EPB tunnelling through clay-sand mixed soils: Proposed methodology for clogging evaluation

The clogging of Tunnel Boring Machine (TBM) tools by soils has long been investigated, owing to the numerous difficulties arising in shield tunnelling as a result. Its occurrence leads to operation delays owing to the frequent and lengthy interventions required to unblock the soil stuck to the excavation tools and screw conveyor. Several authors have proposed laboratory tests for evaluating the clogging potential, however, those include limitations, such as not considering the clay fraction in a soil. One of these methods is the empirical stickiness evaluation, whereby a mixer and a beater are used to define a clogging evaluation parameter. Following an extended test campaign using soils with different clay contents and minerals, it became clear that this method was not adequate to provide reliable information regarding the tendency of a soil to clog in a tunnel drive. A new device was then implemented, which adds to the first method a kinetic energy impulse via dropping of the beater from a certain height. This combination of methods could provide a reasonable approximation of the potential for clogging to occur along Earth Pressure Balance Machine (EPB) tunnel drives. This paper presents the results of the proposed combined methodology for clogging evaluation, as well as the research evolution that led to the addition of the beater dropping stage.