FUNDAMENTAL RESEARCH ON THE SCOURING MECHANICS

Abstract

Scouring is generally a phenomenon that takes place by running water on the movable bed. The external force in this case is given both as the shearing force and the normal force, therefore, various kinds of scouring arise from the combination of these two sorts of force.For the analytical research of the scouring phenomenon, we are required to reveal the happenings when each of these forces works independently.Actually, the scour resulting from the action of the simple shearing stress has hitherto been treated as a problem of critical tractive force. In this case, on the assumption that the drag coefficient of sand grains is similar to that of falling grains in the water at rest, the author derived an experimental formula for the critical tractive force from the published experimental data and the author's own data, but range of application of the analytical research of scour on a particular locality is wider when considering the critical bed velocity along the movable bed rather than when the critical tractive force is considered.According to the above considerations, the author discussed the drag coefficient for rest state of movable bed on the basis of our experimental data, deriving the critical bed velocity formula from considerations of both the tractive force and drag coefficient. The author nominated the scouring phenomenon in this case “scouring mechanics of the first kind”, and these problemes have been alloted for chapter I.If the normal force or pressure changes along the movable bed, seepage force is exerted on the sand grains bringing about scour as in the case of quicksand and piping. On the assumption that permeable layer is uniform, the author discussed the piping phenomenon, and proposed a new creep theory which is proved by his own experiments.The phenomenon of scour due to such seepage water is styled “scouring mechanics of the second kind”, and the piping problem has been alloted for chapter II.The most natural scouring phenomenon takes place when two forces act along the movable bed at the same time, the phenomenon of scour in this case is styled “scouring mechanics of the third kind”.Analytical research of several problems in this case can be performed by replacing the movable bed with the fixed bed, investigating the two external forces, and then assuming the scouring phenomenon. Dealing with the scour from jet, as an example, the writer put it in chapter III.From the above considerations, the phenomenon of scour is analyzed and systematized in this paper, giving us expectation to find a rational method of engineering work for the effective prevention of scouring.