Experimental Investigation into Internal Erosion Potential for Granular Filters

Abstract

Internal erosion is a phenomenon whereby the filtrates under the influence of significant seepage forces accompany the finer fraction from potential internally unstable filters (e.g. broadly- and gap-graded soil), occasionally rendering them ineffective. The filter assessment for internal erosion or instability potential is emphasized through particle size distribution based geometrical criteria ignoring the effect of compaction. In this study, the results of hydraulic gradient controlled internal erosion tests conducted over a wide range of compacted sand-gravel mixtures were used to analyse some of the available geometrical criteria, which interestingly showed partial success in assessing the filter’s internal erosion potential. It was revealed that the occurrence of internal erosion is a combined function of particle size distribution and the relative density of soils that had been ignored in many of the existing criteria. A comparison between the assessments obtained from some of the particle size based criteria and that from a constriction size based technique was reported for a large body of published data. It was observed that the latter criterion, which incorporates the effects of both particle size distribution and relative density of soils in tandem, could assess the reported test results with higher accuracy.