Abstract: When slurry shield boring in high permeability stratum, a large amount of slurry will be lost, which would cause difficult formation of filter cake and even instability of excavation face. The addition of coarseparticle materials can effectively improve the forming effect of filter cake. In order to explore the influence of the addition of coarseparticle materials on the formation of filter cake, the vermiculite with small density is selected as coarseparticle material to carry out slurry permeation and membrane formation test in selfmade slurry permeation experiment equipment by changing its particle size and content. The filtrate loss and filtercake forming time are taken as evaluation indices to analyze the influence of particle size and content on film forming effect, and a rational particle size range of coarseparticle materials is proposed. The experimental results show that: (1) The average formation pore diameter D0〖TX-〗 has reference significance for the addition of coarseparticle materials; the coarseparticle materials with particle size of D0〖TX-〗/2~2D0〖TX-〗 is conducive to the formation of filtercake, and with the increase of the particle size of coarseparticle materials, the filmforming effect increases first and then decreases. (2) When the content of coarseparticle materials is between 20% and 40%, the larger the content of coarseparticle material, the better the film forming effect, but as the content continues to increase (larger than 40%), the effect weakens.  (3) When the content of coarseparticle materials is between 20% and 60%, the influence of the particle size of coarseparticle materials is greater than that of the content, and the increase of particle size will weaken the gain effect brought by the content. (4) It is suggested that the selection of the particle size of coarseparticle materials should be mainly considered in practical engineering.

Key words: slurry shield, high permeability formation, model experiment, particle size of coarse material, coarse material content, filtercake forming effect