Shield tunnelling in sensitive areas: a new design procedure for optimisation of the construction-phase management

摘要

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