Effect of fine fraction on Backward Erosion Piping : factual report of small-scale tests with various fine fractions

The failure mechanism of piping significantly contributes to the high calculated flood probabilities for many dikes in the river area. The presence of a fine fraction, whether or not combined with a broader gradation, in river sands can lead to greater resistance against backward erosion than is currently accounted for in the existing approach. When applying the Decision Support Framework for Piping, it has been shown in various projects in the river area that this additional resistance can have a relevant impact on flood probability. However, research has been mainly focused on tidal deposits so far and experience for river deposits is limited, especially with non-plastic fines in stable samples. Two small scale piping experiments, SSP-300 and SSP-301, were conducted in the Geohal laboratory at Deltares to investigate the influence of fine content on backward erosion piping (BEP). This report presents the factual data of these experiments. The tests were performed using Baskarp B15 sand with silt contents of 5% and 10%, respectively. Prior to the piping experiments, dry density and permeability tests were carried out to characterise the test materials. During the experiments, a hydraulic head difference was applied across the test box and increased in discrete increments while piezometric head and outlet discharge were continuously monitored. Pipe initiation, development, and failure were also documented through a logbook. The critical head difference was determined from the experiment and corrected for local hydraulic losses at the inlet filter and at the outlet opening. The results show that an increase in fine content leads to a higher critical head difference, indicating increased resistance to backward erosion piping. The material with higher silt content also exhibited lower permeability. These findings are consistent with previous experiments with B15 sand mixed with silt and confirm the role of fine content in controlling BEP behavior.