Model Test on the Passive Failure of Slurry Shield Tunneling in Circular‐Gravel Stratum

To study the passive failure behavior of slurry shield tunnels in the circular‐gravel stratum, a set of slurry shield test devices capable of cutter cutting and slurry automatic circulation is designed and developed. The effects of the cover‐span ratio on the evolution process of ground surface displacement, the distribution of earth pressure and the passive failure mode of the face are discussed. The results show that the ground surface is gradually uplifted with the increase of slurry chamber pressure, and the amount uplifted decreases with the increase of the cover‐span ratio. The earth pressure near the face during testing experienced the growth stage, the rapid decrease stage, and the stable stage. The passive failure of slurry shield face experiences excavation face stability stage, slurry film splitting stage and splitting through stage in sequence, and the ultimate supporting force of passive failure of excavation face increases with the increase of the cover‐span ratio. The essentially of the passive failure is a kind of slurry splitting phenomenon in which slurry splits the slurry film and then splits the soil layer. When a passive failure of the excavation face occurs, there is a mud channel formed by the slurry split stratum in front of the excavation face. When the cover‐span ratio is relatively small, the channel extends to the ground surface and forms a hole whose size is basically the same as the diameter of the shield. The hole morphology gradually changes from an ellipse to a circle with the increase of the cover‐span ratio.