Groundwater inflow prediction in urban tunneling with a tunnel boring machine (TBM)

Tunneling in urban areas demands an accurate characterization of the shallow subsurface to minimize risks during excavation. Unexpected high water inflows constitute a major problem because they may result in the collapse of the tunnel face and affect surface structures. Such collapses interrupted boring tasks and led to costly delays during the construction of the Santa Coloma Sector of L9 (Line 9) of the Barcelona Subway. A method for predicting groundwater inflows at tunnel face scale was implemented. A detailed 3D geological and geophysical characterization of the area was performed and a quasi-3D numerical model with a moving tunnel face boundary condition was built to simulate tunnel aquifer interaction. The model correctly predicts groundwater head variations and the magnitude of tunnel inflows concentrated at the crossing of faults and some dikes. Adaptation of the model scale to that of the tunnel and proper accounting for connectivity with the rest of the rock massif were crucial for quantifying the inflows. This method enables us to locate the hazardous areas where dewatering could be implemented.