Three-dimensional monitoring of the tunnel face: development of an innovative tool for preconvergence monitoring

In the traditional design of tunnels excavation, various hypotheses are used for schematizing the mechanical and deformative behaviour of the surrounding rock mass. Due to a large number of uncertainties and to the anisotropic nature of the rock masses, the design assumptions should be validated during the construction phase. This validation is usually carried out employing regular monitoring of the rock mass deformation around the tunnel face. Various studies highlighted the relationship between tunnel convergence, preconvergence, and front extrusion, which are recognized as key parameters to understand the rock mass mechanical behaviour. Often, tunnels are built in critical conditions, such as soft soils and low ground cover, with the risk of interfering with pre-existing buildings or the nearby infrastructures. To better characterize the rock mass mechanical features, solutions and design improvements can be planned by measuring the deformations in a three-dimensional manner, both convergence and preconvergence, caused by the excavation. This approach, in agreement with the “Analisi delle Deformazioni Controllate nelle Rocce e nei Suoli” (ADECO – RS) philosophy, consists of the analysis of the deformation response of the advance core ahead of the tunnel face. In this perspective, the monitoring phase assumes an important role during the tunnel excavation but also during the project design, where continuous monitoring of the rock mass response can help the project manager to take decisions and to modify the project as in the “learn as you go” philosophy. Moreover, such approach allows to contain excavation cost and realization times. The work presented in this thesis focuses on the development of an innovative and automatic tool for the monitoring of the preconvergence deformations, called Pre-Conv Array, developed at the University of Parma. First of all, the general characteristic of the proposed tool are presented, together with the ones of the on-board sensors, and their calibration procedure. Afterwards, the calculation process that defines the recorded displacements is explained, as well as the statistical approach adopted to elaborate the large amount of data obtained during the entire monitoring period