Introduction

With its aggressive conditions, the Great Lakes have always posed a unique challenge for marine design and construction. Conventional materials, like steel and timber piling, have often succumbed to the relentless movements of ice sheets during Minnesota’s cold winters, traditional decay, and the corrosive actions of microorganisms in the fresh water of the harbors. The search for a durable, non-corrosive, and rot-resistant solution was paramount.

Situated on the shores of the Duluth Superior Harbor, AMI Consulting Engineers faced firsthand the challenges posed by the harbor’s aggressive conditions. Principal and Vice President of AMI Consulting Engineers, Chad Scott, notes, “The need for a solution beyond traditional materials became evident as we witnessed the abrasion and decay on conventional wood piling and severe corrosion on steel piling each year.” Determined to find an answer, AMI partnered with Bedford Technology (now Tangent Technologies) to explore a groundbreaking solution: fiberglass-reinforced HDPE plastic piles.

The HDPE Plastic Pile Test

November 2015, AMI installed a 45-foot length 16SP16F14 SeaPile, designed and manufactured by Tangent Technologies, just offshore of AMI’s harbor side office. Within the first month of installation, the pile was put to the test as it resisted the movement of large ice sheets pushed by high winds and late season ocean and lake vessels. After each ice sheet season, the pile demonstrated impressive resilience by returning to its original position.

Durability over Eight Years

Since installation, the HDPE plastic pile has endured eight years of diverse challenges in the Duluth Superior Harbor. Each cycle has seen fluctuating water levels and winters with varying thicknesses of ice sheets. Throughout these extreme conditions, the HDPE pile’s durability has been showcased. Tangent’s commitment to providing durable and sustainable marine solutions was thoroughly tested during this 8-year trial.

Data and Survey Evidence

AMI Engineers collected data and survey evidence to support the HDPE plastic pile’s durability claims over 8 years. Key observations include a consistent vertical angle measurement, no increase or decrease in the top elevation (no ice jacking), and permanent deformation of the center of the pile of less than 0.23 inches over the test period. A small taper at the top of the pile resulted from the pile driving process, which caused the pile to mushroom slightly under the pile driving pressure and did not detract from the pile’s near-vertical alignment, showcasing its resilience. Additionally, no measured variation in circumference was noted from ice or vessel abrasion. The findings collectively underscore the HDPE plastic pile’s robustness and ability to maintain structural integrity under the demanding conditions experienced in the harbor.

Comparison with Steel and Wood Piling

Since 1998, AMI has observed and monitored freshwater steel corrosion in the Duluth-Superior Harbor, highlighting the susceptibility of conventional steel pilings to microbial-induced corrosion in a freshwater environment. A significant benefit of HDPE plastic piles is that they do not corrode or rot, making them an ideal solution to combat the destructive effects of the harbor’s microorganisms.

While steel piles might appear more economical initially, their susceptibility to accelerated corrosion can lead to increased maintenance and eventual replacement in challenging environments like the Duluth-Superior Harbor. To combat steel corrosion, additional treatments such as coatings or pile wraps are required for protection. In contrast, with their notable durability and reduced maintenance requirements, HDPE plastic piles present a compelling long-term value proposition for marine structures, including residential, commercial, and lighter-duty industrial port environments.

Wood pilings in harbor environments commonly decay due to microbiological action, rot, and erosion from ice forces. HDPE piles are carefully engineered with pultruded rebar and reinforcing structural fibers throughout the cross-section, ensuring exceptional strength and durability against ice abrasion.