Influence of chip-size on development of adhesion for conditioned clayey soils

In the excavation of clayey soil with Earth Pressure Balance TBMs, the study of soil conditioning is a complex issue. The presence of a mix of water, clay and foam, often added with polymers, makes the resultant mass a material strongly time-dependent and where the internal forces are given both by mass and surface actions. Differently from granular soils, where the time-dependence of the conditioned mass is ruled by the decay of foam, in clays one the rate of adsorption of the liquid phase of the conditioning into the clay chips cannot be neglected since strongly affects the quality of the conditioned mass in time. This paper compares the difference in the adsorption rate through the evaluation of the dynamic adhesion for different chip size. For each studied clay, different chip size distributions are obtained and tested. The difference in the development of adhesion reflects the different rate of adsorption of the liquid phase showing that the dimensions of the chips should be considered in the choice of clay conditioning parameters with reference also to the estimated time spent by the conditioned material to go through the excavation chamber and to exit from the screw conveyor.