Tunnel face failure induced by combined effect of unloading and cutterhead vibration in unsaturated sand: insights from a scaled model test

When shield tunnelling in unsaturated sandy ground (USG), the tunnel face is inevitably subjected to the combined effects of unloading and cutterhead vibration (CEUV), which adversely affects ground stability. To investigate the underlying mechanical mechanisms, a tunnel face unloading (TFU) apparatus was developed, and a series of physical model tests were conducted. The results indicate that the longitudinal acceleration amplitude induced by cutterhead vibration attenuates exponentially in the ground due to the energy dissipation. The TFU causes a chimney-like global failure in the dry sandy ground (DSG), but produces a localized failure with a cavity above it in the USG. Under vibration, the loosening zone in DSG expands in width, while it enlarges both in height and width in USG. The apparent cohesion enhances the tunnel face stability in the USG and facilitates the formation of the self-stabilized arch (SSA), as reflected in stress evolution. Nevertheless, the vibration weakens the soil arching effect, causing an increase in the limit support pressure. Via comparison with previous model tests and numerical simulations, the experimental results are well validated. The formation and destruction of the SSA and the increase of vertical earth pressure under CEUV are theoretically explained by the improved multi-arch model. In addition, the hysteretic failure of cavities in USG poses significant risks to tunnel engineering. Non-destructive physical prospecting methods are recommended to assist in shield tunnelling control and ground remediation.