Corrosion rate measurement of sheet pile wall in the port of Durban

Abstract

Ports and harbours are typically located along coastal and inland waterways. Harbours refer to water bodies adjacent to the shore that shelter watercrafts from stormy weather and provide anchorage for ships. Ports, on the other hand, denote infrastructure designed for docking vessels that transport passengers and cargo to and from land. In essence, harbours become ports when they are utilised for commercial activities, such as loading and unloading cargo, embarking passengers, or any other revenue-generating operations. At the Port of Durban, steel sheet piles were installed beneath the quay walls to protect against rising sea levels, prevent soil erosion, and support the riprap beneath the deck of the pile quay wall. However, these steel sheet piles have reached the end of their design life. Furthermore, the thickness of the steel sheet piles at several berths has decreased due to corrosion. The aim of this study was to determine the corrosion rate (mm/year) and estimate the remaining thickness of steel sheet pile walls at Island View Berth 3 and Maydon Wharf Berth 12 in the Port of Durban. This investigation provided an estimation of the quay wall’s functionality and determine whether maintenance, reinforcement, or replacement is necessary. Additionally, the determined corrosion rate will inform future designs of steel sheet piles. Island View Berth 3 was constructed in 1993 using the ARBED BZ 7 sheet pile type, while Maydon Wharf Berth 12 Berth was rebuilt in 2012 with the new HZM/AZ combined sheet pile wall system. The latter utilised over 2 800 tonnes of HZ 1180M A-24 king piles and 440 tonnes of AZ 18-700 sheet pile pairs as intermediate piles. A significant challenge at the Port of Durban was the development of excessive sinkholes behind berths, believed to result from erosion caused by deteriorating sheet pile structures. Currently, the Port lacks mechanisms to measure the remaining thickness of the steel sheet piles or perform underwater maintenance of these structures. Furthermore, no system is in place to monitor or track the condition of the sheet piles, making it difficult to determine when replacement or maintenance is necessary. During the inspections, 42 points were examined: 22 in Island View Berth 3 and 20 in Maydon Wharf Berth 12. Island View Berth 3 was inspected over two days in January 2020, while Maydon Wharf Berth 12 was inspected over 14 days. Prior to measurements, divers cleaned 200 mm x 200 mm patches of the steel sheet pile wall. Marine growths were manually removed using a steel scraper, hammer, and wire brush. The outer flanges surface of the steel sheet pile was cleaned from top of pile to the bottom of sheet pile. An ultrasonic thickness (UT) gauge was employed to assess the remaining thickness of steel sheet piles at both berths. This device emits high-frequency sound pulses through a hand-held probe in contact with the metal, measuring the time taken sound waves to travel through the material, reflect off the back wall, and return to the probe. The remaining thickness was determined by calculating the sound speed in steel and using half the total travel time. Corrosion rates were calculated using the formula icorr =(To−Ta)/t, where To is the original thickness, Ta is the actual thickness, and t is the exposure time in years. This data can inform the design of new steel sheet pile structures for ports. At Island View Berth 3, the average corrosion rate 28 years after installation was 0.0516 mm/year. Maydon Wharf Berth 12 showed varying corrosion rates by zone: 0.0545 mm/year (splash zone), 0.0485 mm/year (tidal zone), 0.0345 mm/year (lowwater zone), and 0.0290 mm/year (immersion zone), with an overall corrosion rate of 0.0466 mm/year. This study highlights significant corrosion variability across studied zones and emphasises the need for a comprehensive maintenance plan. These findings provide essential insights for future design and preservation strategies of marine structures at the Port of Durban.