Civil Engineering May 2021 | Vol 29 No 4

12 May 2021 Civil Engineering At Big Bay, the gabions supported a fairly exposed piece of coastline and protected both the pedestrian walkway between Big and Small Bay and a sewer line. Behind the walkway are private residential properties. But abrasive beach stones washing up against the gabions, along with major storm events since June 2017, caused the structures underpinning the gabion terraces to fail, leaving the wall compro- mised and unstable in sections (as shown in Figure 1). The structural failure of the gabion wall posed the risk of the imminent col- lapse of the entire embankment, including a public walkway, a 150 mm municipal sewer, and even the local life-saving club building. This would have had a signifi- cant effect on public health and safety, property, and beach amenities. The City had to act fast and provide a solution. To safeguard those in the area, the walkway was closed off and the City’s Water and Sanitation Department was alerted to the potential risk of damage to the sewer line. They immediately imple- mented mitigation measures. PHYSICAL SITE CHARACTERISTICS AND PROJECT LIMITATIONS Due to the extent of the damage, neither full nor partial repair of the existing gabion wall was an option – replacement was the only way to go. Adding to the complexity, the urgency of the matter meant that there was no time to conduct an environmental impact assessment or even a basic assess- ment. The emergency defence structure therefore had to be designed to fit onto the existing footprint of the collapsing gabion wall. Any coastal protection structure that required deep excavations or a “dry” working area was not feasible either. The list of complicating factors seemed never-ending. It also included the lack of shallow bedrock at the site, as well as aggressive erosion of the beach since 2017. The foundation design of the replacement structure would therefore be vulnerable to scour. The nearshore hydrodynamics and beach sediment dynamics at the site Figure 1 The extent to which the gabion revetment had been damaged over time Due to the extent of the damage, neither full nor partial repair of the existing gabion wall was an option – replacement was the only way to go