Analysis of the Importance of the Filling Material Characteristics Injected Around the Precast Concrete Lining in the Microtunnelling Technology

Microtunnelling is an excavation technique capable of positioning underground pipes with considerable advantages from the point of view of the environment, safety, construction times and also total costs, when the lengths are significant. The technology used is now reliable and is suitable for use in different types of soils and rocks without particular problems. The installation of the concrete lining of the micro-tunnel takes place behind the excavation machine starting from a thrust shaft. A filling material must be injected between the outer surface of the lining and the tunnel wall. This filling material plays a very important role, because it conditions the overall stiffness of the support system and, therefore, influences the radial load applied by the ground to the support system. Therefore, it is necessary to take into account the presence of the filling material in order to correctly size the support system of the micro-tunnel. This paper illustrates a new procedure for analyzing the behavior of the micro-tunnel support system, based on widely used and tested analytical calculation methods. This procedure was applied to a real case, underway in Northern Italy. The results obtained were then compared with those of numerical modeling, which made it possible to simulate the micro-tunnel in detail. From the comparison it was possible to note how the analytical calculation procedure leads to stress states similar (slightly higher) to those of numerical modeling; the first, therefore, can be effectively used in the preliminary stages of the design of micro-tunnels to obtain useful information on the characteristics of the support systems to be adopted to ensure the stability.