Study on the blast mitigation behavior of metakaolin-based foam geopolymer (MKFG) as tunnel cushioning layer against external blasts

In this study, the similarity model contact blast test and numerical simulations were carried out to investigate the protective behavior of tunnels with a metakaolin-based foam geopolymer (MKFG) cushioning layer under blast impacts. In contact blast test, Rock-Foam Geopolymer-Concrete Tunnel (RFGCT) structures with various densities (400, 600 and 800 kg/m³) of MKFG were tested against a blast impact of 100 g TNT. In numerical simulations, several parameters covering TNT equivalent as well as density and thickness of cushioning cladding, were comprehensively discussed. Test results show that the attenuation rate of cushioning cladding to the blast wave is enhanced from 34.7 % to 71.0 % with the reduction of the density of MKFG from 800 kg/m³ to 400 kg/m³. Meanwhile, the reflected tensile wave generated by blast wave falls from 1.85 MPa to 0.66 MPa. When the density of MKFG exceeds 600 kg/m³, the cladding exists obvious defects in energy absorption at the bottom of mid-span and free end, which gradually disappear as the TNT equivalent and cushioning thickness increases. Increasing thickness of the MKFG-400 can lead to excessive overall displacement of the tunnel lining. Full-size uncertainty analysis shows that at TNT equivalents of 2000 kg, the thickness of MKFG-800 as cushion is recommended to be 2–3 times that of the lining.