Large-scale shake table testing of shallow tunnel-ground system

Soil-structure interaction (SSI) is of critical importance in evaluating the seismic response of shallow tunnels. Experimental observations are crucial in assessing the influence of the surrounding and overburden soil near the ground surface. Therefore, this paper presents a series of shake table tests to investigate the seismic response of a scaled shallow tunnel with different backfill material properties and thickness of overburden soil (burial depth). A shallow tunnel was tested incorporating ground representative of realistic backfill conditions using the largescale 1-g outdoor shake table at the University of California, San Diego (UCSD). The tunnel structure was designed at 1/9 scale based on the configuration of an actual recently constructed prototype structure in San Francisco, California. The test results were interpreted in full-scale, which demonstrated that the maximum tunnel deformation generally occurred at the instant of peak ground acceleration (PGA). Relatively large shear strains were developed (up to 6%) and significant reduction of the soil shear modulus were evaluated as the PGA increased during the shaking. The backfill shear strains were lower as the tunnel embedment depth increased because of the increased soil stiffness and strength due to the higher overburden stress. In addition, the tunnel racking deformation became closer to that of the ground as the tunnel embedment increased. At the largest observed soil shear strains, tunnel racking was significantly lower than that of ground deformation due to the decrease in soil. The FHWA simplified solution was used to estimate tunnel racking deformation and wall bending moments. The FHWA solution produced reasonable estimates of ground deformation. However, a discrepancy was noted in estimating the tunnel deformation in the given ground deformation known as racking ratio (Rr). This discrepancy contributed to conservatism in the FHWA racking prediction that was pronounced particularly with the decreased overburden soil stress.