Secondary Grouting Pressure Limit of Shield Tunnels in Loess Strata Based on Bolt Shear Behavior

Abstract

A loess stratum was selected as the research object, assuming that the slurry diffuses in a hemispherical shape during secondary grouting. Based on the unified strength theory describing the failure characteristics of loess and the critical condition of bolt shear failure, the critical pressure value of the secondary grouting in loess strata was determined theoretically. The reliability of the theoretical analysis was verified by the numerical modeling realizing the large deformation of formation and the dynamic injection of slurry. The critical grouting pressure of 821.80 kPa of theoretical calculation was obtained. Comparing the results of the numerical simulation analysis, it was found that the critical grouting pressure calculated using the theoretical equation had a safety index of 1.14. Bounded by βu (the ratio of expansion radius RP of soil plastic zone to the expansion radius Ru of the slurry) equal to 2.65, the water pressure at the grouting port is the main factor affecting the upper limit of critical grouting pressure when it is less than 2.65, and the effective internal friction angle of soil layer is the main factor when it is greater than 2.65. The influence of matric suction on the upper limit of the critical grouting pressure can be ignored.