Inversed Open Granular Filters in Land Reclamation Structures

Abstract

This thesis is the third study of a research line that focuses on the interface stability of a rubble mound sand retaining structure. The type of interface considered is a geometrically open granular filter because of its economical and constructional benefits. The stability of an open filter structure in which the base layer is located on top of the filter layer was investigated by the use of physical modeling. The model used was the same waterproof elongated container as used by Van de Ven (2019), after some adjustments were made. Three compartments were allocated in this model. In Compartment A, the water level was controlled by the use of a plunger. This caused water level differences between the compartments and therefore induced a hydraulic gradient (i) in the filter structure that was installed in Compartment B. The hydraulic gradient was considered as the main load parameter for erosion. The main strength parameter was the stability ratio (SR), defined as the sieve diameter for which 15% of themass of the filter material is smaller divided by the sieve diameter for which 85% of the mass of the base material is smaller. During testing, this hydraulic gradient was increased by the plunger until erosion was noted, defining the critical hydraulic gradient (icr) for the filter-base combination tested. Two loading conditions were distinguished, namely the parallel and combined set-up. Besides the SR and these loading conditions, the influence of the characteristic grain size of the base material and the filter layer height on icr was studied. During this thesis, a new phenomenon influencing the icr was observed, which was not included in the research line yet. This was the initial sand saturation of the filter layer. Therefore, the physical model results were split up into unsaturated and partly saturated tests. For the unsaturated parallel configuration, SR = 5.7, 6.7, 12.9 and 13.1 were tested. Results were found for the upper two values. For the lower SR, no erosion was detected resulting in a bottom limit of icr . The partly saturated parallel tests resulted in a reduction of icr up to a minimumof 73%. Projecting this effect into reality could have a significant influence on the stability of an inversed open granular filter in land reclamation structures. However, further research is advised. All found icr in the parallel tests were higher compared to Van de Ven (2019). Besides, it was concluded that a larger characteristic grain size of the base layer and the filter layer thickness did not influence the icr significantly. Furthermore, three unsaturated combined tests were conducted with SR = 6.7 and 12.9. No erosion was noted for SR = 6.7. Erosion was detected twice for SR = 12.9, once in the first test segment – resulting in an upper limit for (icr ) – and once in the second test segment. A reduction of 93% for icr in the combined set-up compared to the unsaturated parallel set-up was noted, implying that an additional load decreases the critical load for the considered filter structure. However, this could also be the results of model installation differences. Lastly, a new physical model was designed and constructed to facilitate the study on the influence of the superimposed load and the filter inclination on the interface stability. A newly developed numerical model has been made to plot design graphs for this new model.