Shield tunneling in sensitive areas: A new design approach for the optimization of the construction-phase management

摘要

The evaluation of the correct face-stabilizing pressure is a critical element in the various design and construction phases of a tunnel excavated by means of a TBM with Earth pressure balanced shield (EPBS) in difficult geotechnical conditions. During the advancement of an EPBS, a combined use of analytical (probabilistic) and numerical methods is probably the most efficient approach to continuously check the actual conditions encountered and apply the counter-measures in a timely manner. An innovative, and well-applied, procedure for optimising the construction phase management is described in this article. The starting point of this procedure involves the verification of the results of numerical methods obtained from referenced analytical methods. In the first step of the procedure the results obtained through the (Caquot) analytical method are verified by means of a numerical method in order to evaluate the practical consequences in terms of development of deformations and plastic zone. In this manner, the assumed design risk is evaluated for the different methods and the solution that gives the best correspondence with numerical simulation is selected. Then, residual uncertainties and parametric variations are incorporated in the analysis. Monte Carlo simulation is used to calculate the statistical distribution of the face-stabilizing pressure, and the design value is selected on the basis of an acceptable probability of failure. Finite element analysis (FEM) is finally performed on the basis of this value and results of back-analysis to verify the correspondence between numerical results and monitoring data. The application of the procedure to the twin tunnels underpassing the city of Bologna described here starts with determination of the face pressure (as a function of the stability of the face and the need for confinement of pre-settlements).