Multi-point shaking table test of a long tunnel subjected to non-uniform seismic loadings

A 40-m long test was conducted to estimate the seismic performance of the tunnel of the Hong Kong-Zhuhai-Macau Bridge project under non-uniform earthquake loadings. The test used twelve connected model boxes, a synthetic model soil and a scaled model tunnel. The model boxes included four active'' boxes, which were fixed to four isolated shaking tables that worked as excitation sources, and eight inactive'' boxes, which were passively excited by the active boxes through connections. The soil at the tunnel location was simulated with a mixture of sawdust and sand with mass proportion 1: 2.5, that yielded dynamic properties analogous to those of the in situ soil. The tunnel model, at a 1/60 scale, was composed of 98 sections, each with dimensions 600 9 375 9 170 mm, that were made of aluminum, which best approximated the target response of the actual tunnel, given the size of the model, geometry and scaled engineering properties required. In the tests, a non-uniform seismic excitation was provided by imposing the seismic wave to the active model boxes with a time lag equal to the time that would take a seismic wave to travel from one active box to the next along the axis of the tunnel. The test started with the assembly of the tunnel sections and installation of transducers on the tunnel at critical locations. After placement of the soil and the tunnel, the seismic loadings were applied through the active boxes. The test results showed that the acceleration response of the tunnel was larger than that of the surrounding soil. It was also found that the deformation of the tunnel joints under non-uniform excitation was larger than under uniform excitation, to the extent that it could jeopardize the safety of the tunnel had it been designed solely using the uniform excitation. The results of the experiments clearly showed that the effects of non-uniform seismic excitation should be considered for the seismic design of long tunnels.