Experimental investigation on the rheological behavior of bentonite slurries for SS-TBM excavation

Slurry Shield Tunnel Boring Machines (SS-TBM) are soil machines widely employed for underground excavation in urban environments, where excavation at shallow depths in soft, unconsolidated and water bearing-soil, poses major challenges related to face stability and ground settlement. These machines use a bentonite-based slurry to maintain counter pressure, prevent groundwater inflow, and ensure the stability of the excavation face. However, the rheological properties of bentonite slurries can be significantly affected by mechanical degradation during excavation, which may lead to viscosity reduction, problems in pressure transmission and filter cake formation, potentially leading to face instability or slurry losses. This research investigates the rheological behavior of eight different bentonites, used to prepare slurries at varying concentrations, subjected to controlled mechanical stress in order to simulate, at laboratory scale, the stresses experienced by bentonite during SS-TBM excavation. A preliminary characterization was performed through swelling index and liquid limit tests, to identify the main physical properties and classify bentonites based on objective parameters, providing a useful framework for interpreting the rheological results. Subsequently, standard rheological measurements were performed on both unstressed and mechanically stressed slurries using the Marsh cone test, viscometer, and density and temperature assessments. In addition, a rapid evaluation of the slurry-soil interaction was carried out, by investigating filter cake formation and quality, to study the influence of the rheological properties on its development. The study focuses on evaluating how the rheological behavior of bentonite slurries evolves under shear stress, highlighting the relationship between viscosity variation, bentonite type, concentration and applied mechanical stress. These results contribute to a better understanding of bentonite behavior under operational conditions and provide useful guidance for optimizing slurry management in tunnelling applications.