Interpretation of Free-Field Ground Movements Caused by Mechanized Tunnel Construction

This paper summarizes greenfield ground movements caused by the construction of twin 7.1-m-diameter tunnels for London's Crossrail project using earth pressure balance (EPB) tunnel boring machines (TBM). The data include surface deformations from a series of transects in Hyde Park together with subsurface data from one well-instrumented test section. Although far-field ground movements can be well-fitted using established empirical methods or simplified analytical solutions (elastic half-space), nonlinear, inelastic soil behavior is expected to affect measurements close to the tunnel. This paper considers the effects of constitutive behavior on the observed ground movements. Simple [Mohr-Coulomb (M-C)] and more-complex (MIT-S1) soil models are calibrated using results of high-quality laboratory element tests on intact London clay. The models are then used in two-dimensional (2D) numerical simulations in order to optimize three independent cavity-deformation parameters that control the spatial distribution of ground movements associated with the passage of each EPB machine. Simulations using the MIT-S1 model find maximum radial deformations at the crown of the tunnel with very small movements at the soffit, while M-C analyses show minimum deformations closer to the springline. The analyses consistently show that larger volume losses occur for the second, eastbound (EB) tunnel bore [Delta V-L/V-0 = 0.9-1.0% compared to 0.72-0.79 for prior westbound (WB)]. This result may be attributed in part to differences in EPB control parameters and/or interactions between the two tunnels that are not considered in the current analyses. (C) 2016 American Society of Civil Engineers.