Response of Buried Cylinders Encased in Foam

An analytic method is developed to treat the dynamic response of flexible tunnels encased in crushable foams. An approximate soil-structure interaction law is used to account for the inertial coupling between the free field and the tunnel. The method is then used to investigate the response of tunnel-foam ring structures subjected to shock loadings. A series of dynamic experiments were conducted using 5 in. diam steel tunnel models encased in polyurethane foam and the results were compared with the predictions of the analytic method. On the basis of the results generated, the approximate analysis adequately predicts the gross structural response. For tunnels buried in high strength foam linings, a ringing effect will develop if the foam does not crush under load, and this ringing will persist for substantial times, adversely affecting systems housed within the tunnel.