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Mechanised excavation of tunnels and the fabrication of deep foundation elements in soft ground often require fluid support for the temporarily created openings. The supporting fluids are generally mixtures of bentonite and water, together with chemical additives for difficult geological conditions. The properties of the slurries are selected to ensure the stability of the excavation, in particular to provide sufficient support to the coarse-grained soil layers. When these are interlaid with clayey layers, excavation can be hindered by clogging problems caused by the stickiness of the excavated clay, which can be affected both by the clay mineralogy and the composition of the supporting slurry. Laboratory studies were conducted to determine the rheological properties of bentonite slurries and pure polymer slurries containing poly (acrylamide-co-acrylic acid), methyl cellulose or sodium carboxymethyl cellulose. A mixing test was used to determine the clay stickiness ratio and a model tunnel boring machine cutter-head test to determine clogging potential. The results showed that the stickiness of the excavated soil for bentonite slurries correlated with the shear resistance of the slurry, indicating that increased slurry strength, while beneficial for excavations in coarse soils, may lead to increased susceptibility to clogging under mixed face conditions. In contrast, some pure polymer slurries can help to combine high slurry resistance with low clogging potential by protecting the clay aggregate surface from penetration by water., Mechanised excavation of tunnels and the fabrication of deep foundation elements in soft ground often require fluid support for the temporarily created openings. The supporting fluids are generally mixtures of bentonite and water, together with chemical additives for difficult geological conditions. The properties of the slurries are selected to ensure the stability of the excavation, in particular to provide sufficient support to the coarse-grained soil layers. When these are interlaid with clayey layers, excavation can be hindered by clogging problems caused by the stickiness of the excavated clay, which can be affected both by the clay mineralogy and the composition of the supporting slurry. Laboratory studies were conducted to determine the rheological properties of bentonite slurries and pure polymer slurries containing poly (acrylamide-co-acrylic acid), methyl cellulose or sodium carboxymethyl cellulose. A mixing test was used to determine the clay stickiness ratio and a model tunnel boring machine cutter-head test to determine clogging potential. The results showed that the stickiness of the excavated soil for bentonite slurries correlated with the shear resistance of the slurry, indicating that increased slurry strength, while beneficial for excavations in coarse soils, may lead to increased susceptibility to clogging under mixed face conditions. In contrast, some pure polymer slurries can help to combine high slurry resistance with low clogging potential by protecting the clay aggregate surface from penetration by water.

A major hazard in tunnelling with earth pressure balance machines in clay soils is clogging of the shield openings and, in the worst cases, plugging of the machine. The use of soil conditioners, in particular foams and dispersing polymers, results in reduced stickiness and tendency to clogging and to improved soil-machine interaction. New methods are proposed for the quantification of stickiness and adhesion in the absence of standard procedures. The first method involves the determination of an empirical stickiness ratio by weighing the soil adhering to a mixing tool after the soil preparation process. Laboratory tests were carried out using a Hobart mortar mixer with a mixing paddle, and the stickiness ratio defined as the ratio between the amount of soil sticking to the mixing tool and the total weight of soil in the mixer. The second method employs a shear plate apparatus which allows measurement 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. Tests were conducted using soil mixtures containing kaolin, fine-, medium- and coarse grained illite and bentonite clays and two conditioners, one containing an anionic surfactant and the second modified polycarboxylates. The results showed that the stickiness ratio correlated well with the tangential adhesion and sliding resistance, following a logarithmic trend, and that the chemicals exhibited a stronger effect at soft soil consistencies compared with stiffer consistencies. The water content of the mixtures had a significant effect on chemical efficiency, with the adhesion reduction being lower at lower water contents., A major hazard in tunnelling with earth pressure balance machines in clay soils is clogging of the shield openings and, in the worst cases, plugging of the machine. The use of soil conditioners, in particular foams and dispersing polymers, results in reduced stickiness and tendency to clogging and to improved soil-machine interaction. New methods are proposed for the quantification of stickiness and adhesion in the absence of standard procedures. The first method involves the determination of an empirical stickiness ratio by weighing the soil adhering to a mixing tool after the soil preparation process. Laboratory tests were carried out using a Hobart mortar mixer with a mixing paddle, and the stickiness ratio defined as the ratio between the amount of soil sticking to the mixing tool and the total weight of soil in the mixer. The second method employs a shear plate apparatus which allows measurement 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. Tests were conducted using soil mixtures containing kaolin, fine-, medium- and coarse grained illite and bentonite clays and two conditioners, one containing an anionic surfactant and the second modified polycarboxylates. The results showed that the stickiness ratio correlated well with the tangential adhesion and sliding resistance, following a logarithmic trend, and that the chemicals exhibited a stronger effect at soft soil consistencies compared with stiffer consistencies. The water content of the mixtures had a significant effect on chemical efficiency, with the adhesion reduction being lower at lower water contents.