Ultrasonic characterization of expanded polystyrene used for shallow tunnels under seismic excitation

Expanded Polystyrene (EPS) is used as an inclusion to mitigate stresses acting on tunnels. In this study, the efficiency of utilizing EPS in reducing dynamic loads acting on shallow tunnels was studied. To gauge this, dynamic modulus of elasticity (Ed) and shear modulus (G) of EPS with densities equal to 25, 30, and 35 kg/m3 were characterized based on series of ultrasonic tests, where Ed ranged from 3500 to 6578 kPa, and G ranged from 1535 to 2741 kPa. A correlation was developed between EPS density and damping coefficient, which ranged from 1% to 2.3% based on G and the ultrasonic wave decay. A model was developed for a cut-and-cover tunnel with EPS along its side walls, and a parametric study was conducted using EPS thickness to tunnel depth ratio (tx/h1) varying from 0.05 to 0.20. At tunnel critical sections, normal and bending were reduced by 30% and 46%, respectively.