Stickiness and adhesion of conditioned clay pastes

► A procedure for quantification of stickiness of soft clay pastes is presented. ► A device for measurement of tangential adhesion between clay and steel is introduced. ► Tangential adhesion is found to correlate with stickiness. ► Dispersing soil conditioners are reducing tangential adhesion. ► At lower water contents the effect of dispersing chemicals diminishes. One of the main hazard scenarios for tunnelling with Earth Pressure Balance Machines (EPBMs) in clay soils is the clogging of the shield openings and, in the worst case, plugging of the machine. Application of soil conditioners, particularly foams and dispersing polymers, leads to decreased stickiness and clogging tendency and to improved soil–machine interaction in the excavation process. Due to the lack of standard procedures, an attempt is made to propose new methods for stickiness and adhesion quantification. The first one is a simple procedure which enables the determination of an empirical stickiness ratio by weighing the soil adhering to a mixing tool after the soil preparation process. The second method uses a newly developed device, which allows measuring of the tangential adhesion and sliding resistance between a steel plate and soft soil pastes at different applied pressures, as they occur in the pressure chamber of an EPBM. The stickiness ratio was found to correlate well with the tangential adhesion and sliding resistance following a logarithmic trend for all tested mixtures, exhibiting a higher effect of chemicals on stickiness reduction at soft soil consistencies but a lower effect at stiffer consistencies. The results clearly showed a decreased sliding resistance due to chemical application with a strong dependency on the clay mineral encountered. The mixture water content is of major importance for chemical efficiency, diminishing adhesion reduction at lower water contents.