TBM tunnelling-induced settlements in coarse-grained soils: The case of the new Milan underground line 5

In this paper measurements of tunnelling-induced settlements collected during the construction of the new Milan underground line 5 are presented. The twin tunnels are being excavated in coarse-grained soils partially under the water table with an earth pressure balance (EPB) machine. In the following the attention is given only to the monitoring data recorded under free-field conditions (i.e. no structure on the soil surface) during and after the construction of the first tunnel. Settlement measurements are back-analysed using the classical Gaussian empirical predictions, both in the transverse and longitudinal directions, providing a very detailed description of the EPB tunnelling performance under such geotechnical conditions. In particular, a translated Gaussian cumulative curve, to be related to the influence of pressure applied at the face, is proposed to better match the evolution of settlements during tunnel advancement. Finally, the influence of different excavation parameters (such as face pressure, grouting pressure, machine thrust) on the estimated volume loss and shape of subsidence trough is investigated.