Civil Engineering student’s research offers solution to Dolos repairs

[Article by Amber Viviers]

Tianca Olivier, a master’s student from the Department of Civil Engineering at the Faculty of Engineering, is graduating cum laude at the December graduation of Stellenbosch University for her work on the repair of Dolos breakwaters with larger layered Dolos units. Her supervisor is Dr Andre Theron.

The Dolos unit is a South African invention that was developed to protect breakwaters, which primarily shield ports and harbours and allows for safe conditions in the lee of the structure.

Climate change is causing changes in the coastal environment, resulting in exceeded design conditions, which motivated Tianca to research this field.

“These exceedances can often cause damage to coastal structures and therefore, an effective repair method is needed,” she says. “This research was done to find an effective repair method with which Dolos breakwaters can be repaired. The repair options evaluated in this research all used Dolos armour units layered differently over an existing, damaged Dolos breakwater.”

Tianca first did a thorough background study that gave a good foundation for the research in her thesis. “The literature study allowed for the evaluation of alternative repair methods that have been considered, which factors should be considered when deciding on a suitable repair option and how to design Dolos breakwaters effectively,” she says.

She evaluated three potential repair options through a physical model scale study. “The physical model study was a two-dimensional flume model, which observed different wave heights being made on the various breakwater repair options.

“The model Dolos units were cast by hand in silicone moulds with specific relative densities to ensure the relative density of the model units in the freshwater of the testing facility is the same as the relative density of concrete in salt water,” she adds.

The first test was to damage a Dolos breakwater until just before failure. “This damaged structure was recreated several times and repaired using the different options considered. The repairs used three different Dolos sizes – 10%, 30%, and 100% – larger in nominal diameter than the original Dolos units.

“These units were each packed in three different packing configurations, a single layer over the damaged breakwater, a double layer over the damaged breakwater, and the placement of a new underlayer over the damaged structure and then placing a double layer of repair units over that,” she explains.

Her research found that the best repair option was to use the 30% larger armour unit, packed in a double layer over the existing structure. “This was considered the best option as it was both an economical repair and had the lowest damage after damaging waves had been made. This unit protected the breakwater against future wave conditions yet was not unnecessarily over-designed,” she says.

Tianca says some of these breakwaters along the South African coastline have already undergone severe damage and the repair process took very long as there is no generalised repair option. “Hopefully, this repair option will be considered when more Dolos breakwaters need repairs. This repair option can be adapted based on the specific site conditions of the damaged breakwater,” she adds.

With her master’s journey ending, she says she enjoyed learning new skills and finding ways to solve problems the most during her studies. Next year she looks forward to applying her knowledge in practice while also travelling to some of these remarkable coastal structures.

(left) Side view of a Dolos breakwater being damaged.
(insert) Three-dimensional rendering of a Dolos unit used for 3-D printing created in AutoCAD.
(right) Ms Tianca Oliver.