Hydrogen Embrittlement and Stress Corrosion Cracking Risk in High-Strength Steel Tie-Rods at Marine Bulkhead Structures

Anchored steel sheet pile bulkheads represent a significant portion of the aging waterfront infrastructure at U.S. ports and marine terminals. Many of these structures were constructed in the latter half of the 20th century using 150 ksi tie-rods - a material choice that offered cost and weight efficiency at the time but carries long-term risks in aggressive buried marine environments.

The risks are hydrogen embrittlement (HE) and stress corrosion cracking (SCC), and the inability to easily inspect these tie-rods combined with the absence of reliable visual warning indicators prior to fracture makes them particularly challenging to manage. This article addresses the potential failure mechanisms associated with 150 ksi tie-rods, outlines appropriate evaluation and risk mitigation protocols for existing facilities, and provides material selection guidance for new designs.

The Problem with 150 ksi Tie-Rods
The failure mechanisms are hydrogen embrittlement (HE) and stress corrosion cracking (SCC) - two related brittle failure modes that operate silently, without warning deformation, and at stress levels below nominal design capacity.

Hydrogen embrittlement occurs when atomic hydrogen enters the steel matrix through corrosion reactions. Once inside the metal, hydrogen migrates to regions of high triaxial stress - threads, bends, connection points, and surface defects - where it reduces ductility and fracture toughness, enabling brittle crack propagation. The key governing factor is steel strength: published corrosion engineering guidance indicates that steels exceeding approximately 145 ksi are particularly susceptible. At 150 ksi, high-strength steel tie-rods fall squarely into the high-susceptibility range.

Marine buried environments provide the conditions needed to drive this type of failure:

• Saturated soils, at times with saline or brackish chemistry.
• Low-oxygen conditions that promote hydrogen-generating cathodic reactions.
• Corrosion-driven electrochemical breakdown of the rod and its coating.

There is also another factor that is commonly overlooked: the actual stress state of a buried tie-rod is commonly more severe than the design calculations suggest. Fill settlement can induce bending stress at connection points, threads and hardware introduce stress concentrations, and residual stress from fabrication increases the overall stress state.

Why Conventional Inspection Does Not Solve This
The rods are buried and inaccessible, and HE/SCC is localized and time-dependent - it progresses with no external sign until fracture occurs. Once a crack initiates, propagation to fracture is rapid. There is no reliable pre-failure indicator detectable by visual surface inspection.

When the first rod fractures, load redistributes to adjacent rods. If those rods are in a similar state of degradation - which is likely given shared age, environment, and material - progressive fracture can follow. In severe cases, the result is total loss of the bulkhead structure.

Increasing the factor of safety or adding redundancy does not eliminate the risk, because the mechanism is governed by material susceptibility and environment - not simply by nominal stress levels.

Recommendations
New Designs
Avoid specifying 150 ksi tie-rods for buried deadman applications in marine environments. Instead:

• Select steels in the 75-100 ksi yield range with documented marine service history.
• Use articulated connections to reduce bending stress at threaded connections.
• Treat coatings as a supplemental measure, not a primary defense.

Existing Facilities
The following six-step protocol is appropriate for existing facilities:

1. Identify and prioritize. Review as-built records and prior inspection reports. Flag all marine bulkhead structures constructed using 150 ksi tie-rods. Prioritize based on age, environment, surcharge loading, and evidence of prior wall movement or distress.
2. Excavate and inspect. Expose representative tie-rods at the wall connection, mid-span, and deadman. Assess coating condition, corrosion state, connection hardware, and surrounding soil chemistry.
3. Sample and test. Where corrosion or coating breakdown is found, obtain samples for hardness testing, metallurgical examination, slow strain rate or hydrogen susceptibility testing, and fracture toughness testing if warranted. Soil and groundwater samples should be tested for pH, chloride concentration, sulfate content, resistivity, and dissolved oxygen to characterize the aggressiveness of the buried environment.
4. Assess remaining capacity. Where inspection and testing indicate HE/SCC susceptibility or material degradation, remaining system capacity should be evaluated, including load redistribution scenarios if one or more tie-rods are compromised. Environmental data from soil and groundwater testing should be incorporated into the assessment to characterize the ongoing severity of exposure and inform assumptions about the rate and extent of degradation across the tie-rod system. Findings and their structural implications should be formally documented. Continued operation under these conditions represents a risk-informed decision - not a finding of adequate condition - and a defined path forward is required, whether through a risk management plan or remediation.
5. Implement a risk management plan (if remediation is deferred). If remediation is deferred, a formal risk management plan should be put in place. This plan should include defined operational load restrictions, such as reduced surcharge limits or restricted vessel mooring and berthing loads; active deflection and movement monitoring with clearly defined threshold values that trigger escalated response; and a defined re-evaluation interval - not an open-ended monitoring program. It is important to understand what monitoring can and cannot detect. Because HE and SCC fracture provides no reliable visual warning and tie-rod systems have limited redundancy, wall deflection monitoring can detect system-level response to progressive failure but cannot reliably detect individual tie-rod failure before it occurs. Once a single tie-rod fractures, load redistribution to adjacent tie-rods can accelerate progressive failure faster than monitoring can provide actionable warning. A risk management plan extends the decision-making window, but it does not eliminate the underlying risk - it manages it until remediation can be executed.
6. Remediate. When remediation is elected, or when monitoring thresholds are exceeded, several options are available - each addressing the risk to a different degree. Limiting surcharge loads and restricting mooring and berthing operations reduces demand on the tie-rods but does not address the underlying degradation mechanism. Installing additional grouted tieback anchors or a pile-supported relieving platform reduces the loading in the existing tie-rods and can meaningfully lower the risk of progressive failure, but the original 150 ksi tie-rods remain in place and continue to be subject to HE and SCC in the buried environment. Excavating and replacing the 150 ksi tie-rods with lower-strength steel in the 75-100 ksi range is the only option that fully eliminates the HE/SCC risk mechanism. The appropriate choice depends on the severity of confirmed degradation, facility operational requirements, and owner risk tolerance - with a clear distinction between options that manage the risk and the one option that eliminates it.

Conclusion
High-strength tie-rods were installed widely across U.S. waterfront infrastructure in the latter half of the 20th century, and many remain in service today. HE and SCC are credible failure modes for high-strength steels in aggressive buried environments, and both progress without observable warning prior to fracture. The costs of proactive investigation and remediation are significant but are substantially lower than the consequences of undetected failure.
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For new designs, specify tie-rod steel in the 75-100 ksi yield range. For existing facilities where the tie-rod material grade is unknown, verify the grade against available as-built records. Where 150 ksi tie-rods are confirmed or suspected, follow the protocol described above to assess the condition and determine the appropriate risk mitigation measures.

​Author: Bradley A. Syler, PE, SE