Restoration of a jetty destroyed by Accelerated Low Water Corrosion (ALWC)

Introduction

According to the definition of the International Navigation Association PIANC, “Accelerated Low Water Corrosion (ALWC) is one form of MIC”, while MIC stands for Microbial Induced Corrosion. It is known nowadays that this type of corrosion affects different hydraulic structures worldwide.

This paper describes an economically priced method for corrective reconditioning of a jetty severely damaged by ALWC. Nearly half of the drive piles carrying a jetty stretching 0.6 km in the Baltic Sea (Figure 1) were severely damaged by corrosion. The jetty consists of concrete sections standing on steel drive piles The structural stability of the whole structure was put on risk.

The challenge for the engineers of the Federal Waterway Administration was to completely restore the structural stability, safety and usability of the jetty for all  swimming units as well as supply vehicles of the German navy and its visitors. Furthermore, the restoration had to enhance the final life span of the structure and ensure safe operation for the rest of the desired lifetime. Due to the importance of the jetty within harbour operations, it was impossible to take it out of operation. All repair activities had to be carried out with the jetty continually in use.

History

The jetty was erected in 1964 with an expected life span of 60 years. A routine inspection in 1989 showed no corrosion damage, but only ‘normal’ surface corrosion. However, in 1997 corrosion holes (perforations) in many drive piles were detected accidentally by divers.

These surprising results were checked by thorough inspections carried out in 1998. Ultrasonic measurements according to Alberts (1991) showed severe corrosion on all 643 piles of the jetty with perforations on 280 piles. In the same year, microbiological and mineralogical investigations carried out by one of the author’s (Graff)
have proven the corrosion to be MIC, originating and progressing from the inner surfaces of the piles (Graff, 1998, Graff, 2002). The results of the survey are published previously.

In the following time, one of the author’s (Seifert) has developed the restoration program, which was realised in a first campaign in 2000 in order to repair the most damaged 130 piles. This first campaign lasted until 2002. A second campaign for repairing another 150 piles happened in 2004.
Finally, a third campaign for restoration of all the remaining piles in order to prevent further microbial corrosion with perforations on the piles started at the end of 2005.

The jetty

The jetty consists of 18 concrete sections, each standing on 36 drive piles (Figure 2). The piles are Krupp profiles KP 24 with a wall thickness of 10.0 mm and Krupp profiles KP 34 with a wall thickness of 13.5 mm. In total, there are 643 drive piles, 213 KP 34 and 440 KP 24.

The restoration concept

According to the corrosion damage (Figure 3) and the specific load of the different piles, it was decided to enhance the bearing capacity of the affected piles by mounting steel/concrete constructions in the main corrosion area in order to transfer the given tensile and/or pressure loads.

Three measures were developed:

Matthias Graff, TMH – Dr. Graff und Partner, & Olaf Seifert, Wasser- und Schiffahrtsamt Lübeck, Baubüro Kiel, Germany
Edition: Edition 34

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