Crack formation and damage evolution during consolidation in TBM driven tunnel linings in fine-grained soils

The paper deals with the numerical modelling of crack formation in segmental tunnel linings. A series of numerical analyses was conducted using the finite difference code FLAC2D. The primary aims of the analyses were to back-analyse the damage pattern observed in a TBM driven hydraulic tunnel excavated in clayey soils and to evaluate the safety level of the excavation assessing the stress and strain state of the lining. The excavation of the tunnel and the lining installation were simulated in plane-strain undrained conditions, adopting the stress reduction method. To take into consideration the peculiar interaction mechanism, identified as the cause the damages, the stress release was differentiated based on the orientation along the tunnel wall. Two distinct modelling strategies were used to model the tunnel lining: at first, simple beam elements were used, then, small continuum elements and cable elements were employed to represent the concrete and the steel bars respectively. The implemented algorithm allowed to simulate explicitly the formation of the cracks and their progressive development. Finally, consolidation analyses were carried out to assess the evolution of the damage and the long-term stress and strain level of the lining. The numerical analyses allowed to reproduce the observed damage pattern and to reliably evaluate the stress and strain state in the damaged lining. Furthermore, the long-term analyses showed that the consolidation process has a beneficial effect as the equalization of the pore pressures causes a reduction of the load eccentricity on the lining, thus progressively increasing the level of safety over time. The investigation of the causes of the reported damage and its numerical modelling allowed to remark the importance of proper tail void grouting when excavating under high cover depths in squeezing soils.