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

Malan, Christiaan Tomas (2016-12)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

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.

AFRIKAANSE OPSOMMING: Uitskuring word hier gedefineer as die verwydering van korrelmateriaal in die omgewing van kusstrukture deur middel van hidrodinamiese kragte. Dit word beskou as een van die mees algemene oorsake van die swigting van seemure. Hierdie tesis bestudeer die effek van uitskuring van 'n klipfondament onder 'n vertikale seemuur. Die effekte en gedrag wat in hierdie verslag gebruik en voorgelê word, kan ook toegepas word op vertikale golfbrekers gebou op ‘n klipfondament. Studies uit die verlede oor die uitskuringproses word bespreek en laboratoriumeksperimente word uitgevoer om tot gevolgtrekking te kom tot watter aspekte van die klipfondament die mees vatbaarste vir uitskuring is. Klem word geplaas op vlaklaagdikte, toonwydte, kompaksie en byvoeging van sediment tot die fondament tydens konstruksie. Die hipotese word dus na vore gebring dat 'n dikker vlaklaag meer skade sal ervaar in vergelyking met 'n dunner laag. Die losweg verpakde 19mm kliplaag is meer vatbaar vir hidrodinamiese kragte, want hierdie klein klip word maklik uitgespoel deur die oorliggende beskermingslae. 'n Korter toon breedte van klip sal minder uitskuurskade veroorsaak. 'n Langer lengte van klip, wat nie die filterreëls gehoorsaam nie, sal uitspoel deur die oorliggende beskermingseenhede. Kompaktering verminder die ruimtes tussen die klipeenhede en verhoed dat die struktuurelement onder sy eie gewig in die vlaklaag insink. Aggregaat wat per ongeluk tydens konstruksie by die fondasiemateriaal bygevoeg word, sal uitspoel uit die vlaklaag, wat groter ruimtes tussen die individuele klippe agterlaat. 'n Fisiese model studie is uitgevoer by die fasiliteite van die WNNR in Stellenbosch. 'n Vaste-bed, twee-dimensionele fisiese model in ‘n glaskanaal is opgestel om verskeidenheid eksperimente uit te voer om die uitwerking van die uitskuringproses op verskillende fondament tipes en konstruksieprosedures te bestudeer. 'n Metode vir die meet van uitskuring onder ‘n vertikale struktuur is ontwikkel. Saam met gemete golftoestande van elke toets, is bestaande ontwerpkriteria bewys en addisionele kriteria vasgestel. Hierdie tesis bevestig dat die dikte van die vlaklaag moet ontwerp word met 'n minimum-en maksimumvereiste om stabiliteit te verseker. Insig is gegee oor die belangrikheid en gebruik van die filterreëls, wat die vermoë om ‘n klip uit te spoel, bepaal. Die verskil van verskeie konstruksiemetodes en materiale word weergegee, soos verdigting van die vlaklaag en die toevoeging van sediment in die vlaklaag. Die verslag kom tot die gevolgtrekking dat hierdie metodes die stabiliteit van die struktuur teenwerk. Aanbevelings met betrekking tot die uitvoering van ‘n fisiese model met klipfondamente word in die tesis gegee.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/100339
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