Preliminary test of an innovative pre-convergence monitoring tool for traditional tunnels

During a traditional tunnel excavation, the behaviour of the surrounding rock mass is a meaningful evidence of the appropriate design assumptions. Various studies have highlighted the relationship between tunnel convergence and the pre-convergence, which is recognized as the key parameter to understand and control this behaviour. Up to now, the pre-convergence has been assessed with an indirect approach, deriving its values from other parameters. This paper presents a series of laboratory tests performed on a new instrument designed for the direct monitoring of pre-convergence. The tool (called PreConv Array) is based on 3D MEMS sensors and it is designed to be inserted inside an advancing borehole, providing information about the deformation of rock mass ahead the excavation face. The tests were aimed to evaluate the sensitivity, accuracy and uncertainty of the instrumentation, by imposing various deformations to a PreConv Array prototype and comparing the readings with those obtained by a topographic survey.