A method to design microtunnelling installations in randomly cemented Torino alluvial soil

Abstract

The paper is devoted to improve the applicability of the microtunnelling technique to the metropolitan area of Torino, by producing useful tools to estimate the magnitude of the jacking forces required by the microtunneller and the pipeline to advance into the ground. The subsoil in the city is characterised by a sand and gravel deposit, ranging from medium to highly dense, down to a depth of 8–10m; below this depth randomly distributed cemented soil (in cases a conglomerate), due to calcareous deposition processes, is often present. A site scale distinct element numerical model able to simulate the excavation of a microtunnel in the partially cemented soil (from non-cemented to fully cemented) was built to investigate the influence of the cementation degree on soil–pipe interaction and to point out the unstable area eventually generated after the excavation of the microtunnel. This led to the definition of an empirical relationship between jacking forces and the degree of cementation in the ground that can be usefully used at the design analysis stage to predict jacking forces. A case study which exemplifies the suggested procedure is also described.