Reduction coefficient of external water pressure of hydraulic tunnel lining in water-rich loess stratum: Numerical and <i>in situ</i> test analysis

The reasonable value of external water pressure is a common problem about tunnel lining structure design in water-rich loess stratum. Therefore, the rational selection of external water pressure becomes the focus of research. Based on the Hanjiang-to-Weihe water diversion project, the permeability characteristics of Q2 loess stratum were systematically studied by on pumping test in a borehole and laboratory tests. Founded on the model test of lining unit under the condition of limited discharge, the reduction coefficient of external water pressure (RCEWP) was obtained. The fast Lagrangian analysis of continua three-dimensional (3D) (FLAC3D) program was adopted. Meanwhile, the sensitivity analysis of RCEWP for different influencing factors was analyzed. The results showed that the RCEWP increases with the enlargement of soil permeability coefficient in the model test. With the expansion of discharge rate of lining unit, the RCEWP shows a decreasing trend. The error between the RCEWP obtained by the numerical analysis method and the model test was less than 6%, which proved the rationality of the numerical analysis method. The permeability coefficient has the greatest influence on the RCEWP, followed by tunnel limiting drainage rate, and then the grouting thickness and shield tunnel advancing speed. Therefore, the project suggested that the shield tunneling speed V ≤ 9 m/d, the grouting thickness h ≥ 20 cm, and the grouting effect N ≥ 10, and the distribution range of the RCEWP was β = 0.49–0.79. The RCEWP on the shield pressure chamber was 0.73. The calculated results are basically consistent with the field measured results, to provide a reference for other water-rich tunnel projects in the future.