Experimental and numerical analysis of foamed concrete as a damping interlayer in metro sub-track systems

The use of various materials like flexible mats, polyurethane (PU) carpets, isolator compositions and etc. as a sublayer material in urban metro tunnels, while common in many countries, presents several challenges, including difficulties in material procurement, increased construction costs, and performance degradation over time. To address these issues, the present study introduces a novel foam concrete (FC) mixture as a replacement material. Owing to its multi-phased internal structure comprising interconnected pores and voids, foam concrete effectively attenuates train-induced vibrational energies. Experimental mechanical, physical, and numerical investigations demonstrate that a foam concrete specimen composed of 450 kg/m 3 cement, a water-to-cement (W/C) ratio of 0.5, 60% foam content, and 50 kg/m 3 silica fume achieves a 11.3% damping of applied stresses and reduces applied energy by 2.53 J per 50 m of length. A second mixture, composed of 600 kg/m 3 cement, a W/C ratio of 0.5, and 50% foam content, exhibits a damping ratio of 10.8% and reduces applied energy by 5.05 J per 50 m of length. Collectively, the integrated multi-method experimental and numerical investigations confirm the suitability of foam concrete for sublayer construction applications. This material substantially reduces induced vibrations, dynamic energies, and mechanical stresses.