Experimental investigation of the critical tractive force of glass beads under upward seepage flow

Abstract

For soil structures subjected to seepage flows as well as water flows on their surface, such as river levees and the mound beneath breakwater, erosion of the soil materials become a threat which may cause their failures. In order to assess stability or instability of these soil structures, the effect of the seepage flow on the surface erosion need to be clarified. This article investigates the critical tractive force of a cohesionless material subjected to upward seepage flow. The critical tractive force is defined as the bed shear stress initiating the motion of the soil particles forming the cohesionless bed, which is the most fundamental property related to the resistibility against surface erosion. The effects of the seepage flow on the surface erosion and the sediment transport are divided into the following three categories. – Change of the velocity profile and the bed shear stress on the permeable bed. – Decrease or increase of the critical tractive force of the bed. – Decrease or increase of the erosion rate or the sediment transport rate of the bed material. Thus far, the change in velocity profile of the surface flow due to the seepage flow in the permeable bed has been investigated, and modifications were added to the wall-law, such as the logarithmic law, considering the seepage velocity on the bed. The upward or downward seepage flow is called injection or suction, respectively. The critical tractive force of the glass beads under injection has been experimentally investigated. Since the upward seepage force decreases the virtual weight of the bed material, the conventional concept of the dimensionless critical tractive force has shown that the upward hydraulic gradient linearly decreases the critical tractive force and makes it vanish at the quick-sand condition (e.g., Cheng & Chiew, 1999). A test apparatus has been built: a water channel with a rectangular cross section were prepared and a section of the sediment bed subjected to the upward seepage flow were installed at the bottom of the channel. The test results have shown that the critical tractive force of the cohesionless material slightly decreases as the hydraulic gradient of the seepage flow increases, but that it does not drastically decrease even when the hydraulic gradient reaches the quick-sand condition. The accurate determination of the hydraulic gradient in the vicinity of the bed surface and the bed shear stress with the injection is required for the better understanding of the test results.