Numerical analysis of tunnelling with jet-grouted canopy

The complex interaction between the soil and the structural elements of a tunnel built with provisional jet-grouted reinforcement is analysed with two- and three-dimensional FEM models to understand the mechanisms activated by this tunnelling methodology and to quantify the effects of possible simplifications introduced into the predictive analysis. With this goal in mind, the construction of a shallow tunnel is carefully simulated taking the geometry and the construction time sequence from a real case study. The role of soil constitutive modelling is investigated by comparing the results obtained with a linear elastic–perfectly plastic model, a hypo-plastic model, and an improved version of the latter model better reproducing the nonlinearity at the early stages of loading. Since a jet-grouted canopy, provisional sprayed concrete, and a permanent reinforced concrete lining are required to carry loads soon after their installation, the increase in stiffness and strength versus the time produced by the cement hydration has also been simulated. The outcomes of the analysis highlight the three-dimensional nature of the deformation mechanisms taking place near the advancing front, the effects produced by the different tunnelling operations, and the role of the different structural elements. Although the analysis of the settlements at the ground level reveals the importance of performing three-dimensional calculations with an accurate simulation of the nonlinearity of the soil behaviour, these aspects seem to play a minor role in the prediction of structural forces.