Engineered metabarrier as shield from longitudinal waves: band gap properties and optimization mechanisms

Phononic crystals that prevent the propagation of waves in a band gap have been widely applied in wave propagation control. In this paper, we propose the use of a metabarrier, based on a locally resonant phononic crystal mechanism, as a floating-slab track bearing to shield the infrastructure in a floating-slab track system from longitudinal waves from the slab, thereby improving mitigation of ground-borne vibrations. The locally resonant band gap properties of the metabarrier were studied based on the finite element method, and the shielding performance was verified by the transmission spectrum. Simplified models for band gap boundary frequencies were built according to the wave modes. Furthermore, a 3D half-track model was built to investigate the overall vibration mitigation performance of the floating-slab track with the metabarrier. An optimization mechanism for the band gap boundary frequencies is proposed. As the low-frequency ground-borne vibrations induced by subways carry the most energy, multi-objective genetic algorithm optimization was conducted to obtain a lower and wider band gap for a better shielding performance. The results show that the retained vibration isolation performance of the low natural frequency, the shielding performance of the band gap, and the controllability of band gap boundary frequencies all contribute to an improvement in overall vibration mitigation performance. The vertical static stiffness of the metabarrier was close to that of the existing bearing of the floating-slab track. An optimized locally resonant band gap from 50 to 113 Hz was generated using the optimization mechanism.