Geotechnical characterization of natural clays for the prediction of clogging risk for TBM

During mechanical tunnel driving it is known that the excavated soils can stick to the metallic part of the tunnel boring machine (TBM). Despite the fact that several test methodologies have been developed during the last 15 years to provide an assessment or quantification of the clogging risk, no standardized method is available yet. This study evaluated three natural clay samples with different grain size distribution and mineralogical content considering two widespread test set ups for the evaluation of the clogging tendency: the Hobart mixing and the plate pull-out test. After a comprehensive geotechnical characterization of the three clay samples, adherence and plate pull-out force have been measured at different water content/consistency index. Results show that adherence and plate pull-out force in terms of water content w and consistency index I-C follow normal distribution curves which, as the percentage of clay, the content of clay minerals and the plasticity increases, have higher amplitudes and higher peak values. In general, the existence of a correlation between geotechnical classifications parameters and the behavior in terms of potential clogging risk for natural clays makes it possible to predict these risks in the design phase of the tunnel excavation and to plan appropriate mitigation actions.