ABSTRACT
Mechanized tunnelling with EPB-TBMs bases its success on the ability to guarantee a uniform and constant front-face pressure during the excavation, using the excavated soil properly treated with chemical additives. This front-face pressure, measured with pressure sensors inside the working chamber, is kept constant during the excavation phase by balancing the amount of incoming soil, the foam injected, and the amount of soil extracted through the screw conveyor in the working chamber. During the standstill phase, required to install the tunnel lining elements, particularly in the coarse-grained soils, the pressure inside the chamber tends to decrease until the next resumption of the excavation. This effect, mainly linked to the permeability of the soil outside the TBM, tends to increase settlements induced on the surface and, therefore, should be minimized. In this paper, a very simple calculation model proposed by Bezuijen and Gerheim Souza Dias in 2017 for tunnelling in saturated sandy soils, is applied to an Italian tunnelling project. A comparison of model predictions and data recorded real time on the site during the excavation is illustrated and discussed. The predictive ability of the calculation model proposed for saturated sand seems to be useful also for TBM applications in gravelly soil under the phreatic surface, highlighting the weight of the main factors affecting the specific boundary value problem; moreover, it can be a useful tool to address the problem and to individuate technical countermeasures finalized to minimize the phenomenon and its negative effects.
