D) numerical model was developed using the finite difference element code FLAC3D. Most of the parameters of the slurry-type shield machine were simulated. The analyses included the influence of the face support pressure, shield conicity, shield weight, grouting pressure, harden grout, jacking forces, the weight of the backup train and jointed segmental lining. The results indicated that the most critical scenario, both in terms of ground settlement and internal forces in the tunnel lining is when the upper tunnel (UT) is excavated first and followed by the lower tunnel (LT) excavation. On the other hand, the following upper tunnel should be excavated at a lagging distance, which is larger by about 4 to 5 times the tunnel diameter behind the preceding lower tunnel. The research results reviled that designing a too small pillar depth between stacked tunnels should be avoided. This concerns pillar depths of about 0.25 to 0.5 times the tunnel diameter, as indicated in this study. It could cause a great increment in the normal displacements, longitudinal forces, and bending moments in the lower tunnel lining.