Ground-structure interaction of twin bored tunnels and cross-passages in soft ground pressure balance tunneling

Abstract

With the increasing demand for tunnels in sensitive urban environments, pressure balance mechanized tunneling is employed in most soft ground tunnel construction projects. Segmental tunnel lining is used in pressure balance mechanized tunneling as temporary and final support. With common service life requirements of 100 and even 150 years, efficient design of pre-cast concrete segments can have considerable serviceability and economic importance. The aim of this thesis is to improve the understanding of ground-structure interaction of segmental lining in soft ground mechanized tunneling both in the main running tunnels and at cross-passage openings. Incorporating rare field data collected from the Northgate Link project in Seattle together with advanced three-dimensional (3D) finite-difference modeling (FDM), this research seeks to provide insight into segmental lining ground-structure interaction.As part of the effort to reduce surface settlement, pressure outside the TBM shield plays an important role in pressure balance mechanized tunneling. An advanced 3D FDM model is presented for pressure balanced tunneling, where the annulus between the shield and ground is full of pressurized material, simplifying the modeling of the shield. The FDM results show that the final lining loads are controlled by the shield and chamber pressure, and the modeling of the TBM shield is not required, as the ground convergence is smaller than the annulus gap size. Extending the 3D FDM model for cross-passages, an advanced 3D ground-structure interaction model is proposed for cross-passages connecting segmentally lined tunnels. Validated using field data, the results show the difference between the loading processes of the break-in and break-out. At the break-out opening, the loading process is controlled by the cross-passage opening formation, while at the break-in opening, the loading is controlled by the advance of the cross-passage excavation, as the ground confining the break-in area is gradually removed. The most critical points in regard to the load capacity of the cross-passage opening support elements were found to be the segmental lining above and below the openings, and the bicone dowel shear capacity. In addition, this thesis investigates the ground response of a tunnel excavated by pressure balance mechanized TBMs in soft ground characterized by the hardening-soil small-strain (HSS) model. A comprehensive parametric analysis using a series of 3D and axisymmetric FDM modeling is employed. The results show that longitudinal displacement profiles (LDP) assuming linear-elastic or elastic perfectly plastic (EPP) soil behavior compared to the more realistic HSS model can result in overestimation of pre-convergence prior to liner installation. This over-estimation of the pre-convergence by EPP MC is shown to be as much as 20%. A new LDP solution for HSS is proposed for application in pressure balance TBM tunneling in soft ground, and the practical application of the new HSS LDP solution is described.