The wall is experiencing Microbial Induced Corrosion (MIC), particularly in the upper sections near the lake surface. MIC occurs when microorganisms, such as sulphate-reducing bacteria, form biofilms on metal surfaces and accelerate corrosion. This process is common in aquatic environments and has led to perforations in the sheet piles, compromising the wall’s ability to divert water effectively.
Despite the corrosion, is the wall structurally sound?
Yes. Engineering advice from WSP indicates that the king piles (the structural framework) are sound and expected to remain so until 2038, assuming corrosion continues at the current rate. However, the sheet piles (the interlocking steel panels that form the continuous face of the wall) are failing and allowing leakage, which reduces the wall’s effectiveness in diverting water from Lake Rotorua.
Did we anticipate the corrosion occurring? Was it due to the materials used?
The corrosion affecting the Wall (specifically Microbial Induced Corrosion (MIC)) was not anticipated at the time of construction.
MIC is a complex biological process where microorganisms accelerate the deterioration of metal surfaces, particularly in aquatic environments. It was not well understood or widely accounted for in infrastructure design when the wall was built.
While the materials used for construction (steel sheet and king piles) were standard for such infrastructure, and considered appropriate and durable materials at the time, the specific environmental conditions in the Ōhau Channel (including water chemistry, microbial activity, and oxygen levels) have contributed to accelerated corrosion. These factors were not fully accounted for in the original design, and the corrosion mechanism only became evident through monitoring initiated in 2014.