A reduced-order model for the analysis of underground utility tunnel lifelines protection against transverse seismic excitations

A seismic protection strategy is proposed to mitigate the effects of transverse ground motion on the internal contents of utility tunnels. The approach introduces internal support frames for housing utilities, which are partially decoupled from the tunnel structure through visco-elastic isolators and equipped with vibration control devices. To assess the effectiveness of the protection system, a reduced-order analytical model is developed. The tunnel is idealized as a rigid body embedded in a visco-elastic, compression-only soil medium, containing an internally isolated rigid frame coupled with a Hysteretic Mass Damper Inerter (HMDI). The HMDI consists of an oscillating mass connected to the frame through a hysteretic element and an inerter, combining nonlinear energy dissipation with inertia amplification. The model, validated against a detailed finite element benchmark, shows excellent agreement and confirms its suitability for rapid parametric analyses and preliminary design applications. A comprehensive parametric study under harmonic and earthquake excitations demonstrates that the proposed system significantly reduces the absolute acceleration transmitted to the tunnel contents, even for short isolation periods. Increasing the isolation period further enhances acceleration reduction but leads to larger frame displacements. The inclusion of the HMDI effectively limits these displacements and improves the overall seismic performance. Nevertheless, as its tuning cannot simultaneously minimize both acceleration and displacement responses, an appropriate balance must be achieved in the design.