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  Analysis on Floating Mechanism of Shield Tunnel in Liquefied Foundation using Shaking Table Test

  With the increase of metro lines in large cities, the problem that a shield tunnel must cross liquefied foundation may arise. Once earthquake occurs, the shield tunnel faces the potential risk of floating damage. Several shaking table tests were carried out to study the seismic response of liquefied strata around shield tunnel under three different peak accelerations. The developing characteristics of the excess pore pressure as well as the floating displacement of the shield tunnel were analysed. The floating mechanism of shield tunnel were further discussed based on the shaking table experiment. Even if the foundation is not fully liquefied, the tunnel floating movement appears when the buoyancy caused by the excess pore water pressure is larger than the summation of the residual overburden pressure and tunnel gravity. By assuming that the excess pore water pressure of the soil at the top and bottom of the tunnel develops synchronously, a simplified formula for calculating the critical excess pore pressure ratio caused by the foundation liquefaction was derived. The anti-floating ability of shield tunnel can be greatly improved by depressing the tunnel design depth to make the tunnel partially locate in the non-liquefied foundation.

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