Fall velocity of particles in shear flow of drilling fluids

Viscoplastic drilling fluids discharge excavated solid particles in soft soil tunnelling. Deposition of particles in pipes and boreholes should be kept to a minimum. To quantify the fall velocity of particles in shear flow different combinations of fluid properties and particle sizes were tested. It is shown that co-rotation of the particles with the flow and vertical equilibrium of forces determine the fall velocity in these fluids. The use of a Stokes type formula for fall velocity in shear flow of tunnelling fluids is justified.