Browsing by Author "Cairns, Melissa"
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- ItemPhysical model tests on stability and overtopping of new concrete armour unit Cubilok™(Stellenbosch : Stellenbosch University, 2023-11) Cairns, Melissa; Theron, Dr Andre; Holtzhausen, Anton; Wehlitz, Carl; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: Artificial concrete armour units are employed to protect coastal structures and infrastructure such as rubble mound breakwaters, revetments, and artificial headlands. Several concrete armour unit types have been developed over the last few decades, where each specific unit has a unique shape and behavioural properties. As the need for breakwaters deployed in harsher wave climates and deeper waters increased, the need for larger armour units also grew. Where concrete armour units are required, generally, the best value is achieved with a single-layer option, provided that construction conditions allow for accurate placement of armour units. PRDW Consulting Port and Coastal Engineers are developing a new concrete armour unit called the Cubilok™. This unit is defined by four principal dimensions, which can be modified to obtain variations of the Cubilok™ shape. These parameters can also be used to alter the structural robustness of the unit, which is indicated by its slenderness ratio (H’). Two different unit shapes have been tested previously: single-layer (H’ = 1.09) and double-layer (H’ = 0.92). For this study, the shape previously tested as a single layer was modified by removing the tapered ends of the protuberance (or arms). These changes were made to reduce the settlement observed in previous research; however, it also resulted in a unit with higher structural robustness where H’ equalled 0.6. This unit’s viability as a single and double layer was investigated in this study. The overall efficacy of an armour unit during wave attack is determined by the hydraulic stability. This study was the first attempt to understand the modified unit’s hydraulic stability and recommended wave overtopping discharge. The primary objective of this research was to investigate the behaviour of the Cubilok at slopes of 1:1.5 and 1:1.33 (V:H), which involved testing various wave heights and periods. A 2D flume configuration was tested at the Council for Scientific and Industrial Research (CSIR) in Stellenbosch, South Africa. The configuration included, a sloping foreshore of 1:30, and a constant water level measured at the structure’s toe. The wave conditions were measured with capacitance probes, and the overlay photography technique was utilised to capture and examine the armour layer reaction. Overtopping volumes were measured throughout testing and converted to l/s/m to indicate the average rate of overtopping discharge. The test schedule included two test series to determine a suitable storm duration for the steeper slope of 1:1.33 (H:V). Packing densities of ∅ = 0.63 and 0.65 were investigated for the storm duration tests. A repeatability test was also conducted for both slopes with the same wave condition. The findings showed an improvement in stability for the tighter packing density; therefore, the test programme continued with the packing density of ∅ = 0.65. According to the stability test results, the armour layer was influenced slightly more negatively by longer wave periods, with larger movements and earlier displacements. By the end of the study, 17 test series were completed, totalling 102 individual tests. The stability number was found to increase with decreasing Iribarren parameters at the start of damage. The inconsistent results achieved at the start of damage yielded no conclusive influence of the varying slope gradients on the hydraulic stability. The average stability numbers achieved for the milder slope were often greater at failure. Throughout testing, the stability numbers ranged from NS = 2.04 to 4.64. At the start of damage, the average stability number was NS = 3.51, and at failure, it was NS = 4.30. The research revealed that the Cubilok's performance notably improved on steeper slopes, indicating competitive potential against other single-layer units. Based on previous research, the Cubilok outperformed Accropode in terms of no damage and design stability at a 1:1.33 slope. However, on steeper slopes, Xbloc's design parameter exceeded Cubilok's by 7%. The overtopping rate increased significantly for low wave steepness values (sop =0.01). For low wave steepness values, the results indicated that the overtopping rate increases approximately twofold with an increase in wave height. Furthermore, compared to the CLASH results of other single-layer units, the measured rate of overtopping for the Cubilok slope was slightly greater. The increased overtopping rate was most apparent in test results with low wave steepness of sop = 0.01, falling outside the CLASH range of sop = 0.02, 0.035 and 0.05. It should be highlighted that this study was only a preliminary investigation into the behaviour of the modified Cubilok. The effect of the packing density and shape were compared in relation to the settlement of the unit. Further tests are recommended to address variability in test results.