Open Inverted Filters
Impact of fines content, coefficient of uniformity, and effective stress on inverted geometrically open granular filters in terms of parallel and perpendicular gradients
R.M. Besseling (TU Delft - Civil Engineering & Geosciences)
Bas Hofland β Mentor (TU Delft - Hydraulic Structures and Flood Risk)
A. Antonini β Graduation committee member (TU Delft - Coastal Engineering)
D.C.P. van Kester β Coach (TU Delft - Coastal Engineering)
Hugo Lavies β Coach (Van Oord)
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Abstract
This study investigates the capacity of an open inverted filter to stabilize the interface between the sand fill and core fill in a rubble mound breakwater. The strength of this filter relies on the arching mechanism of the base material, which is directly influenced by factors such as stability ratio and effective stresses. These forces arise from porous flow, and its quantification involves both parallel and perpendicular hydraulic gradients.
A full-scale physical model, with a 760 mm open inverted filter, is employed to determine the critical hydraulic gradients required to initiate erosion. The model consists of three compartments, with the central compartment containing the open inverted filter. The two side compartments are filled with water, with one of them featuring a plunger mechanism to induce flow. Pressure sensors and water level gauges are used to measure the hydraulic gradients. In addition to the physical model, a numerical model is developed to gain insights into the flow dynamics within the system.
The tests reveal that an increase in effective stress results in higher critical parallel and perpendicular gradients. Furthermore, the addition of fines at 5% or 10% seems to enhance the critical parallel and perpendicular gradients. A negative correlation was found between SR and πβ₯,ππππ‘, the results were fitted to a hyperbolic function. The findings further indicate that, even after erosion occurs, a stable interface can be reestablished. Therefore, designing solely for the single largest wave is not imperative.