Numerical Evaluation of the Surface Displacements due to Soil Grouting and to Tunnel Excavation

Abstract

The low-pressure grouting with cement mixtures is one of the techniques adopted for improving the mechanical characteristics of granular soils prior to the excavation of underground openings. When applied to shallow subway tunnels, this procedure often induces heaves of the ground surface, which are followed by settlements during the excavation phase. In urban areas, these displacements may lead to appreciable damages of existing buildings. Here, a finite element analysis is presented of the surface displacements recorded during the construction of a subway tunnel in the city of Milan (northern Italy). The critical comparison between numerical and experimental data singles out the limits of an analysis approach used in tunneling practice that simulates the injections through a nonelastic volume strain. To overcome them, an alternative analysis scheme is adopted, based on the application of a neutral pressure within the grouted zone, which seems preferable for estimating the surface displacement caused by the field works.