Finite element analyses on cohesive soil behavior due to advanced shield tunneling

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Virginia Polytechnic Institute and State University


Soil tunnels are usually constructed using a shield with an open face. However, in the past decade, innovations in shield tunneling technology have brought closed-faced shields that provide continuous support to the face and permit tunneling through even the most difficult conditions of soft ground. These new machines are typically operated in such a way that during tunneling the soil at the face is actually heaved away from the shield. This operating procedure has been said to allow greater control of the ground movements around and above the shield, and to minimize detrimental settlements. However, there is little hard evidence to this effect and there is no rational basis to judge the actual influence of the soil heave.

Building on former researchers’ efforts, this thesis is directed towards developing a suitable finite element method (FEM) approach to the advanced shield problem. The FEM program developed includes the Prevost elasto-plastic soil model, allows for analysis of development and dissipation of excess pore pressure, large deformation, and simulation of the construction procedure of advanced shield tunneling.

This is the first time that the Prevost model was applied to a soft clay. Modifications were made, in particular for the parameter determination, to make the model applicable for the soft clay of San Francisco Bay Mud. Examination of two other soil models for the tunnel analysis, nonlinear pseudo-elastic and Cam Clay models, showed the Prevost model to be preferable. Loading procedures were also examined to accurately simulate the heaving and tail void closure effects.

The finite element simulation of the N-2 sewer project, which is the first advanced shield project in the United States, demonstrated that the prediction agreed consistently well with the observations in the field. Further analyses indicated that heave at the face of the shield increases long·term consolidation settlements while it decreases immediate settlements and thus the final settlement may be reduced. The tail void simply increases settlements.

The results suggest that strict control of heaving and elimination of tail void with proper and prompt grouting are crucial for mitigating ground movements with advanced shield tunneling.