Excavation of clayey soils by mechanical tools is often hindered by the stickiness of the clay on metal parts. Hindrance may occur at the cutting tool itself, or during the subsequent transport of the soil through the excavation system. The effects of stickiness were also experienced in a recent tunnel boring project carried out in The Netherlands. In this study the soil parameters that influence stickiness of clayey soils are investigated. Stickiness can be described in terms of adhesion and adhesive friction. These parameters can be determined in the laboratory by undertaking shear box tests during which the clay is sheared over a standard metal surface. The relation of the basic clay classification parameters (Atterberg limits, percentage of clay, clay mineralogy) with adhesion and adhesion friction has been examined. For one clay type, a potters clay, the variation of the adhesion parameters with the moisture content of the clay has been investigated. For a given metal surface, adhesion and adhesive friction are shown to depend on the clay mineral but also on clay fraction, plasticity, moisture content, degree of consolidation and normal stress. Although for an improved understanding of the phenomenon much more testing on different types of clay is required, the first results indicate that prediction of the occurrence of stickiness is possible if the operational parameters of the tunnel boring machine and the adhesion and adhesive friction parameters of the soil material are known. Using the operational parameters, the driving forces in the system can be estimated while knowing the adhesion properties of the clay-metal contact allows the calculation of the resistive forces in the system. The standard soil investigation should therefore include shear box tests to determine these adhesive parameters. During tunnel boring stickiness can occur in three zones: at the cutting wheel, in the mixing chamber and in the slurry line during pipeline transport. Simple force balance models that examine the movement of the clay along the cutting teeth, along the arms of the cutting wheel or along the walls of chamber or pipeline have been made. If the driving forces are smaller than the resistance along the metal-clay surface, no movement will occur along this surface and sticking is assumed to take place. Several factors are important in the analysis: • the cohesion and internal friction of the clay, • the adhesion and adhesive friction between clay and metal, • the shear resistance between clay lumps, • the roughness of the contact surface. @en