Abstract: In order to improve the efficiency of EPB shield tunneling, nonfull chamber tunneling mode is often used in tunnel construction. While the nonfull chamber tunneling mode in waterrich strata often encounters problems such as large overexcavation due to instability of excavation face. Aiming at the seepage stability of excavation face during nonfull chamber tunneling, the study is carried out combined with an instability phenomenon of excavation surface at a bid section of Jinan metro line R2. First, the selfmade excavation surface seepage failure test apparatus are used to study the seepage failure conditions and types under seepage condition and nonfull chamber tunneling. Second, the sand production rules for different seepage failure types are analysed. Last, six types of strata with different gradation are set according to the content of movable fine particles in soil, and the relationship between soil gradations and critical hydraulic gradient is investigated. The results show that: （1）Seepage failure occurs in the formation under the action of seepage, and the critical hydraulic gradients of seepage failure for nondevelopment type and development type are 0.5 and 0.8 respectively. （2）The sand quality of nondevelopment seepage failure increases first and then becomes stable gradually, while the sand quality of development seepage failure continues to increase, which is characterized by gradual failure. （3）The relation between movable filling content Pf and its critical hydraulic gradient is established. 

Key words: EPB (earth pressure balanced) shield, nonfull chamber construction, stability of excavation face, local seepage failure