One mechanism in the behavior of clayey soil might be controlled by the thickness of the diffuse double layer (DDL) which governs the liquid limit. Sridharan and Venkatappa Rao (1979) stated that the liquid limit of soils is mainly influenced by the DDL held water.
The most important conclusions concerning the structure of the double layer as function of the electrolyte concentration (and/or dielectric constant) of the fluid is that the extension of the double layer in solution decreases with increasing electrolyte concentration (or decreasing dielectric constant) as shown in the following equation (van Olphen 1963, Sridharan and Jayadeva 1982, Shang et al 994, & Mitchell and Soga 2005):
For kaolinite, a change in dielectric constant (or electrolyte concentration) does not lead to any noteworthy change in double layer thickness. It can be stated that the DDL for non-swelling clays is very small or non- existant. Hence, the DDL approach does not apply for such clays.
Figure 6 (below, left) shows the liquid limits for Na-smectite and kaolinite mixed with increasing concentrations of NaCl (common salt). It can be recognised that the influence of the double layer is very important for Na-smectite as an expansive clay. For this clay, a salt concentration of 1mol is sufficient to reduce the liquid limit to values close to the minimum. This behaviour corresponds with the data gathered by Di Maio (1996). For kaolinite no appreciable influence of NaCl on the liquid limit is observed.
Starting from these experimental results, the so called ‘cone pull-out test’ as a new laboratory test to determine the clogging behaviour of different fine-grained soils has been developed (Feinendegen et al 2010). In the relevant literature up to now most authors defined the stickiness of different fine-grained soils by a determination of the adhesive forces. For this purpose mainly modified direct shear tests as well as separation tests, typically with steel pistons, have been carried out (Schlick 1989, Beretitsch 1992, Thewes 1999, Zimnik 2000, Burbaum 2009). However, one precondition for an exact measurement of adhesion forces is that there is no adherence of soil to the testing device. Particularly for piston pull tests this cannot be ensured. Furthermore, separation tests do not account for the influence of the soil parameters on the adherence.
Clogging only then occurs, when the resisting forces within the soil matrix are smaller than the bond stress between clay and steel surface.
The sample material is compacted in a standard proctor device; a steel cone is inserted into a pre-drilled cone shaped cavity and loaded for 10 minutes with the magnitude of the applied load between 3.8 kN/m? and 189 kN/m? depending on the consistency. The load is then taken off and the specimen is placed in a test stand where the cone is pulled out with a velocity of 5mm/min. The amount of soils attached to the cone is weighed and divided by the cone surface giving the “adherence” in g/m2 (Feinendegen et al 2010).
New manipulation
The cone pull-out tests were also used to determine the effectiveness of new manipulation techniques to reduce the adhesion and/or clogging of clays to the surfaces e.g. of a TBM. Since kaolinite did not show any variation in LL tests and since ethanol is not recommended for tunneling projects, only smectite was tested mixed with water and NaCl as pore medium at different consistencies (i.e. 0.4-0.55-0.7- 0.85) (figure 7, above). It gets clear how NaCl drastically reduces the adherences to the cone surface. This effect can also be seen in figure 8 (left, top). A possible explanation could be the suppression of the DDL, which causes a closer proximity of the clay particles. With a smaller DDL the clay structure becomes more compact and the amount of pore fluid necessary to induce particle mobilisation is reduced.
An alternative manipulation method may be the use of electro-osmosis. By applying an electric charge to the steel parts of a TBM, water is transported through the clay into the interface between clay and steel. This creates a film of water, the adherences are reduced, and the clay can easily be removed (van Baalen et al 2000). Using again the cone pull-out test, smectite was tested with water by applying a low electric field between the cone as negative pole (cathode) and the proctor mould as positive pole (anode).
After the application of a direct current (2.5V) for 10 minutes the cone was pulled out. The results show a strong decrease of material attached to the cone after this simple treatment.
Electro-osmosis could be easily used in situ, however because of several collateral effects (e.g. corrosion of metallic parts, energy consumption, health and safety, probable disturbance of the TBMs computers) its use in a TBM should be tested on a real scale.
Conclusions
Based on theoretical and experimental works several chemical manipulations of clays were performed. Coupling the modifications obtained in laboratory with the cone pull-out test as a new laboratory test to determine the clogging propensity of different fine-grained soils, a decrease of the adherences has been proved. The variations of the pore fluids lead to an increase in their internal shear strength causing a drop of adhesion to the metal surface of the cone.
However, the pore fluid has a major influence on smectitic clays rather than on kaolinitic clays. Additionally, the applicability of electro-osmosis as alternative way of manipulation has been shown. The laboratory findings have to be applied in situ on real tunnelling projects in further investigations.
soga Figure 6, variation of liquid limit for Na-smectite with increased electrolyte concentration in pore fluids Figure 7, cone pull-out test (after Feinendegen et al 2010); Figure 8, Adherences to the test cone fpr water (left) and 1mol NaCl figure 9, Application of electro-osmosis to reduce clogging in laboratori on a pure smectitie
