Scour of the screed layer underneath a vertical seawall with a rubble mound foundation

ENGLISH ABSTRACT: Scour is here defined as the removal of granular bed material by hydrodynamic forces in the vicinity of coastal structures. It is believed to be one of the most common causes of seawall failure. This thesis studies the effect of scour of a rubble mound foundation underneath a vertical seawall. The effects and behaviours stated in this report may also be applied to vertical breakwaters constructed on a rubble mound foundation. Past studies on the scour process are discussed and several laboratory experiments performed to conclude what component of the rubble mound foundation is most susceptible to scour. Emphasis is placed on screed layer thickness, toe width, compaction and addition of sediment to the foundation during construction. The hypothesis is thus put forward that a thicker screed layer will experience more damage compared to a thinner layer. The loosely packed 19mm stone layer is most susceptible to hydrodynamic forces as it is small stone easily exposed through the overlying armour layers. A shorter toe width of rock should produce less scour damage. A greater area of stone, not adhering to the filter rules, will wash out through the overlaying armour units. Compacting reduces the voids between the stone units and prevents the structure element from sinking into the screed layer under its own weight. Cohesive and non-cohesive soils added inadvertently during construction should wash out of the screed layer, leaving behind larger voids between the individual stones. A physical model study was performed at the facilities of the CSIR in Stellenbosch. A fixed-bed, two-dimensional physical model in a glass flume has been set up to conduct an array of experiments to study the effects of the scour process on several foundation conditions and construction procedures. A method for measuring scour underneath a vertical structure was devised. Together with measured wave conditions of each test, an existing design criterion is proven and additional criteria are stated. This thesis can conclude that the thickness of the screed layer should be designed meeting a minimum and maximum requirement to assure stability. Insight is provided on the importance and use of the filter rules governing the capability of rock being washed out. The significance of various construction methods and materials is stated, such as compaction of the foundation and the addition of sediments to the screed layer. It was concluded that these methods oppose the stability of the structure. Recommendations concerning the execution of physical model tests regarding rubble mound foundations are stated in the thesis.