In case of hydraulic transportation of soil through pipes, the consideration for the flow velocity suited to the characteristics of soil (for instance, specific weight, grain size, shape and the like), the solid content and the pipe diameter is an important factor for the economical transportation of soil.
　At the time of the selection of the flow velocity, those which fundamentally express the transportability of particles will be the settling velocity and the initial stirring velocity in horizontal pipes.
　In this report we investigated the initial stirring velocity of a particle in horizontal pipes and compared it with the theoretica 1 equations derived from the equilibrium conditions between fluid resistances and frictional forces.
　Within our test, we concluded that the influences of Re were small and the initial stirring velocity could be approximately shown as the relation between Fr/μs and K.Besides we also discussed the relation of it to the settling velocity.
　Grain sizes used here ranged from 0.079 to 20mm and pipe diameters were 50, 75 and 100mm.