Twin earth pressure balance tunneling – Monitoring and numerical study of a soft soil urban case

In recent decades, numerical calculations are often used to optimize from a technical and economical point of view the design of underground works. Nowadays, using these tools permits to obtain efficient and affordable designs. The simulation of the tunneling procedure in case of the use of a tunneling boring machine and in urban areas is a complex soil/structure interaction problem. The use of continuum numerical methods is then required. In this paper, a three-dimensional code using finite differences is used. The reference case of this study is the Lyon metro line D project in France where a non-symmetric load is present at the surface due to the presence of a retaining wall. An accurate on-site monitoring system was set up to assess ground movements during the mechanized excavation of twin tunnels. The collected data is used as a reference to show the validity and accuracy of the developed three-dimensional model. A procedure that simulates the segmental lining installation is developed to simulate the tunneling process. Two constitutive models were used to simulate the soil behavior: Plastic Hardening (PH) and linear elasticity with perfect plasticity (Mohr-Coulomb shear failure criterion). The simpler soil constitutive model shows some drawbacks which can be eliminated by using the Plastic Hardening. The existence of a retaining wall induces a non-symmetric load condition; however, the settlement trough predicted by advanced numerical models shows a good agreement with the monitoring data one.