This chapter provides several examples that illustrate the role of petrography and the need for a new direction in aggregate research. It discusses the concept of durability and reviews the information laboratory testing methods of geomaterials. A general introduction, describing the special nature of geomaterials with their varying mineralogical composition and texture, is presented. The geological construction materials that are commonly used in civil engineering construction are mainly natural soil aggregates and crushed rock or rock blocks. The microscopic structure, just like the internal structure of other materials such as steel or concrete, determines to a large extent the mechanical behaviour of the geological materials. The ideal approach to the evaluation of geomaterials would make use of this aspect and incorporate probabilities, based on the variability of observations and test results, rather than fixed values. Weathering can lead to considerable variation of the geotechnical properties within a rock unit.@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

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

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

Review of the application of the Methylene Blue adsorption test (titration spot method) applied to geomaterials (soils and rocks). With this method the Cation Exchange Capacity (CEC) of fine soils or fine grounded rock powder can be determined. Swelling clays have a high CEC and even small quantities (less than 1 mass % can be detected by this method. Applications of the method are discussed including assessment of the durability of armourstone.@en