Monitoring of preconvergence deformations in a road tunnel: Data analysis and validation

Tunnels are complex constructions, generally built in difficult geological contexts. When dealing with underground structures, the study of ground deformations is a key aspect to consider in order to guarantee safety during the tunnel excavation and construction quality. One of the main aspects to investigate is related to the development of preconvergence phenomena in the advance core, i.e. deformations involving the volume of rock mass ahead of the tunnel face. This paper presents the application of a new monitoring tool specifically developed to measure preconvergence effects during the excavation phases with a direct approach. The device, called PreConv Array, consists of a series of 3D MEMS (Micro Electro-Mechanical System) and temperature sensors. The system takes advantage of automated procedures for data acquisition, elaboration, and representation, thus achieving a near-real time monitoring of the ground differential vertical settlements ahead of the excavated face. Monitoring results reported in this paper are related to the installation of a PreConv Array during the excavation phases of a road tunnel located in Northern Italy. The collected data allowed to highlight the displacements of the tunnel crown in correspondence of each step of the excavation works. Moreover, the comparison with theoretical Longitudinal Deformation Profiles (LDP) evidenced the good correspondence between PreConv data and the theoretical curves.