Sandfill-Retaining rubble mound structures

Abstract

Currently, sand retaining rubble mound structures are often constructed with geotextiles, lining the interface between the core material and the sandfill. These geotextiles are placed to make sure the sand from the sandfill is not flushed out through the core by incoming hydraulic forces from the surrounding water. It is proposed that difficulties faced during placement, or uncertainties regarding the correct installation of these geotextiles can be overcome by curtailing the geotextiles. The potential for this abbreviations is stressed by Polidoro et al. (2015) as this author concluded, that at the lower inner corner of rubble mound structures with a closed inner slope, pressures are dampened below a certain estimated critical value. However, no proof was found for the applicability of this critical value. Despite several researches predicting hydraulic loading in rubble mound breakwaters, and studies assessing the stability of sand to stone interfaces, insight in the behaviour of sediments in this particular interface configuration of a sand retaining breakwaters was lacking. Therefore, the aim of this research was to study the behaviour of sediments at the interface to a core in a sandfill. Under supervision of VAN OORD and DELFT UNIVERSITY OF TECHNOLOGY (TU Delft), a physical model is designed to study specifically this interface. The model tests took place in the Fluid mechanics Laboratory at TU Delft, where a relatively small setup (1 * 0.5 * 0.15m) was used to model on a relatively large scale (lambda= 15). An exact scaled representation of a nominal breakwater by Polidoro et al. (2015) was used. In the model both the Development of the interface during placement of the sandfill as the behaviour of the sediments when subjected to hydraulic loading is studied. The research concluded that a stable initial interface was found with a slope of approximately 35degrees. In what extent the infill migrated inward through the core varied depending on the installation
method. When subjected to hydraulic loading, a critical hydraulic gradient was found of 0.05m/m on average and 0.04m/m in the most conservative case. These results were established with a measurement accuracy of » 5% and the consistency over different series of tests was¸ 80%. Sometimes segregation of the stones was observed. The main attribution to the deviations in the measured hydraulic gradients and sediment transport were concluded due to this variation in the positioning of the stones. It is concluded that the current existing literature is able to give reasonable initial approximations of the critical gradient in the system(5¡40% accurate), however, the deviation can be significant and further research by varying more geometrical parameters should conclude if the obtained approximations are constant. Concluding, the results obtained in this research suggest that the critical loading conditions for the interface stability of to a rubble mound in a sandfill are of comparable order to conventional filter criteria and are higher than the currently calculated and measured appearing gradients by for instance Vanneste and Troch (2012) and Polidoro et al. (2015). These results justify the further exploration towards the potential of the abbreviation of geotextiles at the considered interface. In order to guide further research a list of recommendations is given as well as additional model improvements.