Abstract: To investigate the soil fluidity mechanism of a silty fine sand stratum that is improved by bentonite slurry for earth pressure balance shield, a jumping table and crossplate shear tester are used to test the fluidity and shear parameters of the improved soil. Furthermore, the shear stressstrain relationship and rheological model are analyzed. The results are as follows: first, in slurry with bentonite to water ratio of 1∶4 is mixed by a mass ratio of 7% ~ 13%; similarly, that with bentonite to water ratio of 1∶6 is mixed by a mass ratio of 7% to improve the fluidity of the silty fine sand. Second, adding an equal amount of free water does not improve the fluidity of silty fine sand. Third, as the amount of the slurry increases, the ratio of bentonite to water decreases; thereby, leading to an increase in the fluidity of the improved soil. Thus, fluidity is negatively related to shear stress. Fourth, the improved silty fine sand follows the Bingham model, and the decrease in plastic viscosity and yield stress is the primary cause of its increased fluidity.

Key words: earth pressure balance shield, fine sand stratum, soil improvement, fluidity, plastic viscosity, yield stress