Backward erosion piping is driven by seepage forces acting on the soil grains at the downstream end of the seepage path. A new device for the laboratory testing of backward erosion progression was developed and tested. The device consists of a plexiglass prism at which the seepage path has been predefined. The prism was equipped with an inflow consisting of gravel separated from tested sand by a strainer. The hydraulic gradient along the seepage pipe was observed by a set of piezometers and pressure cells, and the seepage discharge was measured volumetrically. The transported sediment was trapped in a vertical cone located downstream from the device. The progression of the seepage path, the piezometric heads and the trapped material was observed by two synchronous cameras. 15 trial tests have been carried out to date, and from these, the interim results are presented.
Empirical formulae are often used in practice to quickly and cheaply determine the hydraulic conductivity of soil. Numerous relations based on dimensional analysis and experimental measurements have been published for the determination of hydraulic conductivity since the end of 19th century. In this paper, 20 available empirical formulae are listed, converted and re-arranged into SI units. Experimental research was carried out concerning hydraulic conductivity for three glass bead size (diameters 0.2 mm, 0.5 mm and 1.0 mm) and variable porosity. The series of experiments consisted of 177 separate tests conducted in order to obtain relevant statistical sets. The validity of various published porosity functions and empirical formulae was verified with the use of the experimental data obtained from the glass beads. The best fit was provided by the porosity function n3/(1–n)2. In the case of the estimation of the hydraulic conductivity of uniform glass beads, the best fit was exhibited by formulae published by Terzaghi, Kozeny, Carman, Zunker and Chapuis et al.