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.
During the realization of underground constructions, an in-depth knowledge of the rock mass behavior represents a crucial factor for the correct execution of the excavation works. In fact, the tunnel design process depends strongly on the structure interaction with the surrounding environment, both during its construction and the subsequent operational phase [1].
For this reason, monitoring instrumentation has progressively gained more and more importance in underground excavation processes. Following the principles introduced by the observational method [2], the monitoring activity should aim to obtain information on ground response, verify design parameters, identify any potential critical trend, and provide an overall control on the construction process [3-5]. In particular, automated monitoring systems presents a series of advantages that makes them an appropriate choice in these scenarios. These include the possibility to manage and control all instrumentation from remote, the removal of uncertainties related to manual operations, and the ability to achieve higher sampling frequencies [6,7].
