Model Testing on Failure Mechanism of Tunnel Face in Sandy Cobble Stratum

One of the universal problems in shield tunneling is face collapse caused by the sudden change of the soil in front of the tunnel face in weak sandy pebble stratum. In order to control the sudden change of the soil, the model test method is used to study the stability of the tunnel face in the sandy pebble stratum. The results indicate that (1) in sandy pebble stratum, the instability mode of the tunnel face can be divided into four stages of slow development stage, transition stage, rapid development stage and instability stage; (2) the influence of the ratio of the soil depth and the support pressure on the instability of the tunnel face can be divided into three stages of the insensitive stage, the sensitive stage and the failure stage; (3)finally, the instability failure shape of the sand–pebble stratum is upward developing in the lower part “chimney shape,” while the upperpart of the surface presents “spiral shape” subsidence and ultimately presents “crater shape”; (4) when excavation face is unstable, the surface horizontal settlement groove meets the normal distribution curve proposed by Peck and is similar to the settlement shape measured by the test. The study also found that the bamboo sticks with sand inserted in the soil in front of the tunnel face can increase the friction between the bamboo sticks and compact the soil, make the soil more prone to form soil arches, and effectively control the deformation of the soil in front of the tunnel face.