Constructed a partially penetrating breakwater to deflect swells from Skagway’s Small Boat Harbor. Involved driving more than 100 steel piles a 100-ft long pedestrian bridge, wood promenade, and structural welding for steel pile caps and other steel structures.
Previously, Skagway’s harbor provided access from the south allowing strong southerly wind surges to enter the harbor and batter the existing float system. The new wave attenuator is an arced flat sheet combination wall supported by 24-in dia. by half-inch-thick bearing piles.
Each of the bearing piles was fitted with SPIN FIN® tips. These specially designed tips are composed of several angled steel plates welded to the bottom of the piles causing the piles to turn when driven increasing compression and tension capacity. This construction method allowed the barrier wall, designed with a flat, closed ended pile, to use the shortest pile possible at a substantial cost saving to the Client. Two-inch-thick solid end caps allowed the pile to be terminated at a shallower depth than open-ended pile. The angled plates provide more bearing and pulling area once piles are locked off by welding to their caps.
This method made pile installation more difficult when hard layers were encountered at depths shallower than anticipated. Close coordination with on-site Engineers allowed the crews to adapt installation as needed. Sheet pile bottom was terminated at elevation -11 feet Mean Lower Low Water (MLLW). The 20-inch diameter pile were driven into the ground for support, but the sheet pile are terminated above the mud line. This innovative solution was chosen because it would provide effective protection while minimizing the footprint of the structure.
The wave barrier wall is composed of 20-inch dia. half-inch thick steel pile with flat sheet pile welded to either side. The pile-sheet pile combinations were installed by threading the sheet pile interlocks together to provide a continuous wall. The finished structure was a suspended 30 foot tall steel curtain 290 feet in length to deflect wave action.
Despite high-tide depths of nearly 70-ft, more than half the pile extended above the practical template elevation resulting in the pile being top-heavy and unstable in the batter position. Pile was offset from the template to stabilize the pile without extending the template height. Once in position, the pile was vibrated vertically several feet into the marine sediment before laying it over to the batter position. Once the pile was confirmed to be in the correct position and batter, the pile was vibrated an additional 40 to 60 feet using an ICE 44B vibratory hammer. Pile was then impacted to final tip with an I-36 diesel impact hammer.
Achieving planned tip elevations was critical to ensuring the flat sheets welded to the pipe sections were all the same grade creating an even-top-of-wall alignment. The installation sequence, generally guided by TESPA, required a stepped installation approach to advance proper alignment and avoid steel binding. Any misalignment by the pile driving crew would make threading the sheets near impossible or, if threaded, result in a steel bind. PPM worked closely with the field engineer to establish driving criteria that allowed various size holes to be cut in the pile and end plate prior to the pile reaching practical vibratory refusal.
The pile driving crew faced high winds and rough seas with a 25-ft tide ranges while driving 140- to 160-ft long 24-in dia. battered bearing piles. To minimize pile spliced in these conditions, most of the pile was driven full length. Driving the bearing pile in one piece reduced the crew’s exposure to the severe elements and decreased time spent staged on over-water templates. By minimizing the need to access work on icy template beams in windy, dark conditions, fall and other risks were significantly reduced.
Due to seasonal restrictions and environmental constraints, work took place during the winter months. Support crews dealt with large snow packs, heavy winds, and low temperatures; average temperatures ranged from 22 to 31 degrees Fahrenheit before accounting for the near constant wind. Daylight was limited during the construction to four hours of daylight in addition to frequent overcast conditions.
In order to complete the project in the short work window, crews worked an average of six 12-hour shifts each week. Days off were scheduled around weather days. If weather was especially severe, work would shut down until safe operations could resume. Balancing the need to maintain operations without compromising our commitment to safety allowed the project to be successfully completely without injury.
Construction was based out of two barges; a designated crane barge and a materials barge. Pile was loaded on a barge in Seattle, WA and towed to Skagway, AK. The planned construction sequence required the material barge to be loaded in a designated order. The larger bearing piles were among the last to be loaded as they were the first to be driven.
PPM also built the promenade on top of the wave barrier. The timber promenade has become a popular attraction, allowing the public to enjoy Skagway’s spectacular landscape and stunning overwater views. The promenade is supported along the curved wall pile cap bearing pile box caps with a handrail around its entire perimeter. The wall pile cap beam was fabricated in approximately 30-foot-long sections to minimize field welding. The handrail was fabricated in sections and the catwalk fabricated and assembled prior to arrival in Skagway with the exception of the timber decking. The configuration is accessed by a 100-foot catwalk mounted on-shore.
The new breakwater has proven effective by a number of recent storm events, successfully blocking waves and greatly improving conditions in the Small Boat Harbor. With the protection of the new attenuator, the Municipality is now free to expand the harbor as desired. A new float replacement with an increased slip count is now planned for the entire basin.
From start of design through all phases of construction, client-partnering was utilized to align the interests of all parties involved. PPM was able to meet the aggressive schedule and complete the project on time, within budget with an outstanding safety and performance record. This is particularly noteworthy because of the high risk nature of steel construction.
For the past 20 years, Skagway has been a tourist destination, drawing people from around the world to experience the pristine landscape of southeast Alaska. The Skagway boat harbor provides berths for 5 cruise ships up to 1,000 feet in length. As many as 10,000 visitors embark every day through this port during cruise season. This project was been given national recognition winning awards such as the Pile Driving Contractors Association 2009 Pile Driving Project of the Year Award.
This project was particularly challenging because of the horizontal curve alignment and small tolerances for pile installation. PPM designed and constructed a complex pile driving template structure in order to install the pile and ensure all the interlocking sheets did not bind. Including King Pile, Spin Fin® piling, and sheet pile.