Three-dimensional numerical back-analysis of a monitored deep excavation retained by strutted diaphragm walls

The urban development often requires the use of the underground space and the number of deep excavation pits in city centers is increasing every year. To minimize the effects of excavations on adjacent structures, it is becoming increasingly important to estimate not only the lateral displacement of retaining walls but also the movement of the retained soil. Empirical methods have been developed on the basis of experimental data as to define the displacements field induced by excavation in greenfield hypothesis. However, numerical analyses can be used when more complex situations have to be analyzed. A case study of interaction between a monitored deep excavation and existing buildings is presented in this paper. The Saint-Agne subway station of Toulouse (France) new line B has been realized with a diaphragm wall supported by up to three levels of steel struts and has been built in an overconsolidated molassic geological context. The set of measurements obtained with different monitoring devices have been compared with the 3D numerical analysis using a finite difference code in which the dewatering is taken into account through an uncoupled flow-mechanical calculation. A good agreement is observed between the numerical results and the monitoring data. The model also gives an insight on the 3D behaviour of the excavation and its impact on nearby structures. Short remarks regarding the prediction of the excavation behavior by means of 2D compared to 3D numerical analysis results are briefly issued.