DEFYING NATURE: HOW COMPOSITE MARINE PILES PERFORMED IN THE DULUTH SUPERIOR HABOR’S HARSH ENVIRONMENT.

CASE STUDY DEFYING NATURE: HOW COMPOSITE MARINE  PILES PERFORMED IN THE DULUTH SUPERIOR  HABOR’S HARSH ENVIRONMENT. 

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 center of pile permanent deformation  of less than 0.23 inches over the test period. A small taper  at the top of the pile was a result of 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 overall findings collectively  underscore the HDPE plastic pile’s robustness and its  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, HDPE plastic piles, with their notable durability  and reduced maintenance requirements, 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. 

Additionally, the material incorporates UV stabilizers  that protect the color and integrity of the base resin,  reducing the need for recurring maintenance— unlike wood and wood-based composite products.  Furthermore, due to the density and smooth nature  of HDPE, there is less adhesion from icecompared to  timber and steel, resulting in less uplift on the piling  from ice forces.. 

FUTURE APPLICATIONS AND  RECOMMENDATIONS 

AMI Consulting Engineers and Tangent Technologies are  thrilled with the 8-year performance test results for the  HDPE plastic pile in the Duluth Superior Habor’s hostile  

environment. The case study provides a compelling  rationale for considering HDPE plastic pilings in  future marine construction projects. The durability,  resistance to corrosion and rot, and ability to withstand  ice movements make HDPE reinforced plastic piles  an attractive choice for environments facing similar  challenges. 

CONCLUSION 

AMI Consulting Engineers and Tangent Technologies  faced a tough challenge with the full-scale test in the  Duluth Superior harbor. The result? The HDPE plastic  piles didn’t just survive; they outperformed conventional  timber materials and provided a real alternative to steel  piling in the right design conditions. Even after eight  years, the HDPE plastic piles’ durability, resilience, and  overall effectiveness make them a promising alternative  to conventional materials in marine construction  projects facing challenging conditions, such as severe  ice and accelerated corrosion present in the Duluth  Superior harbor. As we look to the future of marine  engineering, it’s clear that innovations like this can pave  the way for products that are safer, environmentally  friendly and provide better reliance and sustainability.