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  Evaluation of loosening earth pressure acting on large diameter shield tunnel in deep complex strata

  The shield tunnel with an outer diameter of 14.5m is planned in the Zhoujiazui Road cross-river tunnel in Shanghai. The tunnel is a single tube with two road surfaces. To avoid the deep pile foundation in the dock of Huangpu River, the maximum thickness of overburden soil of shield tunnel is 44.4m in the preliminary design phase. The strata from the crown of shield tunnel to the 1.4D level (D: outer diameter of tunnel) are stiff and the strata from this level to the ground surface are soft. The Terzaghi loosening pressure is often used in sandy soil and rock. It is a dilemma to determine whether the loosening earth pressure can be used in deep complex strata. Even in the Chinese design codes concerning shield tunnel, it is not explicit. Based on the shield tunnel design method, the calculation methods of soil pressure are discussed. The complex strata are divided into soft and stiff ones according to the mechanical parameters of soil. The earth pressures of soft and stiff strata are calculated separately and then the internal forces of tunnel are analyzed.

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