Seismic response of ultra-large diameter shield tunnel in upper-soft and lower-hard site: Shaking table tests and numerical simulations

The shaking table test study on the ultra-large diameter shield tunnel in upper-soft and lower-hard site was rare, due to the limitations of test technology and equipment performance. This study performed a series of large-scale shaking table tests and numerical simulations, addressing this deficiency on the seismic response of an ultra-large diameter shield tunnel in upper-soft and lower-hard site. In shaking table tests, a modified method of similitude ratio designing developed in our previous study was adopted, which made it possible to conduct the large-scale test on a shaking table with general loading performance. The test and numerical results meet a good agreement, showing that an evident inflection occurs at the soft-hard interface in the curves of peak acceleration amplifi-cation factors versus depth. Furthermore, combining numerical simulations on the upper-soft and lower-hard site, pure soft site and pure hard site, the seismic response differences of the tunnel between upper-soft and lower-hard site and pure soft and hard sites are further revealed. The results demonstrate that the largest relative difference of tunnel strains between upper-soft and lower-hard site and pure soft or hard site always appears at haunch, and the haunch strain of the tunnel in upper-soft and lower-hard site keeps being larger than those in pure soft or hard site regardless of excitations. In addition, the deformation mode of the tunnel in upper-soft and lower-hard site is more like the shape of an avocado, while it presents a typical ellipse in pure soft or hard site.