Numerical Assessment of Vibration Mitigation in Cut&Cover Metro Tunnels through Open and Rock Wool-Filled Trenches

Ground borne vibrations from railway tunnels represent a significant challenge for human comfort, structural integrity, and the operation of sensitive equipment. This work explores mitigation strategies for “cut&cover” metro tunnels through the introduction of lateral trenches alongside tunnel walls. The effectiveness of such measure was assessed using two-dimensional finite element models that reproduce the soil–tunnel interaction under dynamic loading. Two alternative solutions were considered: open trenches, sustained by retaining structures, and trenches filled with rock wool, a soft and dissipative material able to carry geostatic loads. Both options were examined at different positions, either adjacent to or detached from the tunnel, and for two different soil stratigraphic profiles. The first stratigraphy assumes a simplified configuration having two layers only, one above and one below the groundwater table, which intersects the diaphragm walls forming the tunnel sidewalls. The second adopts a more detailed profile with multiple layers identified by geophysical survey data. Results are reported in terms of acceleration amplitudes and insertion loss. They show that open trenches directly connected to tunnel walls provide the strongest attenuation, though with significant feasibility issues. Rock wool-filled trenches display less attenuation compared to the previous solution. However, they have the big advantage of reduced sensitivity to their position relative to the tunnel. Overall, the study demonstrates the key role of trench typology and position in vibration mitigation and provides practical insights for the design of control measures around urban metro infrastructures.