There are several precedents of unintended surface settlements resulting in enormous loss of costs and time, even if for the rock medium. The expansion of shield tunneling requires controlling the shield tunneling parameters precisely to reduce the surface settlements. In this study, numerical parametric studies are conducted to evaluate the geotechnical properties, and TBM operational factors on the surface settlements during shield tunneling. The numerical model based on FLAC3D is validated by comparing the results with the literature and field data. Ground stiffness is the dominant factor in the settlement, and the groundwater inflow follows it. The face pressure and tail void grouting pressure show a relatively weak impact on surface settlements because of the higher stiffness of rock mass. The results from this study are expected to contribute to understanding the settlement behavior induced by shield tunneling through the rock mass and the prediction of surface settlement.
Shield tunneling has been extensively applied in urban areas since it can achieve tunnel construction with minimized ground deformation by continuous excavation and support. However, there are several precedents of unintended surface settlements resulting in enormous loss of costs and time, even if for the rock medium. The expansion of shield tunneling requires controlling the shield TBM precisely to reduce the surface settlement. The parameters triggering the surface settlement during the shield tunneling vary; tunnel geometry factors such as the diameter and depth of the tunnel (Melis et al. 2002 and Chakeri et al. 2013), ground properties such as the elastic modulus, cohesion, and unit weight (Selby 1988 and Golpasand et al. 2018), the operational factors such as face pressures and steering gap slurry pressures (Lambrughi et al. 2012 and Comodromos et al. 2014), tail void grouting pressure, the amount of backfills and injection point (Suwansawat & Einstein 2007 and Kim et al. 2018), and other mechanical data from Tunnel Boring Machines (TBMs) (Goh & Hefney 2010 and Kim et al. 2020). All these factors are related to unavoidable gaps or stress imbalances. Among them, the operator can regulate only the support pressure on the tunnel face, along the shield skin, and along the annular between excavated surface and segmental linings. However, the surface settlements that are not directly governed by the pressure balance can be caused by direct ground loss such as the failure of achieving impermeability. As the groundwater inflows during tunneling, it causes the groundwater drawdown resulting in the reduction in pore pressure, which means the increasing effective stress, and the seepage forces occurred at the path of groundwater flow causes the ground deformation locally (Yoo 2016). In this study, several numerical parametric studies are conducted to evaluate the impact of geotechnical properties and TBM operational factors on the surface settlements during shield tunneling. The numerical model based on FLAC3D is validated by comparing the results with the literature and field data. The operational factors selected for parametric studies are face pressure, tail void grouting injection pressure, and groundwater inflow regarding the grout's setting time. It is expected that the order and amount of contribution to the surface settlement can help to understand the settlement behavior induced by shield tunneling through the rock mass in a realistic view.
