Three-dimensional interaction between shallow foundations and mechanized tunneling in soft ground: A case study of Tehran Metro Line 6, South Extension

In recent years, the effects of mechanized tunneling using Earth Pressure Balance Shield (EPBS) machines on existing surface structures have received increasing attention. However, limited studies have addressed the interaction between tunnel step-by-step advancement and the shallow foundations under realistic three-dimensional loading conditions. In this research, comprehensive three-dimensional finite element models were employed to investigate some important aspects of surface footings resting above a mechanized tunnel. The study focused on the ultimate bearing capacity under vertical and inclined loads; the failure envelope under combined vertical (V), horizontal (H), and bending moment (M) loads; the surface displacements; the influence of the tunnel embedment depth; the soil arching effects; and the loosened zone around the tunnel-footing system. A case study of Tehran Metro Line 6-Southern Extension (TML6-SE) was considered in which the EPBS cutterhead was considered at different distances to the surface footing. According to the results, when the cutterhead reached the beginning of the footing, the ultimate vertical bearing capacity was reduced by more than 45% compared to the no-tunnel scenario. As the cutterhead advanced beyond the footing, this reduction in the ultimate bearing capacity gradually diminished, eventually reaching only 3%. At the same relative position of the cutterhead to the footing, the maximum failure bending moment under combined loading also experienced a reduction of approximately 29.2%. Additionally, based on significant variations in the ratio of shear strain to failure shear strain, as a quantitative criterion, the soil was considered loosened within a zone extending approximately to 0.30D (D: tunnel diameter) above the tunnel crown. The bearing capacity of the surface footing was not affected when the tunnel depth exceeds approximately 2D. These findings offer valuable insights for the design, performance evaluation, and risk assessment of shallow foundations in urban tunneling environments.