Influence of EPB-TBM Heading Confinement Pressure on Surface Settlements: Comparison between 3D FE Numerical Predictions and In Situ Measurements

"The most important impact on the environment of a tunnel excavation is the creation of surface settlements. Nowadays, the mechanized tunneling allows minimizing the settlements, applying a confinement pressure which counterbalances the ground de-confinement. It is thus essential to determine the correct amount of confinement pressure. In this paper, the authors present a 3D numerical study simulating the EPB-TBM excavation of a water tunnel, in the Flanders clays (north of France). Firstly, a simplified 3D simulation, based on the concept of instantaneous longitudinal settlement curve, was performed to predict the surface settlements for different values of heading confinement pressures. Secondly, the same pressures were tested by the EPB-TBM in a monitoring zone. The comparison between the prediction settlements and the measurements was very satisfactory and allowed validation of the numerical approach and the choice of the optimal confinement pressure value for a sensitive area of the project. INTRODUCTION The excavation of a tunnel can create displacements of the surrounding soil which may affect existing structures and generate unacceptable dysfunction. The development of the closed shield tunneling has permitted to reduce the volume loss generated around the excavation and consequently the surface settlements. In this case, as explained by different authors (Clough et al., 1985; Kastner et al., 1996; Mair and Taylor, 1997), the ground movements are influenced by the TBM works: the heading confinement pressure, the shield overcutting and conicity, the annular void grout injection parameters, the segment concrete lining rigidity, the consolidation of the grout. Hence, in the tunnel design, it is important to take into consideration these different processes of the excavation works to simulate correctly the tunneling process."