Recently the first full face tunnel boring machines were used in the Netherlands. Several problems have been encountered in tunnelling projects in the Dutch soft soils. One was the adherence of stiff clays to the TBM cutting wheel, mixing chamber and the pipelines, which transport the clay from the front. Adherence of clay can be described in terms of adhesion and adhesive friction. Parameters which influence these two factors are on one hand the mineralogy, texture, the (over)-consolidation ratio and composition of the clay and on the other hand the type of steel and the textural state and condition of the steel surface. To study the adherence of clay, two types of clays were tested. Tests have been performed on Speswhite, which consists mostly of kaolinite and on Boom clay, which is a mixture of quartz, illite and kaolinite. Samples were prepared from clay powders. The adherence behaviour of clay was investigated by measuring the shear stress required to shear the clay over a steel surface in a direct shearbox. Results show a great influence of the steel roughness, contact time and mineral type. This is in accordance with practical experience and previous literature. The shearbox test gives a good quantitative impression of the adhesive shear strength, however it is expected that other devices like the simple shearbox and the ring shearbox are better suited to study the parameter variation during the tests.@en

When boring a tunnel with a tunnel boring machine (TBM) through stiff clays, clay tends to stick to the cutting wheel, to the inside of the excavation chamber and in the slurry transportation line. Several solutions are available to prevent the adherence of clay, but these are not always satisfactory. An alternative solution may be the use of electro-osmosis. By applying an electric charge to the steel parts of a TBM, water can be transported through the clay by electro-osmosis to the interface between the clay and the steel. This creates a film of water at the clay-steel interface and therefore reduces the adherence. Because of this film, the hydraulic force of the slurry can easily remove the clay. To investigate the effect of electro-osmosis on clay adherence, laboratory tests were performed on four different clay soils using two test methods, the tilted plate and the direct shear box. It is shown that both tests can be used to reveal the sensitivity of a clay soil to a reduction in the adherence by electro-osmosis. Apart from a so-called threshold potential below which little reduction in adherence is observed, the shear box test allows the determination of the shear stress drop due to electro-osmosis. Finally, a feasibility study is carried out in order to investigate the applicability of electro-osmosis to reduce the adherence inside a TBM. This depends on three factors: properties of the excavated clay soil, practical limitations or negative side effects and the energy consumption.@en