Physics of adhesion of soils to solid surfaces

Tunnel driving in cohesive soil using slurry- or earth pressure balance-tunnel boring machine often encounters serious problems of clogging of parts of the cutting wheel, the excavation chamber or along the cutting handling and discharge systems. This limits mechanised tunnel driving. Clogging is induced by the adhesion of cohesive soils to solid surfaces. In some cases, clogging brings tunnel excavation to a stand-still. The adhesion of soils to solid surfaces depends on capillary forces of the fluid film in the interface between the soil and the solid surface, and can be understood as a boundary layer effect. This fluid film is mostly present due to the water content of the soil itself, or due to the fluid in slurries or earth mud for face supporting. The capillarity forces increase reciprocally with decreasing thickness of the fluid film. The thickness of the fluid film is not static, but is controlled by the soil that adheres to the solid surface. The thickness of the fluid film depends on two soil properties. The first element is the pore water tension (suction) of the soil, which stimulates fluid flow from the interface into the soil matrix. This reduces the fluid volume in the interface, thus the thickness of the fluid film. Pore water tension depends on the mineralogical properties of the clays as well as on saturation. It is a one-dimensional, transient fluid flow in unsaturated conditions. The second element is the permeability of the soil. This paper provides a physical soil model explaining this process. Results of adhesion tests and pore water tension measurements during adhesion tests on adhesive soils supporting the theoretical approach are presented. Permeability test results on unsaturated clays using a permeameter are given.