Assessing internal erosion with the modal decomposition method for grain size distribution curves

Abstract

The modal decomposition method (MDM) is used to analyse the grain size distribution curve (GSDC) of a soil that was likely to produce internal erosion according to old criteria, and produced internal erosion in laboratory tests. Many previous studies have assumed that such a soil contains a coarse fraction and a fine fraction, the latter being able to migrate within the pores of the coarse fraction. It is shown here that the tested soil had not two fractions, but three grain size modes according to the MDM. The soil was tested in a rigid-wall permeameter under downward seepage. After the experiment, the specimen was divided into four parts, using the lateral piezometers positions to define the parts. The same three grain size modes (same means and variances) were found in the GSDCs of the four parts. In addition, the MDM could quantify internal erosion. The upper part of the specimen had lost the fine mode and had the highest proportion of coarse mode, whereas the medium-mode percentage had little variation with depth. This example explains how the modal decomposition method may help to quantify internal erosion.