Design of rubble-mound foundations for vertical seawalls: scour, screed layer and berm width

Van Wageningen, Esmé (2018-12)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: Coastal erosion has always been a challenging problem to solve for coastal engineers since it is such a complex process that threatens valuable environments and properties along the coastline. Structural erosion, or scour, develops due to the presence of a marine structure that interrupts the natural sediment transport processes. Scour is one of the most common mechanisms of failure of marine structures and therefore it is vital to gain a better understanding of the scour process in order to design marine structures that can withstand the adverse effects of scour. The objective of this study is to gain more knowledge of the scour process that develops in specifically the screed layer directly underneath the concrete elements of a vertical seawall that is protected by a rubble-mound berm. This was accomplished by investigating existing literature on marine structures, their failure mechanisms, the processes of sand and granular scour as well as research on physical modelling. A physical experiment was then set up in a 2D wave flume in the hydraulic laboratory of the CSIR in Stellenbosch, South Africa to test the influences of different aspects of the scour process in the screed layer underneath a model seawall. The influence of the wave period, the rubble-mound berm width, the screed layer thickness, the armour rock stability and the reflection coefficient was investigated. The scour damage in the model screed layer was measured with wooden dowels, as was done in previous research, as well as with a new method developed by the author that uses sonar technology to create a submerged image of the scour pattern that developed in the screed layer. The scour measurements for each different test set-up were analysed. Firstly, it was found that a shorter wave period resulted in more scour damage since the interaction between the incident and reflected waves was more significant near the seawall and occasionally superimposed due to the rapid change in wave direction and orbital velocities, which disturbs the screed material. The rubble-mound berm width, however, did not have as significant an influence on screed layer scour as expected. A wider berm did provide more scour protection, but the optimal berm design must balance protection and construction and material costs and therefore a narrower berm of 4Dn50 is recommended. A minimised screed layer thickness of 100 mm is recommended since it resulted in the least amount of scour whilst still being able to be constructed at an adequate accuracy. As expected, a more stable armour layer resulted in less scour damage in the screed layer. Lastly, it was interesting to observe that a test that resulted in less scour, had a higher reflection coefficient. This is probably due to less wave energy that was absorbed by the berm (causing scour in the screed layer) but reflected back seaward instead.

AFRIKAANSE OPSOMMING: Kuserosie was nog altyd ‘n uitdagende probleem om op te los vir ingenieurs aangesien dit so ‘n ingewikkelde proses is wat die waardevolle omgewing en eiendomme langs die kuslyn bedreig. Strukturele erosie, of uitskuring, is die proses wat ontwikkel as gevolg van die teenwoordigheid van ‘n mariene struktuur wat die natuurlike sedimentvervoerprosesse onderbreek. Uitskuring is een van die mees algemene swigmeganismes van mariene strukture en dus is dit van uiterste belang om die uitskuringsproses beter te verstaan ten einde mariene strukture te ontwerp wat teen die skade wat uitskuring bestand is. Die doel van hierdie studie is om die uitskuringsproses te ondersoek wat spesifiek plaasvind in die vlaklaag direk onder die betonelemente van ‘n vertikale seemuur wat deur ruklip beskerm word. Dit is verwesenlik deur ‘n literatuurstudie te doen oor kusstrukture, hul swigmeganismes en sand- en klipuitskuringsprosesse asook navorsing oor fisiese modelering. ‘n Fisiese eksperiment in ‘n 2D golfkanaal is toe in die hidrouliese laboratorium van die WNNR in Stellenbosch, Suid-Afrika, opgestel om die invloede van verskillende aspekte van die uitskuringsproses in die vlaklaag onder ‘n modelseemuur te toets. Die invloed van die spits-golfperiode, die ruklip se kruinwydte, die vlaklaagdikte, die stabiliteit van die beskermende rukliplaag en die weerkaatsingskoëffisiënt is ondersoek. Soos met vorige navorsingseksperimente, is die uitskuring met houtstokkies gemeet, asook met ‘n nuwe metode wat deur die skrywer ontwikkel is. Hierdie metode behels om sonartegnologie te gebruik om onderwaterbeelde van die uitskuringspatrone wat in die vlaklaag ontwikkel het, te skep. Die uitskuringsmetings is vir elke verskillende toetsopstelling geneem en ontleed. Eerstens is daar gevind dat ‘n korter golfperiode meer uitskuring veroorsaak omdat die interaksie tussen die invallende en weerkaatste golwe superponeerde golwe veroorsaak wat die golf- en orbitaalsnelhede se rigtings vining verander en sodoende die vlaklaag beskadig. In teenstelling het die ruklipkruinwydte nie so ‘n beduidende invloed op die uitskuringsproses gehad soos verwag nie. ‘n Wyer kruinwydte het meer beskerming teen uitskuring gebied, maar die optimale kruinontwerp moet ‘n balans tref tussen effektiewe beskerming en konstruksie- en materiaalkoste en dus word ‘n nouer kruinwydte van 4Dn50 aanbeveel. ‘n Minimale vlaklaagdikte van 100 mm word aanbeveel ten einde uitskuring te beperk terwyl dit nogsteeds moontlik is om dit met voldoende akkuraatheid te bou. ‘n Meer stabiele beskermingskliplaag het ook, soos verwag, minder uitskuring tot gevolg gehad. Laastens, was dit interessant om te let dat toetse wat minder uitskuring gehad het, het hoër weerkaatsingskoëffisiënte gehad. Dit is moontlik omdat minder golfenergie deur die kruin en vlaklaag geabsorbeer is en die energie eerder seewaarts weerkaats is.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/104988
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