Numerical Investigation of Geofoam Effect on the Internal Forces of Cut and Cover Tunnels during Earthquakes

According to increasing demand of box-shaped tunnels in transportation networks, which are mainly built as cut and cover tunnels, the impact of natural hazards such as earthquakes on these geotechnical structures and seismic load reduction methods of these types of structures is more important than before. Nowadays, the use of lightweight materials such as geofoam plays fundamental role in civil engineering and has solved many of the challenges of civil engineering. In the present study, the role of geofoam materials as embankments for cut and cover tunnels on the internal forces of the tunnel structure during the earthquake have been investigated. The numerical modeling was performed by two-dimensional finite difference software, FLAC 2D v7.0. The results of an experimental study were used to verify the numerical model. Time histories of acceleration and bending moments for Kobe and Northridge earthquakes were examined. Based on the analysis, it was observed that the software was able to predict the results of the centrifugal experiment well and provided reliable results. EPS19 has been used as a geofoam embankment in the analyzes. The acceleration time history of the Kobe and Northridge earthquakes with a maximum acceleration of 0.3 g has been used in the analysis. In the numerical model of the present study, the equivalent linear model (ELM) has been used to model the behavior of the geofoam. Mohr-Coulomb elastoplastic model was used in soil behavior modeling for static analysis. The behavior of the soil in dynamic analyses was considered as nonlinear and modeled with hysteretic damping. The results are determined in the form of bending moment and shear force diagrams for the roof and wall of the tunnel. The diagrams were provided for static and seismic loading conditions. According to the results of numerical studies, it was observed that the geofoam model has a very good performance in reducing the bending moments and shear forces in static and seismic conditions. Present study shows that geofoam, as the cover material of the cut and cover tunnel, is able to reduce internal forces of the tunnel structure in static, seismic loading condition compared to the soil. Based on the results, it was concluded that geofoam is a suitable material for retrofitting the cut and cover tunnels during earthquakes.