Overview

Surveying a quay wall or sheet pile structure is not a vessel survey done on land. A permanent marine structure spans several very different environments at once — submerged, tidal, splash and buried — and much of the steel is inaccessible. A realistic survey and remediation plan starts from those constraints rather than fighting them.

The engineering principle

A single steel structure at a quay passes through zones that behave electrochemically differently: the permanently submerged zone, the tidal and splash zones that wet and dry, and the buried zone in the seabed or backfill. Current demand, achievable potential and corrosion behaviour differ between them, and a protection assessment has to consider the structure zone by zone rather than as one uniform surface. The buried portion in particular is governed by soil resistivity, which can vary sharply with depth and location and which strongly affects how far protective current will travel.

Design and selection considerations

Survey design for these structures has to solve for access and variation. Readings are needed across the tidal range and at representative points along the structure, which means planning around tide, weather and safe access. Reference electrode placement has to reach the zone being assessed — measuring a buried section from the waterline is not the same as measuring it where it sits. Soil resistivity testing often accompanies the potential survey because it explains what the potentials are doing. The output is a zone-by-zone picture of where protection is adequate and where it is not, which is what a remediation design needs.

Common mistakes

Treating the whole structure as one uniform surface and averaging across zones that behave differently; surveying at a single tidal state and missing the splash and tidal behaviour; assuming uniform soil when resistivity varies with depth; and under-planning access so that the survey covers only the reachable parts and quietly ignores the rest. A plan that looks tidy on paper but cannot actually reach the buried or tidal steel produces a false sense of assurance.

Inspection and verification

Verification means confirming coverage as much as readings: that each zone was actually assessed, that references were appropriate and checked, and that anomalies were re-measured. For long structures, a sensible strategy samples representatively and then investigates any zone that looks anomalous, rather than assuming uniformity. Repeatable survey points allow future surveys to track change over time.

How CCME-MCPS can help

CCME-MCPS surveys permanent marine infrastructure — quay walls, sheet piles, jetties and dolphins — and produces zone-by-zone assessments and realistic remediation plans that account for access, tide and ground conditions. The ports and harbour sector is covered across our engineering and inspection services.

Important note

This article explains general cathodic-protection engineering principles for education. It is not a design, and it is not guidance for any specific structure — every real system must be engineered to the applicable standards and the specific conditions of the asset. Standards are referred to by name only.