Effects of dispersing foams and polymers on the mechanical behaviour of clay pastes

The use of soil conditioners during excavation with earth pressure balance machines offers a number of advantages, including increased advancing rates, higher safety and settlement minimisation in a variety of ground conditions. Dispersing polymers and foams are added to optimise the strength and improve the deformation properties of the supporting soil paste. In this paper, possible effects of some commercially used products on clays are discussed. Whereas for most clays conditioning is based on lubrication (leading to reduced shear strength), for calcium bentonite a high polymer dosage can lead to the counterproductive effect of increasing the material's strength. In the case of kaolin, the change of mechanical behaviour is much more dramatic than simply reducing the strength; instead, the soil transforms into a viscous material with both liquid and solid properties. This unexpected phenomenon is characterised in detail and the working mechanisms of the used dispersants are identified. Shear-strength measurements at different polymer concentrations, with different kaolins and commercial clay dispersants are presented. The results suggest an enhanced particle-lubrication mechanism, arising from ion exchange and altered electrostatic forces between the clay surfaces and the surrounding water layer; this picture is confirmed by additional adsorption and microscopic measurements.