Mechanism and forecasting model for shield jamming during TBM tunnelling through deep soft ground

Taking the tunnel face effect and time-dependent properties of soft surrounding rock into consideration, the spatial and temporal effects of squeezing deformation in the shield region are studied, and their determining influences on the interaction of contact-squeezing-friction between surrounding rock and shield is analysed. Then the mechanism of TBM shield jamming is revealed as follows: firstly, the radial convergence in the shield area increases with the increasing distance to the tunnel face and the growing stoppage downtime gradually; secondly, the convergence may exceed the overcut gap at a certain distance from the face where the surrounding rock starts to contact and squeeze the shield; as a result, frictional resistance is produced between the surrounding rock and shield; if the thrust supplied by the TBM propulsion system is insufficient to overcome the frictional forces when TBM advances forward, then the shield of TBM will become entrapped. Subsequently, based on the Convergence–Confinement method and the previously established constitutive model for squeezing soft rock, the calculation methods for the interaction curves (Longitudinal Deformation Profile, Ground Response Curve and Support Characteristic Curve) between soft ground and shield are put forward. Then the forecasting model for shield jamming inoculation process, the trigger condition and criterion for shield jamming are established. These forecasting models are illustrated and verified by a case study finally.